Monthly element/Ca trends and inter chamber variability in two planktic Foraminifera species: Globigerinoides ruber albus and Turborotalita clarkei from a hypersaline oligotrophic sea

Abstract. Environmental and biological factors influence the trace element composition (element/Ca) of planktic foraminifer shells. Consequently, the element/Ca measured in these shells (tests) are utilized as proxies to reconstruct past oceanic and climatic conditions. As single shell analyses are increasingly used in paleoceanographic research it is important to understand how proxy systematics change between species, individuals of the same species in a given population, and among chambers of a single individual during its life cycle. Here we present a time series of the chemical composition of planktic foraminifers retrieved using sediment traps between June 2014 and June 2015 at the northern part of the Gulf of Aqaba (aka Gulf of Eilat). Laser ablation ICP-MS element/Ca measurements were performed on single shells and chambers of Globigerinoides ruber albus and Turborotalita clarkei, collected monthly from five water depths (120 m, 220 m, 350 m, 450 m, and 570 m). Sediment trap samples were paired with corresponding data on water column hydrography and chemistry. Pooled means of measured element/Ca display species-specific and element-specific behavior, with generally higher values for T. clarkei phenotypes (‘big’ and ‘encrusted’) in comparison to G. ruber albus. Some element/Ca values measured in water column specimens, such as Al/Ca, vary significantly from core-top specimens. A unique finding is a prominent increase in element/Ca around March–April 2015, during maximum water column mixing, mostly apparent in T. clarkei and to a lesser extent in G. ruber albus. This spring element/Ca increase is observed in most measured elements and is further associated with an increase in inter-chamber variability (ICV). Inter-chamber element/Ca patterns show element enrichment/depletion in the most recently precipitated (final, F0) chamber in comparison to the older chambers (penultimate (F-1), antepenultimate (F-2), etc.). Element/Ca in F0 may also be less sensitive to surrounding environmental conditions. For example, the Mg/Ca of the F-1 and F-2 chambers of G. ruber albus display a positive relationship with mixed layer temperatures while F0 does not. To overcome this effect, we suggest using pooled means from non-F0 fractions as environmental records and paleo proxies.

These results highlight the complexity of proxy systematics that rises from the variability in element/Ca measured among different species and between chambers, caused by ecological conditions and other processes in the water column including physical, chemical, and biological effects.