Evaluating branched tetraether lipid-based palaeotemperature proxies in an urban, hyper-eutrophic polluted lake in South Africa

Organic Geochemistry 53 (2012) 45–51

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Evaluating branched tetraether lipid-based palaeotemperature proxies in an urban, hyper-eutrophic polluted lake in South Africa Supriyo Kumar Das a,⇑, James Bendle b, Joyanto Routh c a

School of Natural Sciences, Linnaeus University, SE-391 82 Kalmar, Sweden School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK c Department of Water and Environmental Studies, TEMA, Linköping University, SE-58183 Linköping, Sweden b

a r t i c l e

i n f o

Article history: Received 30 September 2011 Received in revised form 28 April 2012 Accepted 8 June 2012 Available online 16 June 2012

a b s t r a c t We evaluate the application of the branched glycerol dialkyl glycerol tetraether (br GDGT) based palaeotemperature and palaeoenvironmental proxy to a hyper-eutrophic, polluted and shallow oxic lake. Lake Zeekoevlei is the largest freshwater lake in South Africa, located close to Cape Town. We use published lake-based and soil-based calibration equations, and compare the reconstructed mean annual air temperature (MAT) with regional (South African) and local (Cape Town) instrumental temperature records. The distribution of br GDGTs in the lake sediments is influenced by air temperature. The lake-based calibration equation, which uses the methylation index of branched tetraethers/cyclisation ratio of branched tetraethers (MBT/CBT), formulated for African lakes (Tierney et al., 2010), fits well with the instrumental temperature records. Moreover, the CBT-derived pH likely reflects historic socioeconomic changes in catchment. Our results suggest that a polluted/hyper-eutrophic status and shallow water urban setting do not preclude application of the MBT/CBT-MAT proxy. However, further research is necessary to understand the behaviour of br GDGT–producing bacteria in polluted and highly productive lakes. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Branched glycerol dialkyl glycerol tetraethers (br GDGTs) were discovered in peat deposits (Sinninghe Damsté et al., 2000) and were later reported in coastal marine sediments (Schouten et al., 2000; Hopmans et al., 2004; Weijers et al., 2006). They are thought to be constituents of the membrane lipids of unknown soil bacteria (Sinninghe Damsté et al., 2000; Weijers et al., 2007a) which, in response to soil surface temperature and soil pH, change their core membrane lipid composition by varying the amount of cyclopentyl moieties and methyl branches in the alkyl chain (Weijers et al., 2007a). The variation in the cyclopentyl moieties and methyl branches is expressed in the cyclisation ratio of branched tetraethers (CBT) and the methylation index of branched tetraethers (MBT), respectively. Good correlation between global soil pH and CBT, and MBT with global mean air temperature (MAT) and soil pH (Weijers et al., 2007a) has led to increasing application of the MBT/CBT proxy to ocean margin sediments for inferring past change in terrestrial soil pH and MAT (Weijers et al., 2007b). Moreover, the discovery of abundant br GDGTs in lake sediments has prompted the application of the MBT/CBT proxy to lake sediments for palaeotemperature and ⇑ Corresponding author. Tel.: +46 (0)586217439. E-mail address: [email protected] (S.K. Das). 0146-6380/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.orggeochem.2012.06.005

palaeoenvironmental reconstruction (Sinninghe Damsté et al., 2009; Tierney and Russell, 2009; Fawcett et al., 2011). However, recent studies suggesting an additional in situ source of br GDGTs in lake sediments and water columns (e.g. Sinninghe Damsté et al., 2009; Tierney and Russell, 2009; Fietz et al., 2012) have prompted debate about the application of the global soil calibration proposed by Weijers et al. (2007c). Furthermore, the recent observation of Acidobacteria as a source for (Sinninghe Damsté et al., 2011) strongly suggests the possibility of multiple microbial sources of br GDGTs (Weijers et al., 2009; Peterse et al., 2010). The limitation in the global soil calibration (Weijers et al., 2007c) may also be linked to the fact that changes in air temperature across the globe are not symmetrical and there is considerable variation in temperature on a regional scale relative to the global mean air temperature (Jones and Moberg, 2003). The debate has paved the way for developing an appropriate calibration equation for lacustrine sediments and a number of lake-based calibration equations have been proposed (e.g. Blaga et al., 2010; Tierney et al., 2010; Loomis et al., 2011). It is noteworthy that most of the studies applying br GDGTs to lake sediments have been performed on relatively pristine/oligotrophic lakes. Thus, the objective of this study was to evaluate the behaviour of the soil-based (Weijers et al., 2007b) and lake-based calibration equations (Tierney et al., 2010; Pearson et al., 2011; Sun et al., 2011) in a hyper-eutrophic polluted lake by comparing the reconstructed proxy MAT data with instrumental temperature

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S.K. Das et al. / Organic Geochemistry 53 (2012) 45–51

records. To achieve this goal, we compared the br GDGT derived MAT for a hyper-eutrophic and polluted semi-urban lake (Lake Zeekoevlei), in the outskirts of Cape Town, with regional (South African annual mean air temperature; TYN CY 1.1 dataset) and local (Cape Town; Global Historical Climatology Network Version 2 database) temperature databases.

2. Material and methods 2.1. Site description Lake Zeekoevlei is a perennial, shallow (max. depth 5.2 m), alkaline (pH 9) and freshwater coastal lake on the sandy Cape Flats (Fig. 1). It falls into the sub-tropical Mediterranean (winter rainfall) climate zone. It is a warm water lake, the annual temperature in surface water ranging between 10.3 °C and 28.5 °C. Year round strong winds from the Atlantic Ocean result in complete absence of chemical and thermal stratification (Harding, 1997) and the water remains oxic throughout the year (dissolved oxygen 0.7– 0.9 mmol l1; Das et al., 2009a). Unconsolidated calcareous sand underlies the lake. Milton et al. (1999) reported soil pH ranging from 4.7 in the acidic Fynbos soil along the northern shore to 9.2 in calcareous soil to the southern part of the lake. The lake is fed by the Great and Little Lotus Rivers (Fig. 1), which together drain a catchment (80 km2) covering residential and light industrial areas, commercial farms and open spaces, including vegetated and non-vegetated sections. The rivers are highly polluted with agricultural runoff and sewage effluent (Grobicki, 2001). The lake has been a popular destination for various recreational activities and has undergone several modifications (e.g. damming in 1948 and intensive pondweed eradication in 1951). A number of limnological investigations have been conducted, revealing the role of anthropogenic activity in transforming the lake into a polluted

hyper-eutrophic water body with intense and regular cyanobacterial blooms since the 1950s (Das et al., 2008a, 2009a,b).

2.2. Sampling and br GDGT analysis We collected two cores (32 and 38 cm long) from the deepest part of the lake in 2004 (Fig. 1). The shorter core (32 cm) was used for 137Cs and 210Pb dating, the age of the bottom layer being assigned as 1945 (see Das et al., 2008a,b). The other was sectioned at 2 cm intervals and the sub-samples were freeze dried. Ca. 1– 15 g freeze-dried sediment [depending on total organic carbon (TOC) content] was extracted with CH2Cl2/MeOH (9:1 v/v) using a Dionex 300 Accelerated Solvent Extractor (programmed for three extraction cycles at 1000 psi and 100 °C). The extracts were evaporated to dryness under N2, redissolved in hexane/isopropanol (99:1 v/v) to a concentration of 2 mg ml1 and filtered through a 0.4 lm PTFE filter. We analysed the extracts using high performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry (HPLC–APCI-MS) at the Scottish Universities Environmental Research Centre (SUERC) with a Shimadzu LC-2010EV LC-MS instrument with SIL-20AC autosampler. Separation was achieved in normal phase with an Alltech Prevail Cyano column (150 mm  2.1 mm; 3 ll). The flow rate of the hexane/propanol (99:1 v/v) eluent was maintained at 0.4 ml min1, isocratically for the first 2 min, and thereafter with a linear gradient of 1.8% isopropanol for the next 30 min. The injection volume of the samples was 10 ll. Ion scanning was performed in single ion monitoring (SIM) mode. We quantified the compounds by integrating the area of the [M+H]+ ion. GDGT ratios were calculated by integrating characteristic GDGT peaks using the Shimadzu LC Solution software. Relative abundance of each individual br GDGT was expressed as a fraction of the sum of all nine recovered br GDGTs and was referred to as ‘fractional abundance’. We calculated the

a

b

GREAT LOTUS RIVER LITTLE LOTUS RIVER

SOUTH AFRICA N

54 3 2

S 34°

3 SEDIMENT CORES 5 3 4 2 32 0

CAPE TOWN

RONDEVLEI

ZEEKOEVLEI

5 4 3 2 1 0

ZEEKOE CANAL

18°° 30'

E

FALSE BAY 0

500 m SCALE

5

Bathymetric contour in meters above sea level

Fig. 1. Map showing (a) location of Lake Zeekoevlei; (b) bathymetry map of the lake, positions of influent Lotus Rivers and sampling location.

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S.K. Das et al. / Organic Geochemistry 53 (2012) 45–51

For reconstructing pH, we used the following empirical equations proposed by

MBT and CBT indices by following Weijers et al. (2007b); Roman numerals correspond to the br GDGTs in Fig. 2b.

(a) Weijers et al. (2007b)

½I þ ½Ib þ ½Ic ð½I þ ½Ib þ ½IcÞ þ ð½II þ ½IIb þ ½IIcÞ þ ð½III þ ½IIIb þ ½IIIcÞ

pH ¼ ð3:33  CBTÞ=0:38

ð1Þ ½Ib þ ½IIb ½I þ ½II

RMSE ± 0.8 pH units (Bernhardt, 2011) (b) Tierney et al. (2010)

ð2Þ

pH ¼ 10:32  3:03  CBT

For reconstructing MAT, we used the following empirical equations proposed by

RMSE ± 0.66 pH units We used these multiple equations to compare the results produced by soil-based and various lake-based calibrations, and to identify the best calibration equation for Lake Zeekoevlei. We also calculated the branched and isoprenoid tetraether (BIT) index (Hopmans et al., 2004), which is representative of the relative amount of br GDGTs and crenarchaeol (Fig. 2b) and has been used to estimate the relative input of soil organic matter to lake sediments (Blaga et al., 2009, 2010; Sinninghe Damsté et al., 2009).

(a) Pearson et al. (2011)

T P ¼ 20:9 þ ð98:1  f GDGT IbÞ  ð12:0  f GDGT IIÞ  ð20:5  f GDGT IIIÞ

ð3Þ

Root mean square error (RMSE) ± 2 °C (b) Sun et al. (2011)

BIT ¼ T MBT-S ¼ 3:949  5:593  CBT þ 38:213  MBT

ð4Þ

(c) Tierney et al. (2010)

ð5Þ

RMSE ± 3 °C

T T ¼ 50:47  74:18  f GDGT III  31:60  f GDGT II ð6Þ

We obtained the historical (1946–2004) air temperature data for Cape Town from the Global Historical Climatology Network (GHCN-monthly) Version 2 database. The data were available from National Climatic Data Centre (NCDC; http://www.ncdc.noaa.gov/ oa/climate/ghcn-monthly/index.php). The monthly temperature was recorded at the International Station Meteorological Climate Summary (ISMCS) World Meteorological Organization (WMO)

RMSE ± 2.2 °C (d) Weijers et al. (2007b)

T MBT-W ¼ ðMBT  0:122  0:187  CBTÞ=0:020

ð7Þ

RMSE ± 5.5 °C (J. Weijers, personal communication)

OH O OH OH

O O

IIb

O

O

OH OH

O

O

IIc

O

O

HO

OH

I

Ib

Ic

II

Ib

IIc

IIIb IIIb IIIc

III

Br GDGTs

O O OH O

OH O

b

IIIb

O

O

OH O

O

OH

IIIc

O

O

O

OH O O

O

O

OH

OH

Ib

OH

O

OH O OH O

O

O

O

OH O O

Ic

O

O

II

90 80

O

Relative abundance (%)

a 100

I

ð10Þ

2.3. Instrumental temperature data

 34:69  f GDGT I

40 35 30 25 20 15 10 5 0

½I þ ½II þ ½III ½I þ ½II þ ½III þ Crenarchaeol

The mean duplicate run error (1r; n = 12) for Tp, TMBT-S, TMBT-T, TT and TMBT-W was ±2.8 °C, ±0.03 °C, ±0.03 °C, ±2.7 °C and ±0.02 °C, respectively. The mean duplicate run error (1r; n = 12) for pH [using Eqs. (8) and (9)] and BIT was ±0.04 and ±0.004, respectively. We performed correlation analysis using the software XLSTAT version 2011.5.01.

RMSE ± 4.27 °C

T MBT-T ¼ 11:84 þ 32:54  MBT  9:32  CBT

ð9Þ

O

CBT ¼  log

ð8Þ

O

MBT ¼

O

O OH

O

Crenarchaeol

OH

Fig. 2. (a) Bar plots showing relative abundance of average br GDGTs (n = 16, with 1r errors) in Lake Zeekoevlei sediments. Solid bars represent distribution of br GDGTs in South African soil data (n = 4; Weijers et al., 2007a); (b) structures of br GDGTs and crenarchaeol.

48

S.K. Das et al. / Organic Geochemistry 53 (2012) 45–51 2.2

+

+

2.0 1.8 1.6

+

CBT

1.4

+

1.2 1.0 0.8 0.6 0.4 0.2 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

MBT Fig. 3. Plot of MBT index vs. CBT index for Lake Zeekoevlei sediments (solid circles) and soil samples from South Africa (plus symbols; Weijers et al., 2007c).

station at a US Navy base in Cape Town. We obtained the South African annual mean air temperature from the publicly available TYN CY 1.1 (Mitchell et al., 2004) dataset (http://www.cru.uea.ac.uk/cru/data/hrg/timm/cty/obs/TYN_CY_1_1.html).

3. Results and discussion 3.1. GDGT distribution and source Br GDGTs containing no cyclopentyl moieties (GDGT I, GDGT II, GDGT III) represent 21%, 35%, 19% of total br GDGTs, respectively,

and are more abundant in Lake Zeekoevlei sediments than br GDGTs containing one or two cyclopentyl moieties (Fig. 2a). Sun et al. (2011) have attributed the high abundance of GDGT II and GDGT III to a dominant aquatic source, and high GDGT I and low GDGT III abundances to soil origin. However, a relatively high abundance of GDGT I and GDGT II, and relatively low abundance of GDGT III in Lake Zeekoevlei do not fall into a single category (of br GDGT source) as described by Sun et al. (2011). This indicates that the observations by Sun et al. (2011) for South Asian lakes are not applicable to Lake Zeekoevlei. The fractional abundance of crenarchaeol vs. the br GDGTs, is negligible in Lake Zeekoevlei. Tierney et al. (2010) showed a strong inverse relationship of crenarchaeol concentration with water depth. The near absence of crenarchaeol from Lake Zeekoevlei is therefore probably related to its shallow depth. In fact, the low crenarchaeol concentration results in very high BIT index values (0.98–1), which fall within the range of the highest values reported for lake sediments (0.4–1) and soils (0.9; Sinninghe Damsté et al., 2008, 2009; Blaga et al., 2009, 2010; Peterse et al., 2009; Tierney and Russell, 2009; Bechtel et al., 2010; Tierney et al., 2010; Sun et al., 2011). Hence, we believe the high BIT value reflects an absence of crenarchaeol from the lake and does not represent the relative contribution of aquatic and soil sources of tetraether lipids to the sediment. We did not analyse br GDGTs in catchment soils. Instead, we used the data for South African soils (Weijers et al., 2007a) to infer the source(s) of br GDGTs in Lake Zeekoevlei sediment. Fig. 3 shows a plot of the MBT index vs. CBT index for Lake Zeekoevlei sediments and soil data for South Africa (Weijers et al., 2007a). It shows higher values of MBT and CBT indices in South African soils (0.8–0.9 and 1–2, respectively; Weijers et al., 2007a) than Lake Zeekoevlei sediments (0.3 and 0.4–0.6, respectively). The significant difference between soil and lake sediments is probably not related to preferential degradation of br GDGTs be-

Table 1 Correlation matrix of br GDGT distributions (fractional abundances), reconstructed MAT (TMBT-T, TT, TP, TMBT-W, TMBT-S) and instrumental temperature record (TSouth Africa and TCape Town) in Lake Zeekoevlei sediments including correlation coefficients and p values. f GDGT I f GDGT Ib f GDGT Ic f GDGT II f GDGT IIb f GDGT IIc f GDGT III f GDGT IIIb f GDGT IIIc TMBT-T

TT

TP

TMBT-W

TMBT-S

TSouth Africa

f GDGT Ib

0.712 p = 0.002 f GDGT Ic 0.513 0.526 p = 0.042 p = 0.036 f GDGT II 0.019 0.102 p = 0.944 p = 0.707 f GDGT IIb 0.469 0.565 p = 0.067 p = 0.022 GDGT II c 0.610 0.406 p = 0.012 p = 0.118 f GDGT III 0.648 0.357 p = 0.007 p = 0.175 f GDGT IIIb 0.028 0.053 p = 0.919 p = 0.845 f GDGT IIIc 0.248 0.074 p = 0.354 p = 0.786 TMBT-T 0.325 0.279 p = 0.220 p = 0.296 TT 0.129 0.093 p = 0.635 p = 0.733 Tp 0.295 0.773 p = 0.268 p = 0.000 TMBT-W 0.596 0.024 p = 0.015 p = 0.930 TMBT-S 0.696 0.088 p = 0.003 p = 0.747 TSouth Africa 0.685 0.420 p = 0.003 p = 0.105 TCape Town 0.095 0.418 p = 0.727 p = 0.107

0.423 p = 0.103 0.647 p = 0.007 0.369 p = 0.159 0.386 p = 0.139 0.028 p = 0.917 0.172 p = 0.525 0.395 p = 0.130 0.109 p = 0.689 0.431 p = 0.096 0.180 p = 0.504 0.081 p = 0.764 0.591 p = 0.016 0.029 p = 0.915

0.311 p = 0.241 0.377 0.207 p = 0.149 p = 0.443 0.060 0.544 p = 0.825 p = 0.030 0.623 0.298 p = 0.010 p = 0.263 0.120 0.114 p = 0.658 p = 0.675 0.601 0.471 p = 0.014 p = 0.065 0.467 0.162 p = 0.069 p = 0.548 0.429 0.337 p = 0.097 p = 0.201 0.485 0.230 p = 0.057 p = 0.392 0.420 0.118 p = 0.105 p = 0.664 0.313 0.557 p = 0.237 p = 0.025 0.249 0.172 p = 0.352 p = 0.523

0.483 p = 0.058 0.294 p = 0.269 0.328 p = 0.216 0.118 p = 0.663 0.033 p = 0.904 0.247 p = 0.356 0.306 p = 0.250 0.379 p = 0.148 0.595 p = 0.015 0.051 p = 0.851

0.454 p = 0.077 0.146 0.013 p = 0.589 p = 0.963 0.185 0.096 0.106 p = 0.492 p = 0.725 p = 0.697 0.714 0.849 0.053 p = 0.002 p = <0.0001 p = 0.844 0.208 0.435 0.031 p = 0.440 p = 0.092 p = 0.910 0.380 0.076 0.178 p = 0.147 p = 0.781 p = 0.510 0.453 0.065 0.203 p = 0.078 p = 0.812 p = 0.450 0.612 0.065 0.108 p = 0.012 p = 0.811 p = 0.691 0.211 0.025 0.300 p = 0.434 p = 0.926 p = 0.259

0.339 p = 0.199 0.587 0.661 p = 0.017 p = 0.005 0.951 0.329 0.415 p = <0.0001 p = 0.214 p = 0.110 0.900 0.315 0.328 0.991 p = <0.0001 p = 0.235 p = 0.215 p = <0.0001 0.039 0.103 0.164 0.268 0.360 p = 0.887 p = 0.704 p = 0.544 p = 0.315 p = 0.171 0.082 0.167 0.419 0.076 0.071 0.206 p = 0.762 p = 0.537 p = 0.106 p = 0.780 p = 0.795 p = 0.444

S.K. Das et al. / Organic Geochemistry 53 (2012) 45–51

cause they are generally resistant to degradation (Huguet et al., 2008; Smith et al., 2012; Tierney et al., 2012). Instead, high in situ production of methylated br GDGTs (Tierney et al., 2012) in the shallow lake may result in the difference in MBT and CBT indices for soil and lake sediment (Fig. 3). Consistent with this, lower MBT and CBT values in lake sediments than catchment soils have been reported, and related to significant in situ production of br GDGTs in lakes (Sinninghe Damsté et al., 2009; Tierney and Russell, 2009; Tierney et al., 2010 Sun et al., 2011). 3.2. Reconstructed MAT and instrumental temperature records The TYN CY 1.1 temperature dataset (TSouth Africa) is ca. 3 °C warmer than the mean annual GHCN-monthly air temperature (TCape Town). TSouth Africa correlates (Table 1) significantly (r 0.612 to 0.685) with f GDGT I, f GDGT Ib, f GDGT Ic, f GDGT II, f GDGT IIb, f GDGT IIc and f GDGT III, but does not correlate with f GDGT IIIb (r 0.065) and f GDGT IIIc (r 0.108). TCape Town shows no correlation with br GDGTs except a moderate correlation (r 0.418) with f GDGT Ib (Table 1). Significant correlation of br GDGTs with instrumental temperature (TSouth Africa) implies that br GDGTs in Lake Zeekoevlei

49

sediments relate to regional (South Africa) MAT. The correlation between br GDGTs and TSouth Africa implies that TSouth Africa is a better representative of the MAT for comparison with reconstructed temperature for the Cape Town region. The MBT/CBT derived TMBT-S, TMBT-T and TMBT-w, and the major br GDGT derived TP and TT in Lake Zeekoevlei sediments are plotted with TSouth Africa, TCape Town, the GHCN-monthly summer (Austral summer: December, January and February) and GHCN monthly winter (Austral winter: June, July and August) temperatures (Fig. 4a). TMBT-w (4–6 °C; mean 5 °C) is calculated by using the global soil calibration proposed by Weijers et al. (2007c) and is ca. 8 °C and 15 °C cooler than TCape Town (13–16 °C; mean 15 °C) and TSouth Africa (16–18 °C; mean 18 °C), respectively. The TMBT-w in the lake falls well beyond the RMSE (5.5 °C) of the calibration equation proposed by Weijers et al. (2007c). Similar observations, where the global soil calibration (Weijers et al., 2007c) produces significantly cooler temperature (a ‘cold bias’), have been reported for many lakes covering climate gradients ranging from tropical to temperate regions (e.g. Tierney and Russell, 2009; Tyler et al., 2010; Sun et al., 2011). The cold bias stems from the fact that a significant portion of br GDGTs in lakes is produced in situ, whereas

a

b

Fig. 4. (a) Comparison of MAT reconstruction (solid lines) based on MBT/CBT: TMBT-S (Sun et al., 2011), TMBT-T (Tierney et al., 2010), TMBT-w (Weijers et al., 2007b), the major br GDGTs: TP (Pearson et al., 2011), TT (Tierney et al., 2010), and instrumental local temperature (dashed/dotted lines): TCape Town (mean annual GHCN), GHCN-monthly summer, winter temperatures, instrumental regional temperature: TSouth Africa (mean annual temperature of South Africa; TYN CY 1.1 dataset). Details of the dating (error ± 69 yr) are discussed by Das et al. (2008). (b) CBT/MBT derived estimate of soil pH using equations proposed by Tierney et al. (2010) and Weijers et al. (2007b).

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S.K. Das et al. / Organic Geochemistry 53 (2012) 45–51

the global soil calibration developed by Weijers et al. (2007c) is based solely on the presence of soil-derived br GDGTs (Tierney et al., 2010, 2012; Pearson et al., 2011). TMBT-s (12–14 °C; mean 13 °C) lies mostly between the GHCNmonthly winter temperatures (11–14 °C; mean 13 °C) and TCape Town (Fig. 4a). The 63 °C offset between TMBT-s and TCape Town can be explained with the RMSE (4.3 °C) of the calibration equation, but the difference between TMBT-s and TSouth Africa (3.5–6 °C) falls outside the RMSE range reported by Sun et al. (2011). In contrast to the TMBT-w and TMBT-s, TP (19–22 °C; mean 20 °C) lies close to the GHCN-monthly summer temperatures (20–22 °C; mean 21 °C). TP is 4–7 °C warmer than TCape Town, and 1–4.5 °C warmer than TSouth Africa. The differences lie above the 2 °C RMSE of the calibration equation [Eq. (3)] described by Pearson et al. (2011). TMBT-T (16–19 °C; mean 17 °C) and TT (16–23 °C; mean 18 °C) values are close to each other (61 °C). TMBT-T is 2–3 °C warmer (2.5 °C) than TCape Town, and differs up to ±2 °C vs. TSouth Africa. These differences fall within the ±3 °C RMSE of the calibration [Eq. (5)] used by Tierney et al. (2010). In contrast, TT is 1–8 °C warmer (mean difference 2.8 °C) than TCape Town and differs by 0.1–5 °C vs. TSouth Africa. The TT values mostly exceed the 2.2 °C RMSE of the calibration [Eq. (6)] used by Tierney et al. (2010). In summary, the MBT/CBT-based calibration [TMBT-T; Eq. (5)] proposed by Tierney et al. (2010) provides a better estimate of MAT in the hyper-eutrophic and well-oxygenated Lake Zeekoevlei when compared with instrumental records. The suitability of the MBT/CBT-based calibration for Lake Zeekoevlei is consistent with the fact that Tierney et al. (2010) have proposed the calibration for warm freshwater lakes in Africa. The inapplicability of Eq. (6) (Tierney et al., 2010) for Lake Zeekoevlei may be linked to the exclusion of cyclic GDGTs from the calibration equation. Tierney et al. (2010) considered the cyclic GDGTs as a physiological adaptation to pH (Weijers et al., 2007b), and did not include these compounds in Eq. (6). However, the inapplicability of Eq. (6) to Lake Zeekoevlei suggests that the cyclic GDGTs in highly alkaline lakes are likely influenced by temperature. We do not expect significant offset between air temperature, and temperature in the water column and lake sediments due to the shallow depth and absence of thermal stratification in Lake Zeekoevlei. Therefore, temperature recorded via the br GDGT producing microorganisms in the water column and lake sediment should not show much variation. Nevertheless, the prevailing hyper-eutrophic condition and pollution in the lake implies that these conditions do not affect the temperature recorded via br GDGT producing bacteria. Notably, the instrumental and reconstructed temperature records (Fig. 4a) do not show matching variation, and neither TSouth Africa nor TCape Town show any significant correlation (Table 1) with TMBT-T. Lack of correlation suggests that the reconstructed temperatures have not captured the low frequency variations. We observe neither any trend nor a major change in the highly methylated br GDGTs and MBT values with depth that can be linked to the variation in TMBT-T. Because the water column and sediment have always remained oxic (Das et al., 2008b, 2009b), the variation in reconstructed temperature, especially the post 1982 sharp drop, cannot be caused by a change in stratification/ oxygenation. We believe the change is perhaps connected to the massive cyanobacterial blooms (Das et al., 2009b) that occurred in the lake around this time. Evidently, we need further knowledge about the bacteria producing br GDGTs in order to fully explain the variation in our reconstructed temperature records. 3.3. pH variation The CBT-inferred pH values (7.1–7.6; Fig. 4b), reconstructed using the empirical equation [Eq. (8)] proposed by Weijers et al. (2007c), fall well beyond the range of pH of calcareous (9.2) and acid Fynbos (4.7) catchment soils (Milton et al., 1999) and the pH

of Lake Zeekoevlei water (9–10). However, Eq. (9) (Tierney et al., 2010) provides a better estimate of pH (8.5–9.1), which lies close to the pH of calcareous soil and lake water. The decline in reconstructed pH (Fig. 4b) since the early 1980s coincides with socioeconomic changes in the catchment, especially the mass migration from rural impoverished areas. The mushrooming of an economically poor neighbourhood and heavily fertilized farming since the 1980s in the lake catchment have accelerated raw sewage input and agricultural runoff into the lake (Grobicki, 2001) possibly driving the reconstructed decline in pH. 4. Conclusions Our results suggest that a lake with a polluted/hyper-eutrophic status and shallow oxic water urban setting is suitable for the application of the MBT/CBT-MAT proxy, and the distribution of br GDGTs in Lake Zeekoevlei sediments is influenced by MAT. The reconstructed MAT, estimated using the MBT/CBT inferred lake calibration proposed by Tierney et al. (2010) for African lakes, is in reasonable agreement with the instrumental record. We explain the change in reconstructed pH in terms of raw sewage input and increased use of fertilisers following socioeconomic change in the region. The study yields reasonable temperature estimates although further research is necessary to understand the behaviour of the br GDGT-based paleo-proxy in polluted and highly productive lakes. Acknowledgements A.N. Roychoudhury helped with sampling. The Swedish Research Council, University of Glasgow and SUERC are acknowledged for supporting the study (directly and through grants-inkind). We thank two anonymous reviewers for helpful suggestions that significantly improved this manuscript. Guest Associate Editor—A. Pearson References Bechtel, A., Smittenberg, R.H., Bernasconi, S.M., Schubert, C.J., 2010. Distribution of branched and isoprenoid tetraether lipids in an oligotrophic and a eutrophic Swiss lake: insights into sources and GDGT-based proxies. Organic Geochemistry 41, 822–832. Bernhardt, B.A., 2011. Validation of the MBT–CBT Paleotemperature Proxy: Effects of Environmental and Seasonal Variability in Soils and Lacustrine Sediments. Thesis, University of Minnesota, Minneapolis, 84 pp. Blaga, C.I., Reichart, G.J., Heiri, O., Sinninghe Damsté, J.S., 2009. Tetraether membrane lipid distributions in water-column particulate matter and sediments: a study of 47 European lakes along a north-south transect. Journal of Paleolimnology 41, 523–540. Blaga, C.I., Reichart, G.J., Schouten, S., Lotter, A.F., Werne, J.P., Kosten, S., Mazzeo, N., Lacerot, G., Damsté, J.S., 2010. Branched glycerol dialkyl glycerol tetraethers in lake sediments: can they be used as temperature and pH proxies? Organic Geochemistry 41, 1225–1234. Das, S.K., Routh, J., Roychoudhury, A.N., Klump, J.V., 2008a. Elemental (C, N, H and P) and stable isotope (d15N and d13C) signatures in sediments from Zeekoevlei, South Africa: a record of human intervention in the lake. Journal of Paleolimnology 39, 349–360. Das, S.K., Routh, J., Roychoudhury, A.N., Klump, J.V., 2008b. Major and trace element geochemistry in Zeekoevlei, South Africa: a lacustrine record of present and past processes. Applied Geochemistry 23, 2496–2511. Das, S.K., Routh, J., Roychoudhury, A.N., Klump, J., Ranjan, R.K., 2009a. Phosphorus dynamics in shallow eutrophic lakes: an example from Zeekoevlei, South Africa. Hydrobiologia 619, 55–66. Das, S.K., Routh, J., Roychoudhury, A.N., 2009b. Biomarker evidence of macrophyte and plankton community changes in Zeekoevlei, a shallow lake in South Africa. Journal of Paleolimnology 41, 507–521. Fawcett, P.J., Werne, J.P., Anderson, R.S., Heikoop, J.M., Brown, E.T., Berke, M.A., Smith, S.J., Goff, F., Donohoo-Hurley, L., Cisneros-Dozal, L.M., Schouten, S., Sinninghe Damsté, J.S., Huang, Y., Toney, J., Fessenden, J., WoldeGabriel, G., Atudorei, V., Geissman, J.W., Allen, C.D., 2011. Extended megadroughts in the southwestern United States during Pleistocene interglacials. Nature 470, 518– 521.

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