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

Supriyo Kumar Das, James Bendle and Joyanto Routh

Linköping University Post Print

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Original Publication:

Supriyo Kumar Das, James Bendle and Joyanto Routh, Evaluating branched tetraether lipid- based palaeotemperature proxies in an urban, hyper-eutrophic polluted lake in South Africa, 2012, Organic Geochemistry, (53), 45-51. http://dx.doi.org/10.1016/j.orggeochem.2012.06.005

Copyright: Elsevier http://www.elsevier.com/

Postprint available at: Linköping University Electronic Press http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-87467

Reconstructing short term local air temperature using branched tetraether lipids in

Lake Zeekoevlei sediments in South Africa

Supriyo Kumar Dasa1, James Bendleb, Joyanto Routhc

Department of Natural Sciences and Technology, Örebro University, 70182 Örebro,

Sweden

1Corresponding author. Tel.: +46(0)19303492; Fax: +46(0)19303566;

email: [email protected]

bSchool of Geographical and Earth Sciences, University of Glasgow, G12 8QQ UK

cDepartment of Water and Environmental Studies, TEMA, Linköping University, 58183

Linköping, Sweden

Abstract

The branched glycerol dialkyl glycerol tetraethers (br-GDGTs) based methylation index of branched tetraethers/cyclisation ratio of branched tetraethers (MBT/CBT) proxy was applied for reconstructing the recent local changes in mean air temperature (MAT) in a sediment core collected from Lake Zeekoevlei. Lake Zeekoevlei is the largest freshwater lake in South Africa, and located close to . The lake is shallow, semi-urban, highly polluted and hyper-eutrophic. The reconstructed br-GDGT derived

MAT (TMBT) values closely resemble the historical air temperature data in Cape Town available from the Global Historical Climatology Network (GHCN-monthly) database.

Successful application of the MBT/CBT proxy in Lake Zeekoevlei indicates the suitability of using this proxy even in semi-urban, highly polluted, hyper-eutrophic lakes.

Key words: Lake, br-GDGTs, MBT/CBT proxy, South Africa, sediment, hyper- eutrophic

1 1. Introduction

The branched glycerol dialkyl glycerol tetraethers (br-GDGTs) based methylation index of branched tetraethers/cyclisation ratio of branched tetraethers (MBT/CBT) proxy has been applied successfully in lacustrine sediments for reconstructing past mean air temperature (MAT) and pH (Tierney et al., 2010; Tyler et al., 2010; Zink et al.,

2010). However, there is considerable debate regarding the source of these biomolecules (allochthonous vs. autochthonous or water column vs. lake surface sediments), and the use of appropriate calibration equation (Blaga et al., 2010; Loomis et al., 2011). The debate mainly arises from the fact that the changes in air temperature across the globe are not symmetrical, and there is considerable variation in temperature on a local scale relative to the global mean air temperature (Jones and Moberg, 2003).

Interestingly, most of the studies using MBT/CBT proxy are done on relatively pristine and deep lakes. Thus, the objective of this study is to reconstruct recent local air temperature from a shallow highly polluted hyper-eutrophic semi-urban lake, and compare the reconstructed proxy data with instrumental records. To achieve this goal, we have compared the br-GDGT derived temperature from Lake Zeekoevlei sediments, a shallow lake situated in the outskirts of Cape Town, with the temperature database available from the National Climatic Data Centre (NCDC, http://www.ncdc.noaa.gov/oa/climate/ghcn-monthly/index.php).

Lake Zeekoevlei is a shallow (maximum depth 5.2 m) perennial hyper-eutrophic alkaline (pH 9) freshwater coastal lake (surface area 256 km2) situated on the sandy coastal plains of 20 km south of Cape Town in South Africa (Figure 1). The lake falls in the sub-tropical Mediterranean winter rainfall climate zone, and is a warm- water lake where annual lake surface temperature ranges between 10.3°C and 28.5°C.

2 Year round strong winds result in the complete absence of chemical and thermal stratification in the lake (Harding, 1997), and the lake water remains oxic throughout the year (dissolved oxygen 0.7 - 0.9 mmol l-1; Das et al., 2009b). Unconsolidated sand

(mainly calcareous) underlies the lake. Milton et al (1999) reported soil pH ranging from 4.7 of acid Fynbos soil along the northern shore to 9.2 of the calcareous soil in the south of the lake. The lake is fed by the Great and Little Lotus Rivers (Figure 1), which together drain a catchment (80 km2) covering residential and light industrial areas, commercial farms (Philippi Horticultural Area) and open spaces including vegetated and non-vegetated areas. The rivers are highly polluted with agricultural runoff

(Grobicki, 2001). Moreover, we observed the lake receives overflowing raw sewage effluents from the poor residential areas that lacks proper sanitation. Despite the pollution, Lake Zeekoevlei is a popular destination for recreational activities, and the lake has undergone several modifications (e.g., damming in 1948 and pondweed eradication in 1951) to facilitate year round yachting activities. A number of limnological investigations have been conducted in this lake revealing the consequence of these modifications to the lake ecosystem (Das et al., 2009a; Das et al., 2009b; Das et al., 2008).

2. Material and methods

A sediment core was collected in the northern basin of the lake (Figure 1) in 2004.

Details regarding the sampling, and 137Cs and 210Pb dating in the core have been discussed by Das et al (2008). The core was sectioned at 2-cm intervals, and the sub- samples were freeze-dried. About 1 to 15 g of freeze-dried sediment (depending on the total organic carbon content) was extracted with a mixture of dichloromethane and methanol (9:1 v/v ratio) using a Dionex 300 Accelerated Solvent Extractor

3 (programmed for three extraction cycles at 1000 psi and 100ºC). The extracts were completely evaporated under . The total lipid extracts were redissolved with hexane and isopropanol (99:1 v/v ratio) at a concentration of 2 mg ml-1 and filtered through a

0.4 µm PTFE filter. The extracts were analyzed using high performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry

(HPLC/APCI-MS) at the Scottish Universities Environmental Research Centre

(SUERC) on a Shimadzu LC-2010EV LC-MS with SIL-20AC autosampler. Separation was achieved in normal phase on an Alltech Prevail Cyano column (150 mm × 2.1 mm;

3 µl). Separation was achieved in normal phase on a Grace Prevail Cyano 3µm column

(150 mm × 2.1 mm; 3 µm). The flow rate of the hexane/propanol (99:1 v/v) eluent was maintained at 0.4 ml min-1, isocratically for the first 2 minutes, and thereafter with a linear gradient to 1.8% isopropanol for next 30 minutes. Injection volume of the samples was 10 µl. In order to increase sensitivity and therefore reproducibility, ion scanning was performed in a single ion monitoring (SIM) mode. Quantification of the compounds was achieved by integrating the areas of the [M+H]+ (protonated molecular ion). GDGT ratios were calculated by integrating characteristic GDGT peaks using the

Shimadzu LC Solution software. For reconstructing MAT and pH, we used the following empirical equations proposed by Tierney et al (2010).

MAT =11.84 + 32.54 × MBT - 9.32 ×CBT (1) pH =10.32 - 3.03×CBT (2)

We used these equations because the authors formulated the equations for African lakes, and used lake based calibrations for correcting the inferred temperature and pH conditions. Tierny et al (2010) reported a root mean square error of 2.2˚C using the calibration. We also calculated the branched and isoprenoid tetraether (BIT) index

4 (Hopmans et al., 2004) which is representative of the relative amount of branched and isoprenoid GDGTs in the samples. The average duplicate errors (1σ; n=12) for MAT, pH and BIT were 0.29ºC, 0.05 and 0.004, respectively.

2.1. Instrumental Temperature Data

Historical air temperature data (from 1946 to 2004) were obtained from the Global

Historical Climatology Network (GHCN-monthly) Version 2 database. The data was available from NCDC (http://www.ncdc.noaa.gov/oa/climate/ghcn-monthly/index.php).

The monthly temperature data was recorded at the International Station Meterological

Climate Summary (ISMCS) World Meteorological Organization (WMO) station in a

U.S. Navy site in Cape Town. We used a yearly mean of the GHCN-monthly temperature record in this study.

The GHCN data undergoes rigorous quality assurance reviews, and GHCN-monthly is used by NCDC for long term monitoring of temperature-based trends. The database has been used in the Intergovernmental Panel on Climate Change (IPCC) 4th

Assessment Report, the Arctic Climate Impact Assessment and the "State of the

Climate" report, which is annually published by the American Meteorological Society.

3. Results and Discussion

The MBT/CBT derived MAT (TMBT) ranges from 16ºC to 19ºC with an average of

17ºC (Figure 2). We have plotted TMBT with the GHCN-monthly summer (Austral summer; December, January and February), winter (Austral winter; June, July and

August) temperatures, and the mean annual GHCN-monthly air temperature (TGHCN;

Figure 2). We find that TMBT values are warmer than GHCN-monthly winter temperatures (average 13ºC, range 11ºC to 14ºC), but lie between the TGHCN (average

15ºC and range 13ºC to 16ºC) and the GHCN-monthly summer temperatures (average

5 21ºC, range 20ºC to 22ºC, Figure 2). The difference between TMBT and TGHCN lies within the 2˚C root mean square error estimate of the calibration used by Tierney et al.

(2010). This in addition to an overall similar trend (i.e. overlap of valleys and highs) of

TMBT and smoothened TGHCN (Figure 2) profiles, especially in the upper section of the core, implies that the MBT/CBT proxy provides a reasonable estimate of the MAT in

Cape Town area. Interestingly, the profiles neither indicate any long-term increase in annual mean temperatures despite the increase in global temperature during the last decade of 20th century (Jones and Moberg, 2003) nor show any unusual warming in

Cape Town area during an extreme El Niño event in 1982-83 (Kruger and Shongwe,

2004). Both TMBT and TGHCN have become warmer after 1994, but none of them shows a significant increasing trend. The post-1990 warming is rather less in TMBT in comparison to TGHCN. In particular, the affect of post-apartheid rapid urbanization may play a role in defining the TGHCN temperature trend following 1990. This is because the instrumental temperature data (TGHCN) have been collected from an urban climate station (Cape Town) where a higher increase in minimum temperatures compared to maximum temperatures has been reported due to an increase in urban heat following rapid urbanization (Kruger and Shongwe, 2004). In comparison to the Lake Zeekoevlei catchment, the has experienced substantial post-1990 increase in urban population and industrial development (Kruger and Shongwe, 2004). Hence,

TGHCN should have a much greater effect of urbanization on its temperature trends than

TMBT.

High BIT index values (0.98 to1) suggest an absence of crenarchaeol rather than a terrestrial source of tetraether lipids (Hopmans et al., 2004; Tierney et al., 2010) in Lake

Zeekoevlei sediments. Therefore, in-situ production of br-GDGTs (Damste et al., 2009;

6 Tierney and Russell, 2009; Tierney et al., 2010) cannot be ruled out in the lake. High

CBT-inferred pH values (8.5-9.1), which lies close to the lake water pH (9-10) and the pH (9.2; Milton et al., 1999) of calcareous soil imply a mix source of br-GDGTs. We did not analyse br-GDGTs in catchment soil and particulate organic matter in the lake and are therefore, unable to distinguish the relative contribution from in situ and soil sources. Prevailing hyper-eutrophic conditions and pollution level in the lake implies that these conditions do not affect the temperature signal recorded by the br-GDGT producing bacteria.

4. Conclusions

The br-GDGT derived MAT estimate in Lake Zeekoevlei sediments is in reasonable agreement with the range of instrumental temperatures recorded at a meterological station in nearby Cape Town. Thus, we suggest the suitability of using the MBT/CBT-

MAT proxy even in semi-urban, highly polluted and hyper-eutrophic lakes.

5. Acknowledgements

Alakendra N. Roychoudhury helped us with sampling. The Swedish Research Council,

University of Glasgow and SUERC are acknowledged for funding the study (directly and through grants-in-kind).

6. References

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8 Loomis, S.E., Russell, J.M., Damste, J.S.S., 2011. Distributions of branched GDGTs in soils and lake sediments from western Uganda: Implications for a lacustrine paleothermometer. Organic Geochemistry 42, 739-751.

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9 Figure 1. Map showing a) the location of Lake Zeekoevlei in relation to the African continent, South Africa and Cape Town; b) bathymetry map of Lake Zeekoevlei, the positions of influent Lotus Rivers, and sampling location.

a

b GREAT LITTLE LOTUS RIVER SOUTH AFRICA

N 5 4 3

2

3 SEDIMENT CORE CAPE TOWN S 5 34 4 3 32 2 0 ZEEKOEVLEI 5 HOME BAY 4 3 2 1 ZEEKOE CANAL 0 18 30' E 0 500 m

SCALE 5 Bathymetric contour in meters above sea level

10 Figure 2. Variation in GHCN-monthly summer, winter, and TGHCN, TMBT, and CBT derived catchment soil pH. Bold line in TGHCN represents smoothening of data in

SigmaPlot 8.02. Details of the dating are discussed in Das et al (2008).

9.5

CBT Soil pH ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ±1 σ ■ ■ ■ ■ 8.5 ■

22 GHCN-Monthly

Summer temperature 20

C) C)

º º ( ▲ ( ▲ T in Zeekoevlei ▲ MBT ▲ ▲ ▲ 18 ▲ ▲ ▲▲ ▲ ±1 σ ▲ ▲ ▲ ▲ ▲ ▲ 16 TGHCN 14

GHCN-Monthly

Air TemperatureTemperature Air Air Winter temperature 12

10

8 1946 1954 1962 1970 1978 1986 1994 2002 Year

11