Yiming Luo

Gas hydrate dissociation prolongs acidification of the Anthropocene oceans

Anthropogenic warming of the oceans can release methane (CH4) currently stored in sediments as gas hydrates. This CH4 will be oxidized to CO2, thus increasing the acidification of the oceans. We employ a biogeochemical model of the multimillennial carbon cycle to determine the evolution of the oceanic dissolved carbonate system over the next 13 kyr in response to CO2 from gas hydrates, combined with a reasonable scenario for long-term anthropogenic CO2 emissions. Hydrate-derived CO2 will appreciably delay the neutralization of ocean acidity and the return to preindustrial-like conditions. This finding is the same with CH4 release and oxidation in either the deep ocean or the atmosphere. A change in CaCO3 export, coupled to CH4 release, would intensify the transient rise of the carbonate compensation depth, without producing any changes to the long-term evolution of the carbonate system. Overall, gas hydrate destabilization implies a moderate additional perturbation to the carbonate system of the Anthropocene oceans.

Controls on 231 Pa and 230 Th in the Arctic Ocean

The distributions of 230Th and 231Pa in the Arctic Ocean are not well understood. In order to examine the Arctic 231Pa/230Th system and therefore to shed light on the future use of Arctic sedimentary 231Pa/230Th, we developed a 2-D scavenging model modified from an Atlantic model. Tuned with reasonable parameters that are consistent with Eurasian Basin geographic settings, the model can reproduce most of the features of the 230Th and 231Pa water column profiles as well as the sedimentary 231Pa/230Th distribution patterns and suggests that the sedimentary 231Pa/230Th in the Eurasian Basin is mainly controlled by the deep water circulation. In our attempt to reproduce the sedimentary 231Pa/230Th patterns during the last glacial, we found that circulation strength in the Eurasian Basin at shallower depths may have been stronger than today.

Controls on 231Pa and 230Th in the Arctic Ocean

The distributions of 230Th and 231Pa in the Arctic Ocean are not well understood. In order to examine the Arctic 231Pa/230Th system and therefore to shed light on the future use of Arctic sedimentary 231Pa/230Th, we developed a 2-D scavenging model modified from an Atlantic model. Tuned with reasonable parameters that are consistent with Eurasian Basin geographic settings, the model can reproduce most of the features of the 230Th and 231Pa water column profiles as well as the sedimentary 231Pa/230Th distribution patterns and suggests that the sedimentary 231Pa/230Th in the Eurasian Basin is mainly controlled by the deep water circulation. In our attempt to reproduce the sedimentary 231Pa/230Th patterns during the last glacial, we found that circulation strength in the Eurasian Basin at shallower depths may have been stronger than today.

Future acidification of marginal seas: A comparative study of the Japan/East Sea and the South China Sea

The response of marginal (peripheral) seas to ocean acidification on short and long time scales is not well established. Through modeling, we examine the future acidification of two adjacent marginal seas, the South China Sea (SCS) and the Japan/East Sea (J/ES). Our results illustrate the importance of unique features in determining their acidification. The J/ES basin will become completely undersaturated with regard to calcite rapidly in the next few decades, while the SCS basin will experience relatively slower acidification. During its acidification, the J/ES will continually act as a sink for atmospheric CO2, whereas the SCS will temporarily switch from a source to a sink during the peak pCO2 interval, only to return slowly to being a source again. Marginal sea acidification will be determined by multiple factors, including their connections with the open ocean and their unique physical and biogeochemical dynamics, in addition to the level of atmospheric CO2.

Atlantic deep water circulation during the last interglacial

Understanding how the Atlantic Meridional Overturning Circulation (AMOC) evolved during crucial past geological periods is important in order to decipher the interplay between ocean dynamics and global climate change. Previous research, based on geological proxies, has provided invaluable insights into past AMOC changes. However, the causes of the changes in water mass distributions in the Atlantic during different periods remain mostly elusive. Using a state-of-the-art Earth system model, we show that the bulk of NCW in the deep South Atlantic Ocean below 4000 m migrated from the western basins at 125 ka to the eastern basins at 115 ka, though the AMOC strength is only slightly reduced. These changes are consistent with proxy records, and it is mainly due to more penetration of the AABW at depth at 115 ka, as a result of a larger density of AABW formed at 115 ka. Our results show that depth changes in regional deep water pathways can result in large local changes, while the overall AMOC structure hardly changes. Future research should thus be careful when interpreting single proxy records in terms of large-scale AMOC changes, and considering variability of water-mass distributions on sub-basin scale would give more comprehensive interpretations of sediment records.

Disparate acidification and calcium carbonate desaturation of deep and shallow waters of the Arctic Ocean

The Arctic Ocean is acidifying from absorption of man-made CO2. Current predictive models of that acidification focus on surface waters, and their results argue that deep waters will acidify by downward penetration from the surface. Here we show, with an alternative model, the rapid, near simultaneous, acidification of both surface and deep waters, a prediction supported by current, but limited, saturation data. Whereas Arctic surface water responds directly by atmospheric CO2 uptake, deeper waters will be influenced strongly by intrusion of mid-depth, pre-acidified, Atlantic Ocean water. With unabated CO2 emissions, surface waters will become undersaturated with respect to aragonite by 2105 AD and could remain so for ∼600 years. In deep waters, the aragonite saturation horizon will rise, reaching the base of the surface mixed layer by 2140 AD and likely remaining there for over a millennium. The survival of aragonite-secreting organisms is consequently threatened on long timescales.