Data and methods - Precipitation variability over the last millennium

Chapter 3. Precipitation variability over the last millennium

3.1 Data and methods

3.1.1 Proxy-documentary sources

Southern Africa is relatively sparsely covered by palaeoclimate data, yet there are now several datasets from the region which represent changes in precipitation and/or temperature (Figures 3.1 and 3.2; Table 3.1). Unlike most other regions, however, no dominant data source exists, meaning a diverse range of proxies of varying resolution must be incorporated into any comprehensive analysis of its past climate. As the aim of this investigation is to inform past climate-society interaction, only records with an adequate resolution will be analysed, which is here defined as multi-decadal or higher.

FIGURE 3.1 Spatial distribution of palaeoclimate proxy and documentary data obtained for the southern African region. The shading indicates approximate extent of Portuguese documentary sources. Reference datasets are shown as hollow dots. Labelled datasets are used extensively.

3.1.1.1 Dendrochronological records

Tree-ring studies are frequently the main source of high-resolution palaeoprecipitation indicators in regional investigations, though are relatively few in number in southern Africa. Three records are available from the SRZ, located in KwaZulu-Natal, the Limpopo Valley and northern Zimbabwe (Figure 3.1). The KwaZulu-Natal record is taken from a single Yellowwood (Podocarpus falcatus) tree that was felled in 1916 in the Karkloof forest. Subsequent analyses of the tree-ring growth pattern by Hall (1976) produced a ring-width curve dating back to AD 1320, the fluctuations of which were interpreted as representing past precipitation variability. While this investigation has been widely cited, the only data currently available are 7-year running mean ring-widths taken from the original published set in Hall (1976). Moreover, this record is based on a single tree, which differs from the higher sample size typically used in dendrochronological reconstructions (Jones and Mann 2004). Hall, however, argued that its location on a hill-slope in a mild environment meant that annual precipitation

was the limiting factor for growth, and thus the ring-widths reflected past precipitation conditions. Further north, a 200-year chronology is available based on several samples from Pterocarpus angolensis trees in Mashonaland, northern Zimbabwe (Therrell et al.

2006). This record is made up of samples from three trees, and is significantly correlated with twentieth century summer rainfall in this ITCZ-sensitive region (Therrell et al. 2006). More recently, unpublished carbon isotope ratios of Baobab (Adonsonia digitata) tree-rings from the Limpopo region in northern South Africa have been made available for analysis (Woodborne, unpublished manuscript). This sample is made-up of carbon isotope ratios from six Baobab trees that grew over the last thousand years, and represents the longest tree-ring record in the region.

TABLE 3.1. Metadata of southern African SRZ proxy-documentary records. Resolution: ann. – annual, dec. – decadal, and cen. – centennial.

Site Lat, Lon Start Res. Proxy Reference

Tree

Karkloof -29.3, 30.2 1142 7-yr. Ring-width Vogel et al. (2001) Mashonaland -18.5, 27 1796 Ann. Ring-width Therrell et al. (2006)

Limpopo Several Namaqualand - 1817 Ann. Missionary Kelso and Vogel (2007)

Eastern Cape - 1821 Ann. Colonial Vogel (1989)

Africa-wide 90 regions 1800 Ann. Multiple Nicholson et al. (2012) Other

Cave speleothems, particularly stalagmites, constitute one of the few other natural archives in low or mid-latitude regions that offer the potential for a continuous climate record spanning a long period of time (Talma and Vogel 1992; Holmgren et al. 2001).

The interpretation of climatic variables from these data, however, is difficult. There is inherent complexity in understanding and determining climatic influences on stable isotope records in stalagmites, as multiple processes at different spatial and temporal scales affect their composition (Sundqvist et al. 2013). In southeast Africa, these data have provoked considerable discussion on both the nature and consequences of past climate variability, particularly the records of stable isotope (δ¹⁸O and δ¹³C) growth layers in two dated stalagmites (T7 and T8) from Cold Air Cave, Makapansgat Valley, 30 km southwest of Polokwane in northern South Africa (Figure 3.1).

Records of δ¹⁸O from the T7 and T8 stalagmites were initially interpreted as representing more humid conditions, determined by cave temperature (reflective of annual temperatures outside), and the composition of the meteoric water from which it precipitated. δ¹⁸O composition in stalagmites has also been linked to moisture source, transport distance, amount of precipitation, and regional air temperatures, making its precise palaeoclimate signal highly difficult to determine (Stager et al. 2013). Stalagmite δ¹³C is related to the plants from which the CO2 in the soil is derived (Holmgren et al.

1999). In most semi-arid parts of the southern African SRZ, barring high-altitude mountains, C4 grasses dominate (Vogel et al. 1978). More enriched δ¹³C values are therefore associated with a greater abundance of C4 grasses in vegetation, and thus warmer and wetter summer conditions. Diminished grass cover is more reflective of woody C3 vegetation, such as trees and shrubs, and thus of drier conditions (Holmgren et al. 1999; Lee-Thorp et al. 2001). Consequently, δ¹³C is often directly linked with past precipitation, as this is a prime determinant in the relative abundance of C4-C3

vegetation. Still, enriched δ¹³C can also reflect seasonality, seepage rates, soil water degassing, grazing, and light regimes (Scott and Lee-Thorp 2004; Wang et al. 2010).

Moreover, the hydrological significance of changes in the relative abundance of vegetation types is not clear, as human activity also influenced SRZ vegetation during the late Holocene (Neumann et al. 2008).

An additional indicator from the Cold Air Cave T7 stalagmite is the grey scale colour branding of growth layers. These annual colour variations represent changes in the concentration of humic matter in the drip water feeding the stalagmite (Tyson et al.

2000). Darker branding is suggested to reflect increased temperatures and mobilisation of organic matter from the soil, associated with wetter summers and enhanced grass cover above the cave. By contrast, less colour branding is associated with drier periods and sparse grass (Stager et al. 2013). Holmgren et al. (2003) tested these assumptions, and found significant correlations with both area-averaged annual temperature (r = 0.78, p <0.01) and rainfall anomalies (r = 0.69, p <0.01). Therefore, while Stager et al.

(2013) state that this series provides clear indications of palaeoprecipitation, as it is less likely to represent a wide range of ecological factors, earlier analysis indicates strong relationships to temperature as well. These complexities relating to interpretation mean that it is difficult to tie stable isotope records to single climatic variables, despite these being among the longest, most reliably dated and most finely resolved proxies from

the SRZ. In addition, a subset of last 350 years of the T7 δ¹⁸O dataset was re-analysed by Sundqvist et al. (2013) using high-resolution sampling of stable isotopes and an improved age model. These data will also be considered in the analysis here.

Two other speleothem records exist in southern Africa. The first of these is taken from the Cango Caves, located in the transitional year-round rainfall zone in the south of South Africa (Talma and Vogel 1992). This speleothem record is the region’s oldest, and its stable isotope series has been used to infer both past temperature and precipitation. This record has been found to show limited correspondence with other SRZ records (Tyson et al. 2000), which is thought to be because of its location in the all-seasons rainfall region, an area that is influenced by atmospheric circulation and weather processes affecting both rainfall regions. Consequently, the site is sensitive to even small shifts in the position of the boundary between the two zones (Tyson and Lindesay 1992). This means that at different times, variable or inverse relationships are found between the Cango Caves speleothem record and other SRZ records. No raw data are available for the Cango Cave speleothem, and it will be used as a reference dataset. A further speleothem record has been recently developed from the Dante Caves in Namibia (Sletten et al. 2013). This record spans the last 4600 years, and in addition to oxygen and carbon stable isotope records, it provides data on the relative proportions of aragonite and calcite in layers, which are apparently sensitive to changes between wetter and drier conditions (Sletten et al. 2013). Excluding the T7 stalagmite, each of these records have a variable resolution of between 1-50 years.

3.1.1.3 Lake sediments

Data from lake sediments represent the other main source of palaeoclimate data in the southern African region. SRZ sediment records are taken from three lakes that are hydrologically ‘closed’, where there are no overland inlets or outlets. This means that inputs to their water budgets are entirely dependent on precipitation, which as a result of their location is mostly ITCZ-sensitive. The most recent of these lake records is a diatom record from Lake Sibaya (Stager et al. 2013), located in Maputaland, northern KwaZulu-Natal. This record represents hydrological fluctuations over the last 1800 years with a variable sampling resolution of between 1-22 years, although these dates are at more consistent intervals than the Dante Cave speleothem. Two other lakes, Nhauhache (Holmgren et al. 2012) and Nhaucati (Ekblom and Stabell 2008), are located within 10 km of one another near the town of Vilanculos in coastal Mozambique.

According to Holmgren et al. (2012), these dune lakes respond quickly to climate fluctuations, and may be more suitable for palaeoclimate inference than interior lake basins, as the hydrology of inland lakes may be complex and influenced by larger regional or local aquifers. Nonetheless, uncertainty from zones of rapid and/or discontinuous sedimentation is a factor in these cores (Ekblom and Stabell 2008). Raw data for these two lakes are unavailable, therefore their moisture inferences will be assessed qualitatively (Figure 3.2).

FIGURE 3.2. Summer Rainfall Zone proxies ^AD 800-2000.

3.1.1.4 Documentary reconstructions

As well as natural proxy archives, a number of semi-quantitative documentary reconstructions of nineteenth century precipitation variability exist in different parts of the region. SRZ rainy season reconstructions are available for Lesotho (Nash and Grab 2010), the Kalahari (Nash and Endfield 2008) and the Eastern Cape (Vogel 1989), and were produced from letters, journals and reports written by missionaries and colonial authorities, as well as newspapers, diaries and travelogues. Observations in each of the reconstructions were classified into five categories on a -2 to +2 scale, equating to very dry, relatively dry, normal, relatively wet, and very wet, dependent on the prevalent documented climatic conditions in each rainy season. A further annual nineteenth century semi-quantitative ‘wetness index’ was developed by Nicholson et al. (2012), covering 90 regions across the African continent. This index incorporates rainfall station records, documentary data (often secondary), and other natural proxies, which were analysed and combined to give a ‘wetness’ score on a seven-class scale of -3 to +3 for each geographical region. Each region was determined on the basis of homogenous spatial patterns of inter-annual twentieth century rainfall variability. This record also relies on statistical inference for regions and periods with insufficient data based upon

‘equivalent regions’ in the instrumental record.

3.1.1.5 Other proxies

Among the other sources of palaeoclimate data are the SRZ multi-proxy reconstruction for the last two centuries (Neukom et al. 2014). These records were statistically produced using ensemble-based principal component regression. The data used from across the SRZ include each of the documentary datasets (Vogel 1989; Kelso and Vogel 2007; Nash and Endfield 2008; Nash and Grab 2010), the Pterocarpus angolensis tree-rings from northern Zimbabwe (Therrell et al. 2006), the Ifaty Reef coral record (Zinke et al. 2004), and rainfall station datasets from across the region. Datasets used solely for reference include nitrogen isotope ratios from bone assemblages excavated at archaeological sites in the Limpopo Valley from AD 800 to 1800 (Smith 2005; Smith et al. 2007), and fossilised rock hyrax (Procavia capensis) middens from the margins of the Namib Desert, which offer stable carbon and nitrogen isotope records spanning the last 12,000 years. These latter records have been developed at two sites: the first in the Kuiseb River Basin (Scott 1996), and the latter in the Spitzkoppe range (Chase et al.

2009) (Figure 3.1), but will not be used at the forefront of the investigation into SRZ climate variability, as they are both near the SRZ-WRZ boundary and are located in a considerably drier part of Namibia than the Dante Cave speleothem records. Other natural proxies, such as pollen records from Wonderkrater (Scott 1982), Tate Vondo (Scott 1987) and Wonderwerk (Brook et al. 2010) are too low resolution, or are affected by late-Holocene human influence, and are not used at the front of the analysis. Coral records of SST variability (Zinke et al. 2004) will also not be used in this analysis, as the scope of the thesis rules out any thorough consideration of climate modes.

3.1.2 Supplementary written sources

Sixteenth to nineteenth century European documents are available to supplement proxy-documentary records in the SRZ. These predominantly Portuguese records centre on the area of present-day Zimbabwe and Mozambique, with a particular focus along the coastal areas and Zambezi river (Figure 3.1). The sources are held within translated source compilations in the British Library (Theal 1898-1903; da Silva Rego and Baxter 1962-1975; Freeman-Grenville 1962), and the Department of History at the University of Zimbabwe (Beach and Noronha 1980), the former of which have been assessed for possible climate-related evidence by Ekblom (2004).

Full details on these written source volumes are outlined in Chapter 5, Section 5.1.

Portuguese observers’ descriptions of the environment mostly relate to atmospheric factors, including rainfall, floods, dry and wet spells, storms, frost, snowfalls, climate-related events such as the presence of plague locusts, and frequent references to food scarcity and famines. It should be noted, however, that the data from the Portuguese documents are comparatively sporadic and scanty when compared to those used for the nineteenth century reconstructions in the region. Indeed, while northern Zimbabwe and the lower Zambezi were in contact with the Portuguese from the beginning of the sixteenth century, there is little meteorological information in contrast to references to economic aspects such as gold resources and ivory, the exploitation of which reflected the primary purpose of the Portuguese presence in the region (Beach 1980, 1994; Pikirayi 2003).

3.1.3 Analysing regional precipitation variability

The overriding aim of this analysis is to use the full and best available range of regional data to understand the evolution of SRZ precipitation variability over the last millennium. While assessment of southern Africa’s precipitation history is challenging, the comparison of state-of-the-art, high-resolution records means that dependence on individual records with inherent ambiguity is reduced.

Nevertheless, a number of quantitative and qualitative methods must be applied to adequately assess the moisture inferences of complex palaeoclimate records.

3.1.3.1 Comparison of proxy-documentary data

To establish the general coherence between the records shown in Table 3.1, inter-annual correlations will be conducted where possible. Where records are not of annual resolution, however, the inter-annual record will be degraded to match that of the lower-resolution proxy to facilitate statistical comparison. This means that inter-annual records can be correlated with inter-annual proxies, but non-annually resolved proxies with varying sampling resolutions cannot be compared with each other. While degrading the inter-annual records to match the resolution of unevenly dated proxies may not be a precise measure of their correspondence, the

resultant r values give an idea of the general coherence between these records.

Furthermore, inter-annual records will be smoothed to 30-year running means and correlated to determine the level of agreement with lower frequency variability. As the period common to the longest, highest-resolution proxies is 1036-1890, correlations will be calculated over this period.

The main line of investigation in this chapter concerns the general evolution of precipitation variability over the last millennium. As the relative lack of high-resolution data rules out an integrated SRZ multi-proxy reconstruction, periods of coherence and reduced correspondence will primarily be qualitatively explored to reveal the dominant precipitation signals and periods of uncertainty. This will also show whether precipitation changes were coherent on a large spatial scale, or whether local variability was more prevalent at certain locations or time periods.

This approach follows those of Tyson and Lindesay (1992); Holmgren et al. (1999);

Tyson et al. (2000, 2002); Nicholson et al. (2013) and Stager et al. (2013), also in southern African contexts. Further statistical analysis, such as t-tests, will be conducted on individual inter-annual records to reveal if significant differences exist between centennial periods. The magnitude of centennial wet and dry conditions will also be revealed by examining the number of years ±1 standard deviation from the common mean of the period 1036-1890, while 30-year running standard deviations will be performed to analyse periods of high and low variability within and between the annual records.

The chapter will also examine evidence for, and analyse the temporal characteristics and magnitude of, the regional precipitation manifestations of the MCA and LIA, broadly defined in the literature as AD 900-1200 for the MCA, and either AD

1300-1800 or AD 1500-1800 for the LIA. This is an important proposition to test, as the global extent of these eras are highly debated, while the variability within these periods is not fully understood (Nicholson et al. 2013). Although visual comparison of proxies is helpful in understanding periods of coherence between records, the implications of periods such as the MCA and LIA cannot be adequately compared by this technique alone. To enable a clearer, regional representation of the temporal characteristics and magnitude of the MCA and LIA, this study will analyse the signals of multiple proxies by assessing their temporal characteristics in relation to certain statistical thresholds, in part following the approach of Osborn and Briffa (2006). This first requires inter-annual proxies to be smoothed to remove variations on timescales shorter than 20 years, and then normalised to have zero mean and standard deviation.

Thereafter, the dominant moisture inferences of the proxy records are analysed by plotting the proportion of records where data in any given year exceed ±1 or ±2 standard deviations from the common period mean of 1036-1890, as well as the number of years above or below this mean itself. The proportion of records exceeding these thresholds over the 1200-year period will then enable consideration of the temporal extent and magnitude of the MCA and LIA to be considered. A major

restriction of this analysis, however, is that only four of the proxy records that extend back to ^AD 800 are inter-annual. For the purposes of incorporating the higher number of non-annually resolved proxies into this general assessment, and to understand their dominant signals for societal consideration, a modified variant of this method will also be applied insofar as the dates of the non-annually resolved proxies will here be assumed to be ‘mid-points’. This means that if the dated sample value is above the 1036-1890 mean, an equal number of years either side of this date are also assumed to be above the mean, and it is thus plotted accordingly. While there are uncertainties in these assumptions, there is no other method to integrate the high number of non-annually resolved SRZ data into the analysis, and consequently their incorporation for consideration of society is favoured over their exclusion. Therefore, the results of this analysis for inter-annual records alone will be compared to the same assessment which incorporates the non-annually resolved proxies.

Nineteenth century climate variability will be introduced and compared in this chapter, congruent with the debates over the significance of rainfall variability in political centralisation and conflict in KwaZulu-Natal. To date, only sparse written reference to climatic conditions and the Karkloof tree-ring record have been considered in this debate. This chapter will introduce the range of natural, and particularly documentary data sources published since the last major analysis of climate-society interactions in this period (Eldredge 1992). Furthermore, the Africa-wide precipitation database of Nicholson et al. (2012) will be compared to a range of proxy-documentary datasets to assess their coherence. This comparison will

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