Impacts of autochthonous marine branched GDGTs on related paleo- environmental proxies: a preliminary study

Abstract:

Two proxies derived from branched glycerol dialkyl glycerol tetraethers (brGDGTs)-, the methylation index of branched tetraethers (MBT) and the cyclization ratio of branched tetraethers (CBT), are often used to reconstruct paleo mean annual air temperature (MAAT) and soil pH on the belief of their terrestrial origin. However, mounting evidence indicates the existence of autochthonous brGDGTs in marine environments，which may affect MAAT reconstruction and the use of other related paleoenvironmental proxies. Here we provide high resolution profiles of brGDGTs in a sedimentary core (MD05-2896/7) from the southern South China Sea, which include MBT and CBT indices as well as the branched and isoprenoid tetraether (BIT) index. The BIT results varied systematically with glacial-interglacial cycles, with values distinctly lower (<0.1) during the interglacial periods (MIS 1 and MIS 5) than during the glacial periods (MIS 2, MIS, 3, MIS 4 and MIS 6). Also distinct is the MBT/CBT-derived temperature, which show lower values during the interglacial periods but higher values during the glacial periods. We hypothesize that the lower MBT/CBT-derived temperature during the interglacial periods reflects subsurface water column temperature registered by autochthonous brGDGTs produced in situ marine conditions, whereas the higher MBT/CBT derived-temperature during the glacial periods reflects terrestrial MAAT because of the overwhelming input of brGDGTs from land when sea level was low. Similarly, the CBT-derived pH appears to have been overprinted also by the sea water signal of the interglacials but affected mostly by precipitation during the glacial intervals, showing patterns similar to or as a positive response to the southern hemispheric climate oscillation due to teleconnection. Our study demonstrates the complexity of brGDGT occurrence in marine environments and suggests that the MBT/CBT proxy should not be directly employed for the reconstruction of terrestrial MAAT at marine settings when BIT value is lower than 0.1.