Barium Uptake by Foraminifera: Understanding Past and Present Ocean Processes

Abstract

Oceanic barium can provide multiple insights into the marine environment, an important facet of the climate system. Dissolved barium is removed from near-surface seawater in association with biological productivity and is returned at depth via remineralisation and barite dissolution, imparting a nutrient-like profile similar to that of carbonate alkalinity and silica. Due to the similarity in their distributions, seawater barium and alkalinity display a positive linear relationship globally and different water masses have distinct barium-alkalinity compositions. Benthic foraminiferal Ba/Ca ratios can thus be used as a proxy for past ocean circulation and alkalinity, but this may be complicated by additional environmental influences, particularly in non-pelagic settings. In addition, a specific barium partition coefficient may be required for each individual species. Glacial-interglacial changes have been successfully measured using benthic Ba/Ca ratios but many possible applications have yet to be explored to the same extent, such as comparisons between interglacial periods. Barium isotope ratios (δ138Ba) can also be used to enhance our understanding of the marine environment, but because this is a relatively new technique there is still much to be learned regarding the oceanic distribution of barium isotopes and the relationship between seawater and foraminifer δ138Ba is not yet known. An essential prerequisite to measuring either Ba/Ca or δ138Ba ratios in foraminifera is the removal of particulate barite from their inner and outer surfaces. Although a cleaning technique has been established, this has sometimes increased foraminiferal Ba/Ca ratios, perhaps due to the preferential dissolution of low-barium calcium carbonate. During this project these topics were investigated using sediment cores from the southeast and southwest Atlantic Ocean dated to the Holocene and Marine Isotope Stage 5e (MIS 5e, part of the last interglacial period), as well as seawater and foraminifer samples of modern age from the tropical North Atlantic. Ba/Ca ratios in a continental shelf sediment core remain approximately constant despite apparent fluctuations in primary productivity, suggesting that the Ba/Ca proxy can reliably be used as a proxy for ocean circulation even in relatively shallow and productive regions. The effect of dissolution on Ba/Ca could not be assessed due to the absence of dissolution effects in this sediment core. New barium partition coefficients are presented for the benthic foraminifer species Melonis barleeanus, Oridorsalis umbonatus and Uvigerina peregrina. These are offset from one another, highlighting the potential importance of using species-specific partition coefficients. In other sediment cores from the southwest Atlantic, MIS 5e Ba/Ca ratios are on average significantly higher than those of the Holocene. This may be due to a ‘stagnation event’ in Antarctic Bottom Water (AABW) formation during MIS 5e which led to a build-up of dissolved barium in AABW. In samples from the tropical North Atlantic Ocean, seawater δ138Ba appears to be controlled by conservative mixing at depths below ~500 m, with additional non-conservative controls in the upper ~500 m. δ138Ba ratios in the planktic foraminifer Orbulina universa are consistently isotopically lighter than seawater, but the magnitude of this offset is variable. This variability may be due to morphotype-specific vital effects on δ138Ba ratios. Although cleaning tests provide indirect evidence for barite removal, the impact of cleaning on Ba/Ca ratios remains variable and the cause of this variability remains an open question. Collectively, the findings presented here have implications for the uses of Ba/Ca and δ138Ba ratios as palaeoceanographic proxies, as well as for the possible mechanisms of climatic variability in MIS 5e and the Holocene.

Date of Award	15 Nov 2016
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Kate Hendry (Supervisor)