Temperature calibration for high Mg-calcite planktic Foraminifera shells from the Gulf of Aqaba

Hypersaline, oligotrophic seas have been shown to accommodate planktic Foraminifera (PF) with enriched Mg/Ca in their calcareous shells, compared to other marine environments. Although Mg/Ca is a widely used proxy applied as a paleothermometer, its systematics in extreme hypersaline systems is not well understood.  

We measured element ratios using LASER ablation ICP-MS on the tests of the two abundant PF species, Globigerinoides ruber albus and Turborotalita clarkei, obtained from monthly resolved time series sediment traps at various water column depths in the northern part of the GOA.

Globigerinoides ruber albus shows a positive relationship between Mg/Ca and surface water temperatures of the surface mixed layer down to 60 m water depth. Although T. clarkei does not show the same positive relationship, it exhibits high Mg/Ca during water column mixing (March–April) possibly reflecting the movement of two water masses in the water column.

Using common calibration equations (Anand et al., 2003; Kisakürek et al., 2008; Gray et al., 2018), the high-Mg (5-25 mmol/mol) provided higher than measured in-situ (IS) ambient seawater temperatures (TMg/Ca and TIS, respectively). The high salinity at the GOA (>40) is assumed to be the main reason of the high shell-bound Mg.

Comparing surface dwelling G. ruber albus versus subsurface dwelling T. clarkei may facilitate reconstruction of the absolute and relative seasonal development temperature and surface water stratification. We suggest that species specific Mg/Ca-T calibrations for the GOA provide more accurate regional palaeoceanography and paleoclimate reconstruction of hypersaline environments.