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
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
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
DEDICATION
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
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.
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……….
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
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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,
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………..
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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,
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
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
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
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
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
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
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:
) (
(
)
(
/)
1000sample
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
(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)
%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
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
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
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
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
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
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.
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
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
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‰
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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:
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
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
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