Unusual Red Sea-Type GDGT Distributions during the Early Paleogene: A Proxy for Enhanced Salinity?
Abstract:TEX86, an organic palaeothermometer based upon the distribution of isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) in marine Thaumarchaeota, is regularly used to reconstruct spatial and temporal sea surface temperature patterns, especially in older (Mesozoic and Palaeogene) settings. The TEX86 proxy assumes that Thaumarchaeota in modern oceans are representative of those living in ancient settings, but even in today’s oceans Thaumarchaeotal phylogeny is diverse. In the Red Sea, phylogenetically distinct archaeal communities occur both above and below the thermocline and correspond to core-top sediments in which TEX86 values consistently overestimate satellite-derived sea surface temperature (SST) by 6-8°C . These elevated SSTs are associated with an unusual GDGT distribution characterised by low GDGT-0 (<10%) and high GDGT-4’ (>10%). Here we propose the %GDGTRS index (= ([GDGT-4’]/[GDGT-4’]+[GDGT-0])*100) as a way to identify Red Sea-type GDGT distributions in ancient sediments.
In a Paleogene (65-34Ma) compilation, Red Sea-type GDGT distributions are common and geographically widespread, and could lead to SST overestimates. The underlying ecological controls that govern the occurrence of these distributions remain unclear, but they are especially common during times of elevated warmth (i.e. EECO, PETM). During the EECO and the PETM, the occurrence of Red Sea-type GDGT distributions coincides with the presence of hypersaline and/or lagoonal dinocysts in at least four global localities. We suggest that the high %GDGTRS values during these very hot intervals reflect the presence of unique Thaumarchaeotal or Euryarchaeotal clusters responding to the development of more saline conditions, similar to or perhaps exceeding those observed in the Red Sea today.
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