Identifying the drivers of GDGT distributions in alkaline soil profiles within the Serengeti ecosystem

Surface soil glycerol dialkyl glycerol tetraether (GDGT) distributions are influenced by mean annual air temperature as well as soil pH. However, the controls on GDGT distributions with depth in soil profiles are less well-known. We report a study of soil profiles in warm, carbonate-precipitating, alkali soils in the Serengeti ecosystem, Tanzania. Measurements of temperature, pH, salinity, and complementary data available from carbonates and organics from the same soil pits provide an interpretive framework for the observed patterns in branched (br-) and isoprenoidal (iso-) GDGTs in soil profiles. While brGDGT distributions at the soil surface primarily reflect mean annual temperature, a warm bias at depth indicates additional sub-surface controls on brGDGT distributions. We consider whether degradation or in situ production in response to alkaline pH and salinity also modulate brGDGTs. We find that the Archaeol Caldarchaeol Ecometric (ACE) index correlates with soil salinity, which both increase with depth. These results support in situ microbial production in deeper soil settings, with pH and salinity controlling the microbial community composition. We also compared brGDGT-predicted mean annual air temperatures (MAAT) to published clumped isotope thermometry on carbonates in the same soils and found that the median temperatures of both proxies were the same at 23 °C. We suggest further comparison of proxy performance in carbonate-bearing soils and geological archives. Differences in the nature of the proxy recorders may broaden sample availability for paleothermometry and help to identify confounding factors in each proxy system.