Matthew Dumont - Academia.edu (original) (raw)

Papers by Matthew Dumont

Carbon dioxide (CO 2) is a greenhouse gas, the concentration of which in the atmosphere has a str... more Carbon dioxide (CO 2) is a greenhouse gas, the concentration of which in the atmosphere has a strong influence on the temperature of the Earth's surface. Throughout Earth's history CO 2 has had an important impact on the global climate, driving or amplifying global temperature changes. However, the causes of atmospheric CO 2 variability through time are not fully understood. By looking into the past during periods when CO 2 changed, we can attempt to better understand the processes that drive such variability. This study focuses on the transition out of the last ice age, known as the deglaciation that occurred 10,000-18,000 years before present. Across the deglaciation atmospheric CO 2 rose by approximately 80 ppm, the largest increase in CO 2 for over 100,000 years (aside from the present day CO 2 rise). Here we provide evidence that the CO 2 rise across the deglaciation was caused by a change in the circulation of water within the oceans. Before the deglaciation, the flow and mixing of water from the surface ocean to the bottom of the ocean slowed, effectively isolating a portion of the deep ocean from the rest. CO 2 accumulated within this deep isolated reservoir until the circulation of water abruptly changed. During the deglaciation, this isolated reservoir was carried up to the surface within the ocean that surrounds Antarctica, the Southern Ocean. Part of the reason why this change of water circulation occurred appears to have been the removal of sea ice around Antarctica. In addition, it was demonstrated that the amount and distribution of silicon and nitrogen within the oceans changed across the deglaciation. Chemicals containing these elements are important for the growth of many algae in the oceans, which play an essential role in drawing down atmospheric CO 2. Therefore, changes in the amount of silicon and nitrogen available to the algae can influence atmospheric CO 2 concentrations and hence the climate. Ultimately it was found that an important factor controlling the amount and distribution of silicon and nitrogen across the world's oceans was the flow and mixing of water within the deepest parts of the ocean. This demonstrates that changes in deep water flow and mixing can have a profound influence on ocean chemistry and the global climate.

The Indian Ocean has been proposed as an important source of salt for North Atlantic deep-water c... more The Indian Ocean has been proposed as an important source of salt for North Atlantic deep-water convection sites, via the Agulhas Leakage, and may thus drive changes in the ocean’s overturning circulation. However, while past changes in Agulhas leakage volume have been explored, little is known about this water’s salt content, representing a major gap in our understanding of Agulhas salinity supply. Here, we present new planktonic foraminiferal Mg/Ca-derived sea surface temperatures (SST) and stable isotope-derived salinity reconstructions for the last 1.2Ma from the western Indian Ocean source waters of the Agulhas Leakage to investigate glacial-interglacial changes in surface water properties. We find that SST and relative salinity both increase during glaciation, leading to high salinity and SST during glacial maxima. We show that the onset of surface salinification and warming in the Indian Ocean occurs during a phase of rapid land-bridge exposure in the Indonesian archipelago i...

Progress in Oceanography, 2016

As tall seamounts may be 'stepping stones' for dispersion and migration of deep open ocean fauna,... more As tall seamounts may be 'stepping stones' for dispersion and migration of deep open ocean fauna, an improved understanding of the productivity at and food supply to such systems needs to be formed. Here, the 234 Th/ 238 U approach for tracing settling particulate matter was applied to Senghor Seamount-a tall submarine mountain near the tropical Cape Verde archipelago-in order to elucidate the effects of topographically-influenced physical flow regimes on the export flux of particulate organic carbon (POC) from the near-surface (topmost ≤ 100 meters) into deeper waters. The comparison of a suitable reference site and the seamount sites revealed that POC export at the seamount sites was ~2-4 times higher than at the reference site. For three out of five seamount sites, the calculated POC export fluxes are likely to be underestimates. If this is taken into account, it can be concluded that POC export fluxes increase while the passing waters are advected around and over the seamount, with the highest export fluxes occurring on the downstream side of the seamount. This supports the view that biogeochemical and biological effects of tall seamounts in surface-ocean waters might be strongest at some downstream distance from, rather than centred around, the seamount summit. Based on measured (vessel-mounted ADCP) and modelled (regional flow field: AVISO; internal tides at Senghor: MITgcm) flow dynamics, it is proposed that tidally generated internal waves result in a 'screen' of increased rates of energy dissipation that runs across the seamount and leads to a combination of two factors that caused the increased POC export above the seamount: (1) sudden increased upward transport of nutrients into the euphotic zone, driving brief pulses of primary production of new particulate matter, followed by the particles' export into deeper waters; and (2) pulses of increased shear-driven aggregation of smaller, slower-settling into larger, faster-settling particles. This study shows that, under certain conditions, there can be an effect of a tall seamount on aspects of surfaceocean biogeochemistry, with tidal dynamics playing a prominent role. It is speculated that these effects can control the spatiotemporal distribution of magnitude and nutritional quality of the flux of food particles to the benthic and benthic-pelagic communities at and near tall seamounts.

Silicon isotope records from diatoms can be used as a proxy for the relative consumption of disso... more Silicon isotope records from diatoms can be used as a proxy for the relative consumption of dissolved silicic acid (DSi) in surface waters. Silicon isotopes in sponges provide information on the concentration of DSi within the ambient seawater in which the sponges reside. Here we provide deglacial silicon isotope records from both diatoms (d30Si_diat) and sponges (d30Si_sponge) collected in three piston cores from the Southern Ocean. Isolated diatom samples were produced from bulk sediment by mechanical separation and chemical cleaning. Pure diatom samples were digested in 0.1 M NaOH before removal of matrix via cation exchange chromatography. The samples were analysed by MC-ICP-MS at the University of Edinburgh. Sponge spicules were individually picked from sediment and underwent the same analytical procedure as decribed above. We relate the d30Si_diat records to changes in the deep upwelling DSi supply and diatom demand for DSi in the Southern Ocean surface across the deglaciation. We suggest that during the late deglaciation, the supply increased dramatically, leading to a decline in the d30Si_diat in all records. The d30Si_sponge data were used in conjunction with older previously published data to elucidate how a restructing of the deep ocean - which supplies DSi to the Southern Ocean - during the deglaciation, could explain the apparent increase in DSi supply that is recorded in the d30Si_diat data.

Earth and Planetary Science Letters, 2020

During the last glacial period, the sluggish deep Ocean circulation sequestered CO2 into the abys... more During the last glacial period, the sluggish deep Ocean circulation sequestered CO2 into the abyss leading to the lowering of atmospheric CO2. The impacted of this redistribution on biologically *Revision Notes *Manuscript Click here to view linked References

Nature Communications, 2020

Changes in ocean circulation and the biological carbon pump have been implicated as the drivers b... more Changes in ocean circulation and the biological carbon pump have been implicated as the drivers behind the rise in atmospheric CO2 across the last deglaciation; however, the processes involved remain uncertain. Previous records have hinted at a partitioning of deep ocean ventilation across the two major intervals of atmospheric CO2 rise, but the consequences of differential ventilation on the Si cycle has not been explored. Here we present three new records of silicon isotopes in diatoms and sponges from the Southern Ocean that together show increased Si supply from deep mixing during the deglaciation with a maximum during the Younger Dryas (YD). We suggest Antarctic sea ice and Atlantic overturning conditions favoured abyssal ocean ventilation at the YD and marked an interval of Si cycle reorganisation. By regulating the strength of the biological pump, the glacial–interglacial shift in the Si cycle may present an important control on Pleistocene CO2 concentrations.

Carbon dioxide (CO 2) is a greenhouse gas, the concentration of which in the atmosphere has a str... more Carbon dioxide (CO 2) is a greenhouse gas, the concentration of which in the atmosphere has a strong influence on the temperature of the Earth's surface. Throughout Earth's history CO 2 has had an important impact on the global climate, driving or amplifying global temperature changes. However, the causes of atmospheric CO 2 variability through time are not fully understood. By looking into the past during periods when CO 2 changed, we can attempt to better understand the processes that drive such variability. This study focuses on the transition out of the last ice age, known as the deglaciation that occurred 10,000-18,000 years before present. Across the deglaciation atmospheric CO 2 rose by approximately 80 ppm, the largest increase in CO 2 for over 100,000 years (aside from the present day CO 2 rise). Here we provide evidence that the CO 2 rise across the deglaciation was caused by a change in the circulation of water within the oceans. Before the deglaciation, the flow and mixing of water from the surface ocean to the bottom of the ocean slowed, effectively isolating a portion of the deep ocean from the rest. CO 2 accumulated within this deep isolated reservoir until the circulation of water abruptly changed. During the deglaciation, this isolated reservoir was carried up to the surface within the ocean that surrounds Antarctica, the Southern Ocean. Part of the reason why this change of water circulation occurred appears to have been the removal of sea ice around Antarctica. In addition, it was demonstrated that the amount and distribution of silicon and nitrogen within the oceans changed across the deglaciation. Chemicals containing these elements are important for the growth of many algae in the oceans, which play an essential role in drawing down atmospheric CO 2. Therefore, changes in the amount of silicon and nitrogen available to the algae can influence atmospheric CO 2 concentrations and hence the climate. Ultimately it was found that an important factor controlling the amount and distribution of silicon and nitrogen across the world's oceans was the flow and mixing of water within the deepest parts of the ocean. This demonstrates that changes in deep water flow and mixing can have a profound influence on ocean chemistry and the global climate.

The Indian Ocean has been proposed as an important source of salt for North Atlantic deep-water c... more The Indian Ocean has been proposed as an important source of salt for North Atlantic deep-water convection sites, via the Agulhas Leakage, and may thus drive changes in the ocean’s overturning circulation. However, while past changes in Agulhas leakage volume have been explored, little is known about this water’s salt content, representing a major gap in our understanding of Agulhas salinity supply. Here, we present new planktonic foraminiferal Mg/Ca-derived sea surface temperatures (SST) and stable isotope-derived salinity reconstructions for the last 1.2Ma from the western Indian Ocean source waters of the Agulhas Leakage to investigate glacial-interglacial changes in surface water properties. We find that SST and relative salinity both increase during glaciation, leading to high salinity and SST during glacial maxima. We show that the onset of surface salinification and warming in the Indian Ocean occurs during a phase of rapid land-bridge exposure in the Indonesian archipelago i...

Progress in Oceanography, 2016

As tall seamounts may be 'stepping stones' for dispersion and migration of deep open ocean fauna,... more As tall seamounts may be 'stepping stones' for dispersion and migration of deep open ocean fauna, an improved understanding of the productivity at and food supply to such systems needs to be formed. Here, the 234 Th/ 238 U approach for tracing settling particulate matter was applied to Senghor Seamount-a tall submarine mountain near the tropical Cape Verde archipelago-in order to elucidate the effects of topographically-influenced physical flow regimes on the export flux of particulate organic carbon (POC) from the near-surface (topmost ≤ 100 meters) into deeper waters. The comparison of a suitable reference site and the seamount sites revealed that POC export at the seamount sites was ~2-4 times higher than at the reference site. For three out of five seamount sites, the calculated POC export fluxes are likely to be underestimates. If this is taken into account, it can be concluded that POC export fluxes increase while the passing waters are advected around and over the seamount, with the highest export fluxes occurring on the downstream side of the seamount. This supports the view that biogeochemical and biological effects of tall seamounts in surface-ocean waters might be strongest at some downstream distance from, rather than centred around, the seamount summit. Based on measured (vessel-mounted ADCP) and modelled (regional flow field: AVISO; internal tides at Senghor: MITgcm) flow dynamics, it is proposed that tidally generated internal waves result in a 'screen' of increased rates of energy dissipation that runs across the seamount and leads to a combination of two factors that caused the increased POC export above the seamount: (1) sudden increased upward transport of nutrients into the euphotic zone, driving brief pulses of primary production of new particulate matter, followed by the particles' export into deeper waters; and (2) pulses of increased shear-driven aggregation of smaller, slower-settling into larger, faster-settling particles. This study shows that, under certain conditions, there can be an effect of a tall seamount on aspects of surfaceocean biogeochemistry, with tidal dynamics playing a prominent role. It is speculated that these effects can control the spatiotemporal distribution of magnitude and nutritional quality of the flux of food particles to the benthic and benthic-pelagic communities at and near tall seamounts.

Silicon isotope records from diatoms can be used as a proxy for the relative consumption of disso... more Silicon isotope records from diatoms can be used as a proxy for the relative consumption of dissolved silicic acid (DSi) in surface waters. Silicon isotopes in sponges provide information on the concentration of DSi within the ambient seawater in which the sponges reside. Here we provide deglacial silicon isotope records from both diatoms (d30Si_diat) and sponges (d30Si_sponge) collected in three piston cores from the Southern Ocean. Isolated diatom samples were produced from bulk sediment by mechanical separation and chemical cleaning. Pure diatom samples were digested in 0.1 M NaOH before removal of matrix via cation exchange chromatography. The samples were analysed by MC-ICP-MS at the University of Edinburgh. Sponge spicules were individually picked from sediment and underwent the same analytical procedure as decribed above. We relate the d30Si_diat records to changes in the deep upwelling DSi supply and diatom demand for DSi in the Southern Ocean surface across the deglaciation. We suggest that during the late deglaciation, the supply increased dramatically, leading to a decline in the d30Si_diat in all records. The d30Si_sponge data were used in conjunction with older previously published data to elucidate how a restructing of the deep ocean - which supplies DSi to the Southern Ocean - during the deglaciation, could explain the apparent increase in DSi supply that is recorded in the d30Si_diat data.

Earth and Planetary Science Letters, 2020

During the last glacial period, the sluggish deep Ocean circulation sequestered CO2 into the abys... more During the last glacial period, the sluggish deep Ocean circulation sequestered CO2 into the abyss leading to the lowering of atmospheric CO2. The impacted of this redistribution on biologically *Revision Notes *Manuscript Click here to view linked References

Nature Communications, 2020

Changes in ocean circulation and the biological carbon pump have been implicated as the drivers b... more Changes in ocean circulation and the biological carbon pump have been implicated as the drivers behind the rise in atmospheric CO2 across the last deglaciation; however, the processes involved remain uncertain. Previous records have hinted at a partitioning of deep ocean ventilation across the two major intervals of atmospheric CO2 rise, but the consequences of differential ventilation on the Si cycle has not been explored. Here we present three new records of silicon isotopes in diatoms and sponges from the Southern Ocean that together show increased Si supply from deep mixing during the deglaciation with a maximum during the Younger Dryas (YD). We suggest Antarctic sea ice and Atlantic overturning conditions favoured abyssal ocean ventilation at the YD and marked an interval of Si cycle reorganisation. By regulating the strength of the biological pump, the glacial–interglacial shift in the Si cycle may present an important control on Pleistocene CO2 concentrations.