• No results found

Use of Stable Isotopes and GIS Modeling to Study Late Pleistocene to Holocene Environmental Change in the Waipaoa Sedimentary System, New Zealand

N/A
N/A
Protected

Academic year: 2020

Share "Use of Stable Isotopes and GIS Modeling to Study Late Pleistocene to Holocene Environmental Change in the Waipaoa Sedimentary System, New Zealand"

Copied!
98
0
0

Loading.... (view fulltext now)

Full text

(1)

ABSTRACT

CHILDRESS, LAUREL BIRCH. Use of Stable Isotopes and GIS Modeling to Study Late Pleistocene to Holocene Environmental Change in the Waipaoa Sedimentary System, New Zealand. (Under the direction of Dr. Elana L. Leithold.)

The source to sink investigation of complex sedimentary systems necessitates chemical (stable isotope and elemental) and physical (modeling) analyses to elucidate temporal changes in volume and provenance of sediment supply. The Waipaoa River watershed, New Zealand, represents a system of interest due to its location on an active margin, very large sediment supply, and well known history of environmental

perturbation. Geographic information systems (GIS) modeling suggests large increases in erosion would have resulted from reduction in landcover due to natural volcanic events and anthropogenic disturbances. The biogeochemical record of these changes has been investigated using the stable carbon and nitrogen isotopic of organic matter in three cores recovered from the adjacent continental shelf by the RV Marion Dufresne in January 2006. The cores contain a record that extends to the late Pleistocene, as far back as 14,000 years. Within the shelf cores, gradual isotopic changes attributed to sea-level change are overlain by a distinct, shorter-term excursion to higher 13C/12C ratios that occurred between approximately 2000 and 500 years before present. Possible

(2)

excursion could suggest an increase in marine organic matter burial, but could also be the

result of increased input of degraded, refractory terrigenous organic matter or

(3)

Use of Stable Isotopes and GIS Modeling to Study Late Pleistocene to Holocene Environmental Change in the

Waipaoa Sedimentary System, New Zealand

by

Laurel Birch Childress

A thesis submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the Degree of

Master of Science

Marine, Earth, and Atmospheric Sciences

Raleigh, North Carolina

2009

APPROVED BY:

__________________________ __________________________ Dr. H. Mitasova Dr. D. J. DeMaster

__________________________ __________________________ Dr. E. L. Leithold Dr. C. L. Osburn

(4)

DEDICATION

(5)

BIOGRAPHY

Laurel Birch Childress hails from the mountains of southwestern Virginia, but spent the

majority of her formative years along the Carolina coastlines of Kill Devil Hills,

Charleston, SC and Wilmington, NC. She attended Topsail High School in Hampstead,

NC where she was a member of the school’s first Electric Vehicle class and performed

hundreds of hours of community service in Key Club. She graduated as Valedictorian in

2002, going on to attend North Carolina State University as a Park Scholar. She received

her B.S. in Marine Sciences (Geology Concentration) in 2006. She continued her

research at NC State, pursuing a Masters Degree in the same field.

Her love of the outdoors and interest in world travel brought her to the study of geology

and marine sediments. She has eclectic musical tastes, a love of animals, enjoys playing

disc golf during her travels, and is always at home outdoors. Laurel’s mountain heritage

has a geological connection evidenced in the exquisite rock churches built by her great

(6)

ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Elana Leithold, and my committee, Drs. David DeMaster, Christopher Osburn, and Helena Mitasova. Without their invaluable insight and help I would not have been able to complete this project. I would especially like to thank Dr. Lonnie Leithold for providing me not only with an amazing graduate project and opportunities to travel around the world, but for allowing me to pursue an area of research I love dearly.

(7)

TABLE OF CONTENTS

LIST OF TABLES………. LIST OF FIGURES………... INTRODUCTION………. CHAPTER I. ISOTOPIC ANALYSIS OF SOIL PROFILES AND

CONTINENTAL SHELF CORES………..………….. Methods………...……….. Sample Collection……….………...…….. Sample Preparation... Analysis Procedures... Results and Discussion... Soils………...……… Radiogenic Isotope Analysis………. Mass Balance Calculations.………... Core Interpretation.……… Conclusions……….... CHAPTER II. GEOGRAPHIC INFORMATION SYSTEMS EROSION

MODELING………. Methods………. Approach………... Data……… RUSLE3d Methods………... USPED Methods………... Results and Discussion………..

RUSLE3d Model………... USPED Model………... Model Comparison……… Model Accuracy……… Summary and Conclusions……….………... REFERENCES……….. FIGURES………... TABLES……… APPENDIXES………... Appendix A. Soil Profile Results………..………. Appendix B. Two Source Mass Balance Results.………. Appendix C. Three Source Mass Balance Results………. Appendix D. Core Isotope and Elemental Results……….

(8)

LIST OF TABLES

Table 1.1

Table 1.2

Table 2.1

Table 2.2

Waipaoa watershed soil sample locations and excavation depths……….

Results of 14C analyses for bulk sediments……….

Erosion values for RUSLE3d modeling………

Percentage of erosion/deposition for each land cover scenario in USPED model……….

77

78

79

(9)

LIST OF FIGURES Figure 1.1 Figure 1.2 Figure 1.3 Figure 1.4 Figure 1.5 Figure 1.6 Figure 1.7 Figure 1.8 Figure 1.9 Figure 1.10 Figure 1.11 Figure 1.12 Figure 1.13 Figure 1.14 Figure 1.15 Figure 1.16 Figure 1.17 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6

Map of the Waipaoa River watershed, North Island, New Zealand………... Map of sites for continental shelf cores MD3007, MD3004, and MD3006………..……… Percent nitrogen and nitrogen isotope composition of core MD3007 of both untreated and acidified samples...……….. Acidified and untreated soil, rock, river, and marine

samples from East Coast, North Island, NZ..……… Organic carbon and nitrogen isotopic values for soil

profiles………...……… Carbon and nitrogen isotope values of terrestrial and marine sediments…..………. Percent organic carbon and nitrogen for soil profiles ……... Bulk organic matter 14C measurements, core MD3007... Terrestrial fraction and 13Corg values for cores MD3007,

MD3004, and MD3006……….. Fractions marine and terrestrial for cores MD3007,

MD3004, and MD3006……….. Fractions marine, terrestrial, and kerogen, core MD3007…. Percentages of marine, terrestrial, and kerogen carbon in core MD3007………. Grain size (Brulet, 2009), 15N, and 13Corg values for cores

MD3007, MD3004, and MD3006………. Percent carbonates and 13Corg values for cores MD3007,

(10)

LIST OF FIGURES

Figure 2.7

Figure 2.8

Figure 2.9

Erosion simulated by RUSLE3d under current land cover scenario……….. Erosion simulated by USPED under pre-anthropogenic land cover scenario……… Erosion simulated by USPED under current land cover scenario………..

74

75

(11)

INTRODUCTION

Reconstructing the long term evolution of coupled terrestrial – marine sedimentary

systems requires the use of multiple tools, including chemical and physical analyses. The

stable isotopic composition of organic matter in continental margin sediments provides a

useful, long-term record of environmental change, while geographic information systems

(GIS) modeling provides a platform on which to evaluate past erosional processes. The

Waipaoa River watershed, New Zealand, represents a system of interest due to its very

large sediment supply and well known, relatively recent history of anthropogenic

disturbance. Located on the North Island of New Zealand the Waipaoa River watershed

measures only 2205 km2, yet has a sediment yield of 6750 tons/km2/year (Hicks et al.,

2000; Figure 1.1). Small mountainous watersheds, such as the Waipaoa, have been

shown previously to be an underestimated source of global sediment supply, with those in

Oceania alone accounting for as much as 20% of the sediment flux to the world’s oceans

(Milliman and Syvitski, 1992).

The Waipaoa watershed lies adjacent to the Raukumara Range and empties into Poverty

Bay. As seen in Figure 1.1, the underlying bedrock can be informally grouped into four

categories: Cretaceous to Oligocene in-place sedimentary rocks, Early Cretaceous to

Oligocene displaced sedimentary rocks of the East Coast Allocthon, Miocene to Pliocene

sedimentary rocks, and Quaternary unconsolidated sediments (Mazengarb and Speden,

(12)

limestone. Pliocene and Miocene rocks are poorly consolidated and consist of sandstone,

siltstone, mudstone, and limestone.

High erosion rates within the watershed are attributable to weak soil cover, frequent

seismic activity, heavy yet infrequent rainfall, limited land cover, and steep slopes. These

factors combine with chemical weathering (Lyons et al., 2005) to produce high rates of

landsliding and gullying, resulting in the large sediment volumes delivered to the river.

These hillslope failures are enhanced by extreme precipitation events such as cyclones,

and by anthropogenic influences within the watershed. Approximately 700 years B.P. the

now native inhabitants, Maori, arrived (Jones, 1988) and began clearing small volumes of

indigenous forest, however, mass clearing did not begin until the arrival of European

settlers in the early 1800s (Pullar, 1962). Following the removal of most indigenous

forest by both Maori and Europeans, much of the deforested land was covered with

pasture and the nation of New Zealand now supports approximately 40 million sheep. It

is estimated that less than 3% of the indigenous primary forest remains today, although

some small efforts with reforestation have been made (Hicks et al., 2000).

From the uplands eroded material is carried in the river as bedload and suspended load

before reaching Poverty Bay. Circulation patterns within the bay transport sediment and

organic matter to the continental shelf (Stephens et al., 2002), which is on average

approximately 25 m wide and has several canyons along the outer shelf and slope. Along

(13)

exist, including the northerly flowing Wairarapa Coastal Current and the southerly

flowing, more offshore, surface East Cape current (Carter et al., 2002). The sedimentary

organic matter deposited on the continental shelf is a mixture of terrestrial and marine

components, each of which has a characteristic carbon isotopic signature, which enables

the determination of changes in source material and processes through time. Marine

organic matter, including that derived from phytoplankton and zooplankton, mixes with

terrestrial organic matter delivered to the shelf via rivers within the water column and

after deposition on the continental shelf.

Consequently, the isotopic record is dependent upon productivity levels at the ocean’s

surface, which may be nutrient or climate driven, as well as sediment and organic matter

delivery from the watershed, which can vary based on the dominant terrestrial processes.

Terrestrial processes may be driven by large scale events such as cyclones and climate

change or localized activity such as burning and agriculture. Vegetation changes within

the watershed, such as a shift from C3 to C4 plants, can be the result of a changing climate

or the result of anthropogenic alteration of land cover for agricultural purposes. Other

possible changes recorded in continental shelf sediments include: storm frequency or

ocean currents as the result of climate changes, and alterations in fire patterns as the

result of volcanic eruptions or anthropogenic land clearing. Large volcanic eruptions

result in tephra deposits in the both terrestrial and marine environments. The main source

of tephra to the region is the Taupo Volcanic Zone located west of the watershed

(14)

occurred during the past 10,000 years and resulting tephras within the continental shelf

sediments at the mouth of the Waipaoa include the Taupo (1718 B.P.; Sparks et al., 1995,

Carter el al. 2002), Waimihia (3410 B.P.; Carter, et al., 2002; Hajdas et al., 2006), and

Whakatane (5530 B.P.; Carter, et al., 2002; Hajdas et al., 2006) layers (Gerber, 2009).

Phillips and Gomez (2007) state that land cover is a key and perhaps underestimated

control on sediment erosion throughout the Waipaoa watershed. Modeling of erosion

through geographic information systems (GIS) can be used to address the impact of land

cover change throughout watershed history. By analyzing the impact of varied land cover

on the erosion rate of the watershed and quantifying the impact of anthropogenic

influence on erosion rates, erosion processes as a result of land cover changes can be

resolved for periods prior to any anthropogenic influences and current inhabitation.

The temporal isotopic changes of the Waipaoa recorded in offshore stratigraphy are

highly influenced by terrestrial processes, of which erosion is a major contributing factor

(Carter et al., 2002; Phillips and Gomez, 2007). Significant increases in watershed

erosion can lead to delivery of a greater fraction of terrestrial material to the continental

shelf, thus significantly altering the isotopic composition of the total deposited organic

matter. The goal of this study was to couple carbon and nitrogen isotopic data from

continental shelf cores with watershed erosion models to better resolve the record of

environmental change within the Waipaoa sedimentary system during the late Pleistocene

(15)

CHAPTER I. ISOTOPIC ANALYSIS OF SOIL PROFILES AND

CONTINENTAL SHELF CORES

Methods

Sample Collection

Continental Shelf Cores. MD3004, MD3006, and MD3007 were collected by the MATACORE group aboard the research vessel Marion Dufresne in January 2006 using

the Calypso coring system. Core MD3004 was collected at 38º 51.49’S, 178º 04.90’E at a

water depth of 52 meters and had a total length of 16.36 meters. Core MD3006 was

collected at 38º 57.60’S, 178º 10.92’E at a water depth of 122 meters was 25.54 meters

long. Core MD3007 was collected at 38º 43.53’S, 178º 14.00’E at a water depth of 61

meters and had a total length of 22.47 meters (Figure 1.2). Once aboard the vessel, cores

were split lengthwise for archive and research purposes. Cores were stored refrigerated at

approximately 4 ºC until they were sampled in July of 2007. Sediment samples were

taken at 20 cm intervals through the entire length of each core and at 10 cm intervals

within the topmost 100 cm. Samples were immediately frozen for transport. Some

thawing potentially occurred during transit from New Zealand to North Carolina State

University, however all samples arrived cold and were immediately returned to a frozen

(16)

Soil Profiles. Soil samples were collected in the Waipaoa River watershed during July 2007. Sites sampled include those mapped in Figure 1.1. Soil classifications are based on

maps of the NZ Fundamentals Soils Layer developed by Landcare Research, NZ. Soil

profile OR070725 is located at approximately 432 m elevation and extends to a depth of

100 cm through an Orthic Pumice Soil resting on gravel. The first 12 cm extend through

light brown soil, followed by dark brown soil to a depth of 20 cm. Between 20 and 60 cm

exists dark red/brown soil, and below 60 cm depth light red/brown soil. Profile

TD070726 is located at approximately 576 m elevation and extends to a depth of 60 cm

through Brown Soil. The profile is very distal to the river mouth and is located near the

Tarndale Slip. At the surface of the profile is significant vegetative cover, while the base

of the profile rests on bedrock. Profile TA070729 is located at approximately 14 m

elevation and extends to a depth of 50cm through Brown Soil to Pliocene bedrock. This

profile is inferred to be a relatively recent soil, based on the absence of tephra layers.

Obvious hillslope failures are located in the field above the profile. Profile PU070729 is

located at approximately 146 m elevation and extends to a depth of 130 cm through

Orthic Pumice type soil. This profile is located on a terrace and is a composite of

overbank silts overlain by developed soils. Tephra layers are visible within the profile.

Volcanic pumice from Rerewhakaaitu, Waiohau, and Waimihia are seen at depths of 95,

80, and 32 cm, respectively (A. Palmer, personal communication; Eden et al., 2001). Also

of note, this profile was collected in a sheep pasture. At each site a soil profile was

excavated and exposed to the depth indicated in Table 1. Samples were then taken at the

(17)

some thawing potentially occurred during transit from New Zealand to North Carolina

State University, however, all samples arrived cold and were immediately returned to a

frozen state upon arrival and remained thus until analysis.

Sample Preparation

Drying Procedures. Samples were prepared for 15N, %C, and carbonate analyses by weighing approximately 1-2 gm of bulk sediment or soil into tared 50 mL beakers.

Samples were then dried for 24 hours in a Labconco Freeze Dry System/Bulk Tray

Dryer. Immediately following drying, samples were re-weighed, ground, and refrozen for

storage until analysis.

Acidification Procedures. Samples were prepared for 13Corg, %OC, and 15N (core

MD3007 only) analyses by first drying as described above. Samples were then

submerged in 5ml of 4N HCl (aqueous) for a period of 96 hours to ensure complete

removal of carbonates. Samples were not rinsed after acidification to prevent the possible

loss of labile fractions. After 96 hours, samples were freeze dried in a homemade vacuum

system at room temperature to remove the remaining acid. Samples were usually

completely dry within 24 to 48 hours and were then re-weighed, ground, and frozen for

(18)

Analysis Procedures

Elemental Analyzer and IRMS. Acidified samples were analyzed for %OC and %N. Samples which were not acidified (untreated) were analyzed for %Ctotal and %N. Both

acidified and untreated samples were analyzed using a continuous flow Flash 1112

Elemental Analyzer with a standard error of 2%. Using tin boats, bulk sediment and soils

samples were weighed (approximately 20-350 mg used) and then combusted at 900º C.

Samples analyzed for 13C, 13Corg, and 15N were then passed to a Thermo Delta V Plus

IRMS, with a precision of 0.1‰, coupled with the above mentioned elemental analyzer

via a Conflo III.

The carbon isotopic composition of samples is expressed as:

) (

(

)

(

/

)

1000

/ / tan 12 13 tan 12 13 12 13 13 ×         − = dard s dard s sample C C C C C C C δ sample C C 12 13

/ is the ratio of the heavy isotope, 13C, over that of the light isotope, 12C, of the

sample. Cstandard

12 is Pee Dee Belemnite, PDB (Faure and Mensing, 2005).

The nitrogen isotopic composition of samples is expressed as:

) (

(

)

(

/

)

1000

(19)

sample

N N 14

15

/ is the ratio of the heavy isotope, 15N, over that of the light isotope, 14N, of

the sample. Nstandard

14

is atmospheric nitrogen, which has a 15N value of 0.0‰ (Faure

and Mensing, 2005).

With respect to nitrogen, literature differs on whether samples should be analyzed

acidified or untreated. Kennedy et al. (2005) report a significant difference between 15N

values of acidified and untreated marine sediment samples, with acidified samples being

more negative, however, they cannot identify what processes might be driving this

difference. Ryba and Burgess (2002) report large differences in %N with treatment. They

reported that acidification by HCl vapor resulting in large increases while direct aqueous

HCl acidification resulted in significant decreases in nitrogen content and thus

recommend the use of untreated sediments for %N analysis. Therefore, in this study

nearly all samples from core MD3007 were run both acidified and untreated for 15N and

%N to determine if acidification impacted elemental composition or isotopic values

(Figure 1.3). While general trends between acidified and untreated samples held in

measurements of %N and 15N, some differences between measurements were apparent.

For 15N measurements acidified and untreated samples varied by as much as 0.4‰ at

1166 cm depth, and by as little as 0.0‰ at 32 cm depth. The maximum isotopic

difference of 0.4‰ (averaged over three sample runs) is outside of instrument error

specifications of 0.1‰. Furthermore, samples analyzed specifically from core MD3007

(20)

(n=36) returned more positive values with acidification and the mean 15N value of

acidified samples was within 0.1‰ of the untreated samples. Thus, based on the analysis

of core MD3007 (p = 0.59) there is no significant difference between acidified and

untreated samples. However, analysis of terrestrial soil samples using both acidified and

untreated samples revealed differences of nearly 1.5‰, indicating some form of isotopic

alteration of sample during acidification (Figure 1.4). Therefore, despite the seemingly

small differences between 15N values within core MD3007, untreated samples were used

for 15N analyses of samples from MD3007 and the other cores, as well as all terrestrial

samples to preclude procedural alteration of nitrogen isotopic values. All 15N data

presented from this point forward represent untreated analyses. However, the samples

analyzed within core MD3007 (p = 0.49), soil profiles, and those presented in Kennedy et

al. (2005) showed no significant difference between acidification and no treatment for

elemental analysis so acidified values will be presented.

Values for percent inorganic carbon, carbonate, and 13C values for carbonate were

calculated from total percent C measurements. Percent inorganic carbon was calculated

by the percent carbon of untreated samples (total carbon) minus measured percent

organic carbon. Percent carbonate was then calculated by:

(

% 3.34

) (

% 4.0

)

%

(21)

which represents percent inorganic carbon added to percent inorganic carbon multiplied

by 3.34 (40 mol Ca ÷ 12 mol C) added to percent inorganic carbon multiplied by 4.0 (48

mol O3 ÷ 12 mol C). 13C of carbonate was estimated by:

inorg

org org

total carbonate

FractionC

C FractionC

C

C ( )

13 13

13 δ δ

δ = − ×

14

C Collection. Samples to be analyzed for 14C were combusted using the elemental analyzer. Oxidized gasses were then passed to a vacuum trapping system where the CO2

was trapped cryogenically in glass tubes. The tubes were then shipped to the National

Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility at Woods Hole

Oceanographic Institution (WHOI), where the CO2 was converted to graphite before

analysis for 14C content.

Biogenic Silica. Sediment samples were analyzed for biogenic silica content by an

extraction procedure following DeMaster (1979). Samples were weighed (30-50 mg) into

50 mL tubes and a 1% sodium carbonate solution was added to fill. Tubes were then

placed in a water bath at 85ºC. At intervals of 1, 2, 3, and 5 hours the tubes were mixed,

allowed to settle for 1 minute, and 1 mL of each sample’s solution and 9 ml of distilled

water were combined with 4 mL of a molybdate solution and allowed to stand for 10

minutes. The molybdate solution was formed by mixing 4 g analytical reagent quality

(22)

volume was then made to 500 mL with distilled water. A reducing agent was made by

mixing 100 mL of metol-sulphite solution (6 g anhydrous sodium sulphite, 500 mL

distilled water, 10 g metol (p-methylaminophenol sulphate); filtered through No. 1

Whatman filter paper) with 60 mL oxalic acid solution (50 g analytical reagent quality

oxalic acid dihydrate, 500 mLdistilled water) and 60 mL 50% suphuric acid solution (250

mL concentrated reagent quality sulphuric acid, 250 mL distilled water). Following the

ten minute interval 6 mL of the reducing agent were added and the solution was left to

stand for 2-3 hours. Samples were then analyzed using a spectrophotometer to measure

absorbance. From the absorbance the weight percent of biogenic silica extracted was

calculated by converting the absorbance to percent silica using a correction based on a

calibration curve, made simultaneously with standards, and correcting this value based on

sediment sample weight.

Results and Discussion

Soils. Values for 13Corg in soil profiles range between -27.3‰ and -22.4‰ (Figure 1.5,

Appendix A). Values for 15N in soil profiles range between 0.4‰ and 8.3‰ (Figure 1.5,

Appendix A). 13Corg and 15N values within soil profiles of disparate geographic location

vary greatly (Figure 1.6). Within soil profiles OR070725 and TD070726 carbon and

nitrogen isotope values both trend toward more positive values with depth. In profile

OR070725 13Corg values at the surface are as low as approximately -27.0‰ and at depth

as positive as approximately -25.0‰. 15N values are approximately 2.0‰ at the surface

(23)

that of soil profile OR070725 measuring approximately -27.0‰, however at depth only

increase to approximately -26.0‰. At the surface of TD070726, 15N values are

approximately 3.0‰ and shift to values as positive as 5.0‰ at approximately 10cm depth

and then return to nearly 3.0‰ near the bottom of the profile. All four soil profiles follow

a trend toward less %OC and N content with depth, although specific values vary by

profile. (Figure 1.7, Appendix A) Within soil profile OR070725, OC near the surface (10

cm depth) measures 5.75%, while at the base of the profile the OC content is as low as

1.71%. Percent N within the profile near the surface measures 0.43% and at the base of

the profile N content is 0.10%. Profile TD070726 resembles profile OR070725 in both

%OC and N content. OC for TD070726 is 6.41% at the surface and 1.64% at the bottom

of the profile. Percent N content is 0.38% at the surface and 0.11% at the bottom of the

profile. The isotopic trends of OR070725 and TD070726 in combination with the

decreasing %C and N with depth suggest these soils have formed in place. Ehleringer et

al. (2000) concluded that the cause of increasing 13Corg values with decreasing %OC

with depth in a soil on which C3 plants grow is the result of microbial incorporation in

conjunction to a small degree with decreasing 13C ratios of atmospheric CO2. Microbial

incorporation of altered atmospheric CO2 results in more positive 13Corg values (Boström

et al., 2007).

Soil profiles TA070729 and PU070729 also show a large loss in %OC and N with depth

but do not show the increase in carbon isotope values (Figure 1.7). Profile TA070729 has

13

(24)

approximately -23.0‰, with a final value of approximately -24.0‰ in the deepest

sample. This could suggest that instead of having formed in place from bedrock, this soil

formed as an accrual profile with sediments from surrounding areas building the profile

through time. This is supported by physical evidence at the location of the soil profiles

collection, where there is obvious hill slope failure present above and near the profile.

Further support comes from percent organic carbon and nitrogen values. Surface OC

content is only 1.74% and decreases to 0.11% at the base of the profile. Percent N values

are similarly low, decreasing from only 0.17% at the surface to 0.02% at the bottom of

the profile. These low values of organic carbon and nitrogen could suggest the soil is

formed as a composite of other soil profiles. A hillslope failure could release soil from

depth with lower OC and N values which could then be formed downhill into a profile

such as TA070729. Of note is that despite the decrease in 13Corg there is an increase in

15

N values, with surface values as low as nearly 0.0‰ and values at depth as positive as

approximately 4.5‰. This could possibly be the result of microbial or biologic plant

processes present in the soil (Ehleringer, et al., 2000; Boström, et al., 2007). Soil profile

PU070729 has a more negative 13Corg profile than TA070729, but remains more stable

with depth. Surface 13Corg values measure approximately -24.0‰ and 13Corg values at

the bottom of the core equal approximately -25.0‰. The site of collection for this profile

is known to be an accrual area of sediments representing a terrace accretion area. This is

supported by the presence of tephra layers within the soil profile. The formation of this

soil represents to compilation of previously formed soils which leads to the irregular

(25)

exhibit values at both the top and bottom of the profile approximately 4.0‰ and the most

positive values of approximately 7.0‰ between 40-60 cm depth. In terms of percent

organic carbon and nitrogen profile PU070729 follows the same trends as the other three

profiles, but shows higher OC and N contents at the surface, 7.9% and 0.64%

respectively. At the bottom of the profile, OC and N contents are only 0.16% and 0.03%

respectively. Extremely high percentages of organic carbon and nitrogen within the upper

10 cm of the profile could be the result of contamination from the sheep pasture.

Radiogenic Isotope Analysis. From core MD3007, 11 bulk sediment samples were analyzed for 14C. Ages were calibrated using Calib version 5.0.1 software

(http://calib.qub.ac.uk/calib/). Calibrated values reported represent 1-sigma deviations

(Table 2). Bulk sediment calibrated ages do not correlate well with known tephra ages

from within the cores but appear to be substantially older (Figure 1.8). These results are

expected as samples from the contemporary Waipaoa sedimentary system contain a

mixture of modern and ancient marine and terrestrial OC (Leithold et al., 2006; Blair et

al., in review) Despite their old ages, bulk sediments in MD3007 do indicate sediment

accumulation rates similar to rates calculated by Brulet (2009) using wood fragments

from the same samples as well as tephra layers. From the base of the core to a depth of

approximately 666 cm the accumulation rate is 273 cm/ky using bulk sediment compared

with 293 cm/ky based on wood and 231 cm/ky based on tephra (Brulet, 2009). Between

approximately 666 cm and the surface of the core the accumulation rate is approximately

(26)

cm/ky based on tephra (Brulet, 2009). The similarity of bulk, wood, and tephra

accumulation rates suggests little alteration to the volume of modern material mixed with

aged through time.

Mass Balance Calculations. To better evaluate the sources of organic matter within the cores, several mass balance calculations were performed. A two source model was used

to estimate the proportion of marine and terrestrial organic matter in each sample using

13

Corg. (Figures 1.9 and 1.10). For the two source 13Corg model, endmember values of

-19.0‰ (Brackley, 2006; Thompson, 2009) and -26.7‰ were for marine and terrestrial

organic carbon, respectively. The riverine organic carbon end-member value was based

on an average of suspended river samples taken at Kanakanaia (Figure 1.1) in 2003 and

2004 (Leithold et al., 2006; Blair et al., in review). Fractions terrestrial and marine were

calculated using the equation:

l terrestria l

terrestria marine

marine

sample f C f C

C 13 13

13 δ δ

δ = +

From fractions marine and terrestrial, the percent marine and terrestrial carbon at each

depth can be calculated as the percent organic carbon multiplied by fraction marine or

terrestrial. Calculated terrestrial fraction and percent marine and terrestrial carbon for

each core are presented in Figure 1.9 and Figure 1.10, respectively. Raw values are

presented in Appendix B. Discussion and interpretation of results follows in the section

(27)

A three source model was also employed to estimate the proportion of marine and

terrestrial organic matter, as well as kerogen content in each sample using 13Corg and

14

C (Figures 1.11 and 1.12). For the three source 13Corg and 14C model, end-member

values of -19.0‰ (Brackley, 2006; Thompson, 2009) and -26.7‰ were again used for

marine and terrestrial organic carbon, respectively. An end member value of -25.7‰ was

used for 13C of kerogen (Blair et al., in review). Values for 14C end members of

marine, terrestrial, and kerogen were 100, 100, and -1000, respectively. Fractions marine,

terrestrial, and kerogen were calculated with the equations:

ogen ogen

l terrestria l

terrestria marine

marine

sample f C f C f C

C 13 13 ker 13 ker

13 δ δ δ

δ = + +

ogen ogen

l terrestria l

terrestria marine

marine

sample f C f C f C

C 14 14 ker 14 ker

14 = + +

From fractions marine, terrestrial, and kerogen the percentages at each depth can be

calculated as the percent organic carbon multiplied by fraction. Calculated fractions and

percentages marine, terrestrial, and kerogen carbon for core MD3007 are presented in

Figure 1.11 and Figure 1.12, respectively. The three source model was only performed on

core MD3007 due to limited 14C data. Raw values are presented in Appendix C.

(28)

The calculations incorporate several assumptions, the most significant being the

assumption that values of marine and riverine end-members have remained constant over

the entire temporal duration of the cores. Such consistency through time is certainly

unlikely, but in the absence of known past end-member values contemporary values

represents the best available data. These calculations also use an average of current

end-member values, which may not accurately represent the entirety of material suspended

within the river or deposited on the continental shelf. Preferential suspension of any

fraction of river material during measurements could skew values obtained for

calculations. On the continental shelf marine organic matter measurements could

represent a limited portion of available biota and thus over or underestimate values.

These calculations show a decrease in terrestrial organic matter during a positive 13Corg

excursion to be discussed in the following section. In all three cores marine organic

matter contributions remain relatively stable throughout the length of the cores.

Core Interpretation. By using the bulk stable carbon and nitrogen isotope values in conjunction with %C, OC, and N within the three cores, a more complete environmental

history of the watershed and continental shelf can be developed. General values for

marine and terrestrial 13Corg include: -22.0‰ to -19.0‰ for phytoplankton (Fry and

Sherr, 1984; Prahl et al., 1994), -28.0‰ to -25.0‰ for C3 plants (Fry and Sherr, 1984),

-12.0‰ for C4 plants (Smith and Epstein, 1971; Fry and Sherr, 1984). Specifically within

the Waipaoa watershed rocks range from -26.5‰ to -22.8‰ (Leithold et al., 2006), and

(29)

MD3004, and MD3006 range between -25.7‰ and -23.2‰ (Figure 1.13, Appendix D).

Within core MD3007 there is a trend from approximately 2200 cm to 1900 cm depth

toward more positive values, which is then followed by a trend from 1900 cm to 203 cm

depth toward more negative values. Within this interval 13Corg values gradually move

from approximately -24.5‰ at the most positive to -25.5‰ at the most negative, for a net

change of ~1.0‰. These trends suggest a period of progressively increasing proportions

of marine organic matter followed by increasing terrestrial flux. This pattern would be

consistent with a sequence of relative sea level rise and fall. Lewis (1973) presented

evidence of sea level rise between approximately 15 and 9.5 ka B.P and Wolinsky et al.

(in review) and Rose (2008) present evidence for significant shoreline progradation

within the past 6,000 years, supporting evidence within the cores of a fall in sea level

corresponding to the above mentioned interval. 14C values from collected wood within

this interval (Brulet, 2009) indicate these sediments fall within this proposed age of seal

level rise and progradation. This 13Corg isotope trend is replicated to a small degree by

the 15N values, although the nitrogen isotopic signal is not as consistent. Terrestrial

sources of nitrogen isotopes are known to be more depleted in 15N, with typical 15N

values ranging between 0.0‰ to 4.0‰, and averaging around 2.5‰ (Mariotti et al., 1984;

Kuwae et al., 2007). Marine sources of nitrogen are generally more positive, with typical

values ranging between 6.0‰ to 8.0‰ and being measured as high as 9.0‰ in the open

(30)

The trend toward more negative 13Corg isotope values is also visible in the lower part of

core MD3004, however it is shifted toward slightly more positive values relative to those

of MD3007. Within core MD3004 from approximately 1600 cm to 300 cm depth there is

a gradual trend toward more negative values. 13Corg values shift from approximately

-23.5‰ to -25.0‰ within this interval, although this core lacks the preceding trend toward

more positive 13Corg values seen in core MD3007. This could possibly be due to the

shorter length of core MD3004. Core MD3004 is positioned slightly south of the

Waipaoa River mouth, and Poverty Bay (Figure 1.2). Prevailing anticlockwise currents

within the bay and northerly shelf currents could cause preferential deposition at site

MD3007 and isotope signals may not be as well recorded to the south of the river mouth

(Stephens et al., 2002). This is evident in core MD3006 which is not only located further

south, but also at a greater water depth. Core MD3006 does not exhibit either the trend

toward more positive or negative 13Corg values between the base of the core and the

Taupo tephra layer but remains relatively constant, only fluctuating between values of

-24.0‰ and -23.5‰. This steady, more positive isotopic depositional signal is possibly

due in part to the water depth of core MD3006, which receives less terrigenous organic

matter (Figure 1.13) and is dominated by much more positive 13Corg values, indicative of

marine organic matter. Some reduction in isotopic signal strength at MD3006 could be

the result of the more southerly position relative to the river mouth.

Values for 15N in marine cores MD3007, MD3004, and MD3006 range between 4.3‰

(31)

cores toward more negative 15N values with decreasing depth in the seabed. There is

also a trend toward generally more positive 15N values with increased water depth and

distance from shore. In core MD 3007 15N values measure approximately 5.1‰ at the

bottom of the core. The trend towards more negative values continues up-core and at the

top 15N values measure as low as approximately 4.3‰. Within core MD3004 15N

values measure approximately 6.0‰ at the base of the core and as low as approximately

4.5‰ at the top of the core. In core MD3006 the total variation in 15N values is

significantly less than in the other cores, with the most positive value of approximately

6.0‰ measured near the base of the core and the most negative values of approximately

5.1‰ measured at the top of the core. 15N values are similar to 13Corg values in that

more positive values indicate more marine organic matter and more negative values

indicate a greater portion of terrestrial organic matter (Peters, et al., 1978; Liu, et al.,

2006; Kuwae, et al., 2007; Lambert, et al., 2008). The trends in 15N seen in all three

cores support evidence from 13Corg values that there has been a decreasing contribution

of terrestrial organic matter followed by a relative increase.

In core MD3007 %C increases from the base of the core up to approximately 1800 cm,

where it reaches a maximum of 1.5%. This coincides with the increase in percent

carbonates seen near the base of core MD3007 (Figure 1.14). Following this maximum,

%C values trend up-core toward lower values through 52 cm depth. Percent OC values

are relatively steady throughout the length of the core. Percent OC content at the base of

(32)

Between 500 cm and 200 cm there are occasional points that reach %OC values as high

as 0.8%. Percent N values within MD3007 trend upward very closely with %OC values

to a depth of 268 cm, with values approximately one order of magnitude less than that of

OC (Figure 1.15). Trends in %C, OC, and N within core MD3004 are nearly identical to

those observed in MD3007. C, OC, and N content increase from the base of the core up

to approximately 1000 cm depth, where values reach a maximum of 1.39%, 0.54%, and

0.06% respectively. Above this level, C, OC, and N content progressively decrease until

approximately 55 cm depth where C, OC, and N reach values of 0.79%, 0.26%, and

0.03%, respectively. Core MD3006 %C content is relatively constant throughout the

length of the core. Percent C values at the base and top of the core measure 1.38% and

1.4% respectively, and values throughout the core hold within 0.2% with the exception of

two values measuring approximately 1.0% C content. The upward trends in %OC and N

contents within core MD3006 are similar. Percent OC and N contents are lowest at the

base of the core measuring 0.33% and 0.04%, respectively. The upward rate of change in

OC and N is fairly low up to 90 cm depth, with OC and N only increasing by 0.3% and

0.03%. These values are consistent with observed changes in isotopic values in the lower

portions of the cores.

Following the sequence of carbon isotopic trends suggesting gradual relative sea level

rise and fall in the lower portions of the cores is a positive 13Corg excursion, visible in all

three cores but most pronounced in MD3007. In MD3007 from approximately 203 cm to

(33)

values increasing from approximately -25.5‰ to as positive as -23.8‰ for a net change

of ~1.5‰. In MD3004 an isotopic excursion similar to that seen in MD3007 is observed,

with 13Corg values ranging from -25.0‰ to approximately -23.3‰. Core MD3006

exhibits a similar, yet lesser magnitude excursion toward more positive values, with

13

Corg values increasing from approximately -24.0‰ to as positive as nearly -23.0‰. Of

note, in all three cores this isotopic excursion tracks reasonably well with grain size data

from Brulet (2009) (Figure 1.13). Several possibilities exist for this isotopic excursion

and include: 1) an increase in marine organic matter from a pulse of productivity, 2)

preferential preservation of marine organic matter, 3) an increase in flux of a heavy

terrestrial fraction, and 4) a decrease in the proportion of terrestrial organic matter due to

dilution of the fluvial discharge with volcanic ash.

If this excursion toward more positive 13Corg values is the result of an increase in marine

organic matter, then an increase in marine productivity might be implicated (Figure 1.10).

With an increase in productivity an increase in percent organic carbon would be expected

simultaneously, however this does not occur within cores MD3007 and MD3004, and in

fact a decrease in %OC is apparent (Figure 1.15). This could possibly be the result of

winnowing of this increased organic carbon to more distal portions of the shelf. The

possibility of winnowing is supported by a small increase in organic carbon and overall

smaller grain size (Brulet, 2009) in the most distal core, MD3006, during the excursion

(34)

proportion of marine organic carbon as calculated using previously mentioned mass

balance equations (Figures 1.10 and 1.12).

An increase in marine productivity could potentially increase the biogenic silica content

of the sediment if the productivity was dominated by siliceous diatoms. Core MD3007

was chosen for biogenic silica analyses due to the strong excursion signal shown relative

to the other cores. Two samples were analyzed from above the excursion and two

samples were analyzed from within the excursion. Results are reported in terms of weight

percent biogenic silica of the bulk sediment sample. Weight percent of biogenic silica

extracted for samples above the excursion reached values of 2.25 and 2.15 percent.

Weight percent of biogenic silica extracted for samples within the excursion reached

values of 2.1 and 2.31 percent. For comparison, a tephra sample which should contain no

biogenic silica was also analyzed and found to have a value of approximately 2.8 percent.

Thus, the biogenic silica analysis of core MD3007 revealed no significant change in

biogenic silica content between samples taken from above and within the excursion

(Figure 1.16). Furthermore, values measured indicate relatively low biogenic silica

content when compared with other shelf environments, such as North Carolina, where

there is significant biogenic silica content (DeMaster, 2002; Figure 1.16). This does not

support the hypothesis that an increase in biologic productivity is responsible for the

13

Corg excursion. It must also be noted that some biogenic silica tests are more resistant

to dissolution than others (DeMaster, 1979) and that these measurements do not take into

(35)

an increase in calcareous foraminifera an increase in percent carbonate would be

expected. There is, however, no increase seen in carbonate values during this excursion to

support the hypothesis of an increase in calcareous biologic debris (Figure 1.14). There is

also the possibility that biogenic silica tests initially present within the sediment were

dissolved during early diagenesis. Bennett et al. (1999) note than biogenic silica (opal)

often dissolves within the upper 5 cm of the seabed. This process may lead to poor

biogenic silica preservation. Preferential preservation of marine OC by absorption onto

clay particles is another possible cause for the 13Corg excursion. However, during the

excursion there is a notable increase in grain size in conjunction with decreasing clay

content, as shown by Brulet (2009).

Another possible explanation for the apparent increase in marine organic matter is that

there is a shift in the nature, volume, or provenance of terrigenous material delivered to

the continental shelf. C3 and C4 plants have very different isotope values (Smith and

Epstein, 1971; Fry and Sherr, 1984). In many Quaternary floral communities a shift from

C3 to C4 plants can be seen with time as the result of climate change (Ehleringer et al.,

1997). Within the late Holocene record of the Waipaoa system there is no evidence for

any natural shift in plant communities (Wilmshurst et al., 1999), however very recent

anthropogenic activities (past several decades) have introduced C4 agricultural crops to

the watershed. Such recent addition of C4 plants would not influence a positive isotope

excursion located at greater than 50 cm deep in all cores, as based on 14C dates these

(36)

Shifts in river path and stream capture could also account for the excursion in carbon

isotope values. Soil profiles from disparate areas of the watershed have distinct 13Corg

and 15N profiles (Figure 1.6). The soil profile taken from Te Arai, a tributary of the

Waipaoa shows 13Corg values which closely resemble that of marine organic carbon

signature and it is known that the Te Arai was captured following the Taupo eruption

(Pullar and Penhale, 1970; Gomez et al., 2007). While the Te Arai does not currently

contribute a significant volume of suspended sediment to the Waipaoa River (Hicks et al.,

2000), it is possible that previous erosion within the Te Arai was greater, and the

tributary a more important source of sediment to the river.

An increase in the input of volcanic ash to the river as the result of the Waimihia and

Taupo eruptions and/or mobilization of older volcanic material by shallow landsliding, is

another possible explanation for the isotopic excursion. Pollen records indicate that

vegetative cover was disturbed in the watershed for decades to hundreds of years

following the eruptions (Wilmshurst, 1999; Carter et al. 2002), which may have

temporarily accelerated delivery of ash to the fluvial system. In addition, Gomez et al.

(2004) have suggested that a period of increased storminess associated with the onset of

the modern ENSO regime may have lead to increased shallow landsliding of the

dominantly volcanic soils in the Waipaoa watershed after about 4000 years ago. An

increased contribution of carbon-poor volcanic material to the river load would dilute the

(37)

explain the apparently more marine-dominated isotopic values in the interval above the

Taupo tephra. This is supported by the three source mass balance equations, which show

a decrease in kerogen. Less carbon from kerogen could suggest a dilution of the

terrigenous signal by volcanic ash.

Within the positive 13Corg isotope excursion the 15N values also change. Within core

MD3007 15N values also become less positive within the limits of the 13Corg excursion.

While not as prominent as the 13Corg excursion, the 15N values within the excursion

are divergent from surrounding 15N values. It is known that during times of high nutrient

supply and primary productivity the organic matter deposited from the decay of

phytoplankton will have a relatively low 15N value (Schubert and Calvert, 2001).

However, the negative 15N excursion seen in MD3007 is not seen in MD3004 or

MD3006. Within MD3004 and MD3006 15N values are becoming more positive during

the excursion. Increasing 15N values in conjunction with decreasing grain size can

indicate an increase in marine organic matter (Kienast, et al., 2005), however the grain

size within the excursion increases between approximately 30 – 75 um in cores MD3007

and MD3004 and remains constant in MD3006. The decrease in 15N in conjunction with

the increase in grain size could indicate an increase in the input of terrestrial organic

matter. Terrestrial organic matter is generally associated with soil and plant matter which

have a less positive 15N signature in addition to being associated with generally larger

grain size. However, mass balance calculation indicate less terrestrial fraction of organic

(38)

The lack of a consistent trend of 15N values within the 13Corg excursion of the cores has

several possible explanations. One major consideration is the impact of the nitrogen

cycle. Nitrogen cycling within the water column and upper sediment is known to

complicate the nitrogen signal preserved in sediments (Schubert and Calvert, 2001;

Elliott and Brush, 2006). In addition, it has been noted that differences in water pH,

temperature, species, and growth rate associated with seasonal and spatial variations can

significantly alter nitrogen isotope composition and complicate the nitrogen signal (Wu,

et al., 2006; Liu, et al., 2006). Finally, microbial interactions within the water column and

sediment-water interface can alter the organic pool causing mineralization and

diagenesis, resulting in enrichment in 15N (Liu, et al., 2006). This could lead to the

decomposing organic matter having a low percentage of nitrogen with a much enriched

15

N value (Liu, et al., 2006). It is also possible for microbes to alter the 15N of

terrestrial particles, actually increasing the 15N value from -4.0‰ to +9.0‰ (Caraco, et

al., 1998; Liu, et al., 2006). The lack of a consistent 15N trend between cores could also

be attributed to the incorporation of inorganic nitrogen. Above the depth of 268 cm %N

content becomes more variable, alternating between values of 0.05% and 0.08% on an

approximately 40 cm scale. 15N values of inorganic nitrogen have been shown to be

generally isotopically lighter, and thus incorporation of inorganic nitrogen could alter the

observed signal. While the data are not well clustered within MD3007, the non-zero

intercepts of Figure 1.17 indicate the inclusion of inorganic nitrogen within samples.

Based on these plots, inorganic nitrogen comprises approximately 45-87%, 16-23%, and

(39)

With such large possible percentages of inorganic nitrogen there is the potential for

significant alteration of the nitrogen isotopic signature which may not be fully

understood.

Following this excursion to more positive values the 13Corg record trends up-core toward

more negative values. Values eventually reach -25.5‰ at the top of MD3007. Within the

top 150 cm of MD3004 an upward trend toward more negative values culminates in a

value of -25.5‰ at the top of the core. The 13C of organic matter at the top of core

MD3006 is approximately -24.0‰. This trend is observable in all cores and is possibly

the result of increased anthropogenic influence on sedimentation and carbon burial on the

margin. Anthropogenic impacts on the Waipaoa watershed, beginning with the Maori and

followed by the European settlers, included mass burning and deforestation of the

landscape which could have resulted in the destabilization of many hillslopes and

formation of deep gullies and consequent increased input of terrigenous plant material

and/or rock carbon to the shelf. 15N values following the excursion also trend toward

more negative values, in cores MD3004 and MD3006, reaching surface values of 4.5‰

and 4.0‰, respectively. This trend toward more typically terrestrial 15N values supports

the evidence from 13Corg that increased anthropogenic activity caused increased delivery

of terrigenous plant or rock-derived organic matter to the continental shelf. 15N values of

rocks within the neighboring Waiapu decrease with time before present, and Cretaceous

aged rocks measure as low as 1.0 – 3.0‰ (Thompson, 2009). If rocks of the Waipaoa

(40)

carbon delivery to the shelf could explain the decreased 15N values seen. Elliott and

Brush (2006) state that distinct nitrogen isotopic values exist for pristine forested land

(less than 3.0‰) in contrast with lands altered by anthropogenic activity (greater than

8.0‰) including pastures and septic systems due to waste contributions (Mayer et al.,

2002; Elliott and Brush, 2006). These distinct nitrogen isotope values are pervasive, and

present in soil, groundwater, and surface water and it is therefore possible that increases

in fertilizer, depending on the type, from agriculture or manure from pasturelands could

be altering the 15N signature in the upper portions of the cores. However, without more

detailed compound specific analysis to determine origin and alteration of nitrogen

compounds it is difficult to say what possible influence these factors may have on the

15N signal within the cores. Core MD3007 does not show any discernable trend in 15N

values above the 13Corg excursion with surface values measuring 4.3‰. Within core

MD3007 the 13Corg excursion is closer to the surface of core than in MD3004 and

MD3006. With such a small interval between the excursion and the surface it is possible

that the sedimentation rate was not great enough to express a shift toward more terrestrial

nitrogen values. Within cores MD3004 and MD3006 there is a lag between the end of the

13

Corg excursion and the shift in 15N toward more negative values. A lag in the 15N

shift coupled with the high position of the 13Corg excursion could result in the lack of

15

N shift toward more negative values. There is also potential for compaction and

disturbance at the top of the core which could also complicate the 15N signal at the top

(41)

Conclusions

The bulk stable isotope and elemental record preserved in continental shelf core

sediments provides a unique perspective on environmental processes within the Waipaoa

River watershed during the late Pleistocene and Holocene.

o Long term trends in decreased followed by increased accumulation of terrestrial

organic matter relative to marine organic matter on the continental shelf offshore

of the Waipaoa River are recorded through positive and negative trends in the

13C

org and 15N record.

o An isotope excursion toward more positive (marine) values exists near/above the

Taupo tephra layer. The most likely explanation is a shift in the quantity or

character of terrigenous material delivered to the continental shelf. Two possible

explanations are the dilution of the riverine supply by organic-matter-poor

volcanic material and the capture of other tributaries, such as the Te Arai, which

could alter the signature of organic material being eroded from the watershed.

o Above the positive isotope excursion, carbon and nitrogen isotope values trend

toward more negative values. This trend indicates increased input of terrestrial

material. Such increased inputs are likely influenced partly by anthropogenic

(42)

o Nitrogen isotope values generally support interpretations based on the carbon

isotopic data. The complex nature of the nitrogen cycle and isotopic response can

complicate the signal. Nitrogen cycling within the water column as well as within

the seabed by microbial agents can lead to enriched terrestrial nitrogen isotope

values no longer representative of true terrestrial values.

o Two soil profiles analyzed are typical of soils formed in place with respect to

isotopic and elemental values. Two other soil profiles analyzed are typical of soils

formed by accumulation of material mobilized by landsliding and episodic

(43)

CHAPTER II. Geographic Information Systems Erosion Modeling

Methods

Approach. Two distinct model systems were employed to model the erosion of the Waipaoa using geographic information systems (GIS). Previous efforts to model the

sediment production of the watershed include work done by Phillips and Gomez in 2007.

They used weathering rate equations of Phillips (2003) and mass balance calculations to

compute the total suspended sediment discharged from the river and concluded that land

cover is indeed an important control of sediment erosion. Other efforts with erosion

thresholds and suspended sediment yields include Lyons et al. (2005) and Hicks et al.

(2000) using the LOWESS model. To our knowledge modeling efforts using GIS

technologies within the Waipaoa watershed have not been published. This technology

represents an untapped resource in the spatial modeling of erosion within the watershed.

Land cover and land use change has been cited by numerous authors as a serious source

of erosion change within the watershed (Milliman and Meade, 1983; Hay, 1994;

Milliman, 2001; Syvitski and Milliman, 2001; Phillips and Gomez, 2007). The modeling

included in this chapter will attempt specifically to simulate the anthropogenic alteration

of land cover regimes. This mainly includes the clearing and deforestation by European

settlers, which constitutes the majority of land use change within the watershed within the

last 800 years (Marden et al., 2005; Parkner et al., 2006). If current erosion volumes can

be accurately modeled, it is then plausible to alter land cover in the model to

(44)

Pleistocene and Holocene. These erosion values could then potentially be correlated with

offshore sedimentation rates and relationships between land cover, erosion, and shelf

sedimentation rates can be elucidated.

Modeling equation RUSLE3d was used to simply model the maximum erosion occurring

within the watershed. USPED was used to model both the erosion and re-deposition

possible within the watershed. To evaluate the effects of land cover, each of these models

was run under two independent land-cover scenarios. The first land cover used was that

of pre-anthropogenic times, and consisted of mostly indigenous forest in the upper

reaches and some grassland along the river banks. The second land-cover scenario

represented current conditions, following the arrival of European settlers. In this modern

land-cover scenario much of the indigenous forest was removed and replaced with

pastureland. Some grassland was left along the floodplain.

Data. A digital elevation model (DEM) of the watershed with 90 m resolution (cell size) was obtained from New Zealand’s National Institute of Water and Atmospheric Research

(NIWA), Wellington. Unsuccessful efforts to obtain other data elements led to the use of

the DEM to create the remaining data necessary for the analysis. Necessary for erosion

modeling are soil, land cover, and topographic factor layers. A soils layer (Figure 2.1)

was created by limiting areas near the river (less than 100 m elevation, less than 1 degree

slope) to be defined as silt (soil erodibility factor (k) = 0.52) and all other areas of the

(45)

layer (Figure 2.2) was created as a function of flow accumulation and slope, both derived

directly from the DEM. Each land cover layer for each successive anthropogenic

influence was created from the DEM as a function of elevation and slope.

Pre-anthropogenic land cover (Figure 2.3) was defined as grassland (land cover factor (c) =

0.0075) for areas under 200 m elevation and with a slope of less than 1 degree. All other

areas were defined as indigenous forest, (c = 0.001). Land cover during European/current

times (Figure 2.4) was defined as grassland for any areas under 200 m elevation and a

slope less than 1 degree. Areas with an elevation greater than 1000 m and a slope greater

than 10 degrees were defined as indigenous forest. All areas in between these two values

were defined as pasture (c = 0.05). These limits were chosen to represent the significant

deforestation and conversion to pasture land under European inhabitation during the 20th

century.

RUSLE3d Methods. The model was run three times. The model was initially run under no land cover, c = 1.0, in order to determine the maximum erosion potential of the

watershed, and assess if the model was capable of producing as much erosion as is

currently estimated. The model was then run under the three different land cover

scenarios. The basic equation used for the RUSLE3d model is:

(46)

E is the erosion (ton/acre/year), R (hundreds of tons/acre/year) is the rainfall intensity, K

(tons per acre per unit R) is the soil erodibility factor, LS (dimensionless) is the

topographic factor, and CP (dimensionless) is land cover and preventative measures. The

topographic factor is computed by the equation:

LS = (m+1) [ A / a0 ]m [ sin b / b0 ]n

where A is the upslope contributing area, b is the slope, ao is 22.1 m, b is 0.09, m is 0.75,

and n is 1.4 (Moore and Burch, 1986). The watershed is characterized by periods of long

and sustained, but low intensity rainfall. Occasional large rainfall events, such as

cyclones, make the rainfall intensity difficult to calculate. Therefore, the value of R was

45, similar to that of Seattle, WA, USA was used. This area has similar rainfall intensities

for much of the year. Preventative practices such as grading, mulching, and replanting

forest were not included. All other values are as indicated previously.

USPED Methods. This model was also run under the three different land cover scenarios. The basic equation used for the USPED model is:

dy a T d dx

a T

d

(47)

where the erosion/deposition is calculated as a derivative of the sediment transport

capacity (T) and the terrain aspect (a). The sediment transport capacity is calculated by the equation:

T = R K CP Am(sin b)n

where all elements are as indicated in RUSLE3d methods section with the exception of

Am(sin b)n, which is equivalent to the LS factor with m=1.65 and n=1.3.

Results and Discussion

RUSLE3d Model. The initial model run was with no land cover. The purpose of this run was to determine if the model could actually accommodate measured erosion volumes

under the greatest erosion possible within the constraints of the model. This

parameterization resulted in 38,717,010 ton/year and 124.11 ton/cell/year being eroded

from the watershed (Figure 2.5). The current estimated erosion is approximately 6750

t/km2/yr (Hicks et al., 2000), which results in approximately 15,000,000 ton/year ± 25%

which yields 55.05 ton/cell/year, therefore the model is obviously able to create enough

erosion to meet current estimates. Following this analysis the land covers were added in

temporal succession. Under the pre-anthropogenic land cover model, erosion rates were

relatively low (Table 2.1), not even reaching 2 percent of currently estimated values. This

result is not entirely surprising as native forest canopy within the watershed would offer

(48)

erosion in many areas of the watershed, especially those covered by forest in the land

cover model. However there is some erosion around the riverbed which is most easily

explained by the lack of forest cover and higher silt content of those areas of the model.

Under European/current time land cover there is significantly greater erosion modeled

within the watershed (Figure 2.7). On the map it is apparent that there is a significantly

greater area of the watershed dominated by serious erosion (Figure 2.7). Areas not on

steep slopes and with significant distances from the river are showing erosion not seen in

the earlier models. This result indicates the model is responding to the land cover changes

and removal of native forests as an erosion barrier. Erosion modeled by RUSLE3d under

European land cover is at least 45 times greater than that modeled under native forest

land cover.

Prior to European arrival, sedimentation rates as recorded within core MD 3007 between

666 cm and 113 cm were approximately 90 cm/ky as determined by 14C of bulk

sediments (Chapter 1). Between the depth of 113 cm and the surface of MD 3007 the

sedimentation rate increases to approximately 107 cm/ky. This apparent increase in

sedimentation rate correlates well with the increased erosion predicted by the model

during this time period. The increased erosion suggested by the model also supports

isotopic evidence from Chapter 1. During the uppermost 50-100 cm of all three cores

there is a decrease seen in both 13C and 15N, suggesting a greater terrestrial influence

Figure

Figure 1.1 Map of the Waipaoa River watershed, North Island, New Zealand. Also  shown: underlying lithologies of the Waipaoa River watershed (Mazengarb and Speden, 2000) and sites for soil profile sampling (OR070725, TD070726, PU070729, TA070729)
Figure 1.2 Map of sites for continental shelf cores MD3007, MD3004, and MD3006. Bathymetric isolines are at 50m intervals
Figure 1.3 Percent nitrogen and nitrogen isotope composition of core MD3007 of both untreated and acidified samples
Figure 1.5 Organic carbon and nitrogen isotopic values for soil profiles OR070725,  TD070726, TA070729, and PU070729
+7

References

Related documents

Outa (2011) opined when examining the impact of internatinal financial reporting standards (IFRS) adoption on the accounting quality in Kenyan listed companies

To plan and control the duration of the operating cycle at each stage or the number of days at each delay centre, and, to take a more sophisticated approach towards the

1924 Women join IBM’s first Quarter Century Club, created to honor employees with 25 years of service.. Linkenhoker organizes IBM’s first customer

Funded  Applicants   Applicants  Not  Funded   No  Applicants   Adams Antelope Arthur Banner Blaine Boone Box Butte Boyd Brown Buffalo Burt Butler Cass Cedar Chase

UEFA club competition: These are the official statistics considered valid for communicating official records in UEFA club competition defined as the European Champion Clubs' Cup,

social service agencies, hospitals and cultural organizations which are using marketing concept.. and techniques to improve the marketing of their services (Fox and