Seasonal changes in marine archaeal tetraether lipid export depth linked to net primary productivity

Abstract

Marine ammonia oxidizing archaea (AOA) produce isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) which form the basis of seawater temperature proxy TEX86. Although most studies use TEX86 for sea surface temperature (SST) reconstructions, maximum GDGT production occurs at and below the nitracline. Moreover, sedimentary GDGT-2/GDGT-3 values >5 reflect GDGT contributions of a deeper dwelling (>100 m) clade of AOA, fueling debate on export mechanisms and the water depth of TEX86 proxy sensitivity. We investigate subseasonal variations in GDGT export depth using 4 new and 19 previously published marine sediment trap records and compare results to real-time biogeochemical and physical hindcast/reanalysis model outputs. Due to the complexity of the biological pump and the often short time series of sediment trap records, no globally consistent pattern emerges. However, GDGT-2/GDGT-3 ratios imply seasonal changes in GDGT export depth, and often correlate to changes in the depth (negative) or intensity (positive) of maximum net primary production (NPP). Increased/shallower maximum NPP also correlate positively with GDGT flux at several sites, indicating that NPP exerts a strong control on GDGT origin depth. Mechanistically, increased/shallower maximum NPP incorporates relatively more shallow clade GDGTs into sinking particles, leading to lower GDGT-2/GDGT-3 values. Additionally, GDGT-2/GDGT-3 values correlate with TEX86 in most sediment traps, suggesting that, on sub-annual timescales, mixing of shallow and deep-clade derived GDGTs rather than SST changes causes variations in TEX86. TEX86-based temperatures are up to 0.5°C lower per unit increase in GDGT-2/GDGT-3, but the North Atlantic deviates from this global trend with a positive correlation.