Oceanic Anoxia
How do oceans become anoxic? Where does this happen? Will it happen in the future?
These are all questions we hope to address by studying anoxia in the geological record
Figure showing some of the driving mechanisms of oceanic anoxia. Credit: Levin 2017 Annual Reviews of Marine Science
Modern anoxic environments and proxy calibration
Today, many regions of the world's oceans have little or no dissolved oxygen in the water. In some areas, high levels of primary productivity cause an oxygen depletion because of the aerobic decay of dead organic matter as it falls through the water column. This is most important in areas of high nutrient inputs (such as upwelling regions like the Namibian and Peru Margins) or restricted basins that have little oxygen supply due to slow circulation, like the Black Sea. The same thing is seen in lakes and rivers due to excess fertilizer inputs.
Temperature also plays a role, as oxygen is less soluble when it is warmer. Hence, as temperatures warm we expect lower oxygen concentrations in the ocean. This is already having an impact on marine life, notably some of those areas that are used as fisheries.
Sample transects across the Namibian margin allow us to examine proxy behaviour under low oxygen conditions. In this study, led by Zhiwei He, we have measured both pore-waters and solid sediment phases for a range of metal isotopes (U, Mo, Zn, Ni, Fe), giving unique insights into how geochemical signatures are recorded in sediments, and how this influences the global isotopic mass balance of the metal. I am collaborating with researchers at the University of Otago to do similar studies in the anoxic fjords of Norway
Ancient Oceanic Anoxic Events
Oceanic Anoxic Events (OAEs) are periods of Earth's history that are defined by widespread de-oxygenation of the oceans. The classic examples of these occurred during the Mesozoic, and can be identified by the deposition of distinct organic rich sedimentary layers known as 'black shales' (see right). OAEs were likely driven by the emplacement of massive volcanic systems known as Large Igneous Provinces (LIPs). It is thought that the CO2 emitted by these LIPs caused an environmental cascade that results in oceanic anoxia.
My work has focused on trying to reconstruct the global extent of anoxic waters using uranium isotopes and comparing these records to other proxies of environmental change.
Ferruginous oceans
When oceans become anoxic they can also be euxinic, with dissolved H2S in the water. H2S is toxic to most organisms and is an important driver of mass extinctions. When H2S is not present, anoxic waters can instead have lots of dissolved Fe(II), a state known as ferruginous conditions. The difference between euxinia and ferruginous conditions is important for global biogeochemical cycles and therefore links to the rest of the Earth system.
I use a sequential leaching technique known as Fe-speciation, which can distinguish between ferruginous and euxinic conditions. I have also helped to calibrate the proxy for appliciation to carbonate sediments.
Comparing hyperthermals
As part of my recent EU funded Marie Sklodowska-Curie Fellowship, I have been trying to compare different global warming events ('hyperthermals') to understand the different responses of the Earth system to a range of carbon emission scenarios (rate and magnitude). Some, like OAEs are characterised by longer CO2 inputs and widespread anoxia. Others, like the Paleocene Eocene Thermal Maximum (PETM), were more rapid and are characterised by ocean acidification but also show evidence of low oxygen conditions.
I am interested in quantifying the differences and similarities between these types of events.
