Chromium isotopes in foraminifera and their application to assessing past ocean deoxygenation

Date: 
Tuesday 24 April 2018 - 15:00 to 16:00
Location: 
Other venue (see below for details).
Other Venue: 
Ray Beverton Room (044/11)
Speaker: 
Dr. Serginio Remmelzwaal, University of Bristol

Abstract:

As greenhouse gas emissions into the atmosphere persist, both oceanic and atmospheric temperatures are projected to continue rising with major consequences for the marine environment. One of the major environmental hazards of this century is the spread of low-oxygen environments or ocean deoxygenation. Past ocean deoxygenation can be used as an analogue to shed light on oceanic dissolved oxygen responses to global warming. The potential of foraminiferal chromium (Cr) isotopes as a new redox proxy was assessed by determining element partitioning and isotopic fractionation of Cr by foraminifera using a variety of analytical techniques (LA-MC-ICP-MS, (MC-)ICP-MS, nanoSIMS). While Cr is distributed throughout the foraminiferal test in both fossil and modern samples, sediment (fossil) core-top samples have up to two orders of magnitude more Cr than non-sedimentary and culture samples. Iron and Cr cross-plots suggest although at least part of the Cr signal in foraminifera is primary, the Cr signal is overprinted by the uptake of Cr in bottom and pore waters. In sediment samples, there is no interspecies isotope fractionation and the Cr isotopic composition of tests is related to the size of the test through surface area/volume ratio effects on secondary Cr incorporation. Chromium isotopes in carbonates were applied to study how climate change influenced ocean deoxygenation during the Pleistocene, Palaeocene-Eocene Thermal Maximum, Eocene Thermal Maximum 2, Ocean Anoxic Event 2 and Ocean Anoxic Event 1a. Open ocean deoxygenation can at least partly be attributed to rising temperatures in intermediate ocean waters. Deoxygenation through the direct effects of temperature on the solubility of dissolved oxygen do not account for the inferred expansion of low-oxygen conditions. Indirect effects of elevated temperatures (e.g. enhanced microbial metabolic rates and remineralisation) are needed in addition to fully account for the episodic deoxygenation during the Cenozoic.

Seminar category: 
G3 Seminars