Assessing past oxygen in the ocean using Cr isotopes as a palaeo-proxy

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. While modern hypoxic environments are closely monitored, the mechanisms leading to these conditions are not fully understood in the context of an Earth system. Past ocean deoxygenation can be used as an analogue to shed light on oceanic dissolved oxygen responses to global warming. The aim of this study is to ground truth the palaeo-redox proxy of the Cr isotopic composition of foraminifera and bulk carbonates to elucidate the processes that led to and the eventual extent of hypoxic and anoxic environments during past climatic events.

The potential of foraminiferal Cr isotopes as a new redox proxy was assessed by determining element partitioning and isotopic fractionation of Cr by foraminifera using a variety of geochemical techniques (LA-MC-ICP-MS, (MC-)ICP-MS, nanoSIMS). To date, Cr isotopes in biogenic carbonates have been interpreted to record the seawater δ53Cr composition at the site of test mineralisation in the surface ocean. 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. This study concludes that Cr in foraminifera is mostly post-depositional and records bottom/pore water signals.

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. Chromium isotopes in carbonates record local seawater deoxygenation during these events. Open ocean deoxygenation can mainly be pinned to rising temperatures in intermediate ocean waters.

Date of Award	19 Jun 2018
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Ian J Parkinson (Supervisor), Daniela N Schmidt (Supervisor) & Fanny M Monteiro (Supervisor)