Temperature reconstructions for the Eastern Indian Ocean based on organic-geochemical proxies (UK’37 and TEX86)

Gesine.Mollenhauer [ at ] awi.de


Sea surface temperature (SST) is very important for studies of the Earth’s climate system owing to the linkages between SST and various climatic processes. A reliable estimation of past SSTs is one of the main goals for paleoclimatologists to improve our understanding of oceanic and atmospheric dynamics and their connection to the global climate. Furthermore, the tropical SSTs play a key role for rapid climatic changes, because large amounts of heat and water vapor were transported from the tropics to the high latitudes. Warm SSTs at low latitude result in more evaporation and could thus induce increased ice sheet size and decreased temperatures at northern high latitudes. Establishing SST evolution in the tropics is crucial for understanding the mechanisms behind abrupt climate changes in the past. In this thesis, the main objectives are to evaluate the applicability of the UK’37 (alkenone unsaturation index) and TEXH86 (tretraether index of glycerol dialkyl glycerol tetraether with 86 carbon atoms) in the tropical Indian Ocean as well as to investigate their control mechanisms for reconstructing temperatures in the past. All studies presented herein are based on 36 surface sediments, a sediment trap covering an annual cycle and a gravity core in the eastern Indian Ocean. To assess the applicability, surface sediment samples from the Indonesian continental margin off west Sumatra, south of Java, and off the Lesser Sunda Islands are measured. In the non-upwelling regions, the results show that the UK’37 temperature estimates are up to 2 °C lower than World Ocean Atlas 2009 (WOA09) during the entire year, likely due to the reduced sensitivity of the UK’37 proxy beyond 28 °C. However, the temperatures based on TEXH 86 are consistent with mean annual temperatures from the WOA09. In the upwelling areas, the UK’37-based temperature estimates reflect the SST during the upwelling season, whereas the TEXH86-based temperature estimates are up to 2 °C lower than UK’37-based temperature estimates suggesting GDGT export from greater and colder water depths around 40-50 m. In the following work, an annual time series sediment trap study was conducted in the central upwelling region off south of Java. A pronounced seasonality of alkenone flux is observed, whereas GDGT flux displayed a weaker seasonality in comparison. The calculated flux-weighted average UK’37-based temperature estimate is similar to the SE monsoon SST rather than mean annual SST. The average is based on those samples only that permitted a reliable SST estimate, i.e., mainly the samples from the SE monsoon period. On the other hand, the flux-weighted average temperature based on the TEXH86 is consistent with mean annual temperature at 50 m depth, indicating TEXH86-temperatures reflect the mean annual subsurface temperature instead of the surface temperature. These observations support the findings concluded in the surface sediment samples study. Based on results from sediment trap and surface sediment samples, the application of the two SST indices on samples from a gravity core is in order to investigate the climatic evolution covering the past 22,000 years off south of Java. In this study, the temperature reconstruction suggest a 3-4 °C cooling during the last glacial maximum (LGM) compared to modern conditions. The results also show that the TEXH86 temperature estimates are up to 2 °C warmer than SST-UK’37 during the last 22ka except during the LGM and during the late Holocene. The differences between the two SST indices are paralleled by G. bulloides percentages as a proxy for upwelling intensity, implying that the offset between two temperature proxies could be considered as the potential for reconstructing the upwelling intensity in the study area. In addition, the initial timing for the deglacial warming of GDGT temperature estimates started at ~18 ka, whereas the lowest UK’37 temperature estimates appeared in the middle of the Younger Dryas period (YD, ca. 12 ka) and the late Heinrich Stadial 1 period (HS1, ca. 15 ka). Our records reveal that the seasonal SSTs and mean annual subsurface temperatures were closely linked to climate changes occurring in both hemispheres, respectively. Thus, the combination of UK’37 and TEXH86 records and their difference give the complementary feedbacks on sea-water temperature developments in the past evolution in the tropical eastern Indian Ocean.

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Chen, W. (2015): Temperature reconstructions for the Eastern Indian Ocean based on organic-geochemical proxies (UK’37 and TEX86) , PhD thesis,

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