Organic and faunal proxy based sea surface temperature reconstruction for sediment core MD03-2607

Several proxies have been developed to reconstruct past sea surface temperature (SST), but different proxies may reflect temperatures of different seasons and each proxy is characterized by certain uncertainties. Therefore, a multiproxy approach is preferred to precisely reconstruct SST. Here, we reconstruct SST of the ocean offshore southeastern Australia (Murray Canyons area) for the last ~135 ka using three independent organic proxies (TEXH86 based on glycerol dialkyl glycerol tetraethers (GDGTs), UK'37 based on alkenones, and LDI based on long-chain diols) in addition to foraminiferal faunal assemblages. The organic proxy records show similar trends, with the highest temperature (21°C for UK'37 and TEXH86, and 25°C for LDI) during the last interglacial and lowest temperature (8°C for TEXH86, 10°C for UK'37, and 12°C for LDI) during the Last Glacial Maximum. However, the differences in absolute SST estimates obtained by the organic proxies varied over time with differences of up to 9°C between LDI and TEXH86. The seasonal SST reconstructions based on the modern analogue of foraminiferal assemblages also show similar trends as the organic proxies with highest temperatures during the last interglacial (23°C for the warmest month SST, 20°C for mean annual, and 18°C for the coolest month) and lowest temperature during the Last Glacial Maximum (14°C for the warmest month, 11°C for mean annual, and 9°C for the coolest month). Down core comparison between the reconstructed SSTs of the organic and inorganic proxies shows that LDI-inferred temperatures compare well with the temperature of the warmest month, TEXH86 with the temperature of the coolest month, and UK'37 with mean annual temperature. An increase in TEXH86 SST estimates relative to those of other proxies during deglaciations and interglacials suggests that either winter temperatures rapidly warmed, possibly due to an invigoration of the Leeuwin Current over the core site, or there was a change in the growth season of the Thaumarchaeota, the source organism of GDGTs. Our study shows the benefits of a multiproxy approach in the interpretation of SST proxies, leading to a more robust knowledge of past ocean temperature changes.

目前已开发出多种代用指标（proxy）用于重建古海表温度（sea surface temperature, SST），但不同代用指标所反映的季节温度存在差异，且每种指标均带有一定不确定性。因此，采用多代用指标联用的方法以实现精准的古SST重建，是更为优选的方案。本研究针对澳大利亚东南部近海（默里峡谷区域）过去约13.5万年的古SST展开重建，除利用有孔虫化石群组合（foraminiferal faunal assemblages）外，还采用了三类独立的有机代用指标：基于甘油二烷基甘油四醚（glycerol dialkyl glycerol tetraethers, GDGTs）的TEXH86、基于烯酮类化合物的UK'37，以及基于长链二醇的LDI。有机代用指标的记录呈现出一致的变化趋势：末次间冰期（last interglacial）时温度达到峰值（UK'37与TEXH86均为21℃，LDI为25℃），末次盛冰期（Last Glacial Maximum）时温度降至最低（TEXH86为8℃，UK'37为10℃，LDI为12℃）。但不同有机代用指标得到的绝对SST估算值随时间存在波动，其中LDI与TEXH86的估算值差值最大可达9℃。基于有孔虫化石组合的现代类比法的季节SST重建结果，同样与有机代用指标呈现出相似的变化趋势：末次间冰期温度最高（暖月SST为23℃，年平均温度为20℃，冷月SST为18℃），末次盛冰期温度最低（暖月SST为14℃，年平均温度为11℃，冷月SST为9℃）。对有机与无机代用指标的重建SST进行岩芯垂向对比后发现：LDI反演的温度与暖月温度匹配度较高，TEXH86反演的温度与冷月温度更吻合，而UK'37反演的温度则与年平均温度一致。在冰消期与间冰期阶段，TEXH86的SST估算值相较于其他代用指标出现升高，这表明两种可能性：一是冬季温度快速升高，这可能与研究站位所在海域的卢因海流（Leeuwin Current）增强有关；二是GDGTs的源生物——奇古菌门（Thaumarchaeota）的生长季节发生了改变。本研究证实了多代用指标联用方法在SST代用指标解译中的优势，可为古海洋温度变化研究提供更为稳健可靠的认知。