Dissertation Abstract
Plio-Pleistocene nutrient consumption in the polar oceans reconstructed from diatom-bound nitrogen isotopes
Studer, Anja S 2013
Department of Earth Sciences, ETH Zurich (Switzerland), 175 pp.
The North Pacific and the Southern oceans represent remarkably similar polar ocean environments. Both regions are characterized by net upwelling of nutrient- and CO2-rich deep waters and high residual surface nutrient concentrations, which arises from iron and/or light limitation on phytoplankton growth. The incomplete nutrient uptake introduces a degree of inefficiency to the global biological pump, thereby contributing to the net transfer of CO2 to the atmosphere.
Several hypotheses have been put forward to explain the lower observed glacial atmospheric CO2 concentrations, with many of them focusing on the Southern Ocean as the primary driver. Among others, a decrease in the exchange between the polar surface ocean and the ocean interior has been proposed, a condition, which has been described as “polar ocean stratification”. This physical mechanism would have reduced the evasion of deeply sequestered CO2 and would have rendered the Antarctic relatively less important in ventilating the ocean interior, contributing to lower atmospheric CO2. It also would have restricted the gross nutrient supply to the polar surface oceans, enhancing nutrient consumption and thus lowering the amount of preformed nutrients returning to the ocean interior.
In order to reconstruct relative nutrient consumption and thus the efficiency of the biological pump in the high latitudes, sedimentary nitrogen (N) isotopes have proven a useful tool, as they record the extent to which nitrate has been taken up in the sunlit surface ocean. To circumvent potential biases associated with diagenesis and allochthonous N input, diatom-bound N isotopes (d15Ndb) have been developed, as they are physically protected from alteration and thus considered to reflect the pristine signal of the organic N originally emplaced in the siliceous frustule.
The aim of this dissertation was to reconstruct nitrate consumption in the subarctic Pacific and in the Southern Ocean over glacial/interglacial cycles and across the Plio-Pleistocene climate transition to assess the leverage of the high latitude oceans in modulating atmospheric CO2 concentrations on various timescales.
Work in the subarctic North Pacific focused on the transition from the Pliocene Warm Period into the Pleistocene epoch of ice ages, to test the hypothesis that the polar oceans stratified when Northern Hemisphere glaciation intensified in association with global cooling. We show that phytoplankton export production permanently declined 2.7 Ma ago. In combination with a general increase in nutrient consumption, these observations strengthen the case that the gross nutrient supply to the surface ocean decreased after 2.7 Ma, resulting from the development of a perennial halocline in the subarctic Pacific at that time.
From the Pacific sector of the Antarctic, we report high-resolution d15Ndb data showing the strongest correlation to date between nitrate consumption in the Antarctic and global climate across the last two glacial cycles. We show that Antarctic surface nitrate concentration declined in two steps across the last glacial inception, coeval with Antarctic cooling and associated atmospheric CO2 decline.
Altogether, the results of this dissertation contribute to advance our understanding on the role of the polar oceans in modulating atmospheric CO2 on various timescales.
Several hypotheses have been put forward to explain the lower observed glacial atmospheric CO2 concentrations, with many of them focusing on the Southern Ocean as the primary driver. Among others, a decrease in the exchange between the polar surface ocean and the ocean interior has been proposed, a condition, which has been described as “polar ocean stratification”. This physical mechanism would have reduced the evasion of deeply sequestered CO2 and would have rendered the Antarctic relatively less important in ventilating the ocean interior, contributing to lower atmospheric CO2. It also would have restricted the gross nutrient supply to the polar surface oceans, enhancing nutrient consumption and thus lowering the amount of preformed nutrients returning to the ocean interior.
In order to reconstruct relative nutrient consumption and thus the efficiency of the biological pump in the high latitudes, sedimentary nitrogen (N) isotopes have proven a useful tool, as they record the extent to which nitrate has been taken up in the sunlit surface ocean. To circumvent potential biases associated with diagenesis and allochthonous N input, diatom-bound N isotopes (d15Ndb) have been developed, as they are physically protected from alteration and thus considered to reflect the pristine signal of the organic N originally emplaced in the siliceous frustule.
The aim of this dissertation was to reconstruct nitrate consumption in the subarctic Pacific and in the Southern Ocean over glacial/interglacial cycles and across the Plio-Pleistocene climate transition to assess the leverage of the high latitude oceans in modulating atmospheric CO2 concentrations on various timescales.
Work in the subarctic North Pacific focused on the transition from the Pliocene Warm Period into the Pleistocene epoch of ice ages, to test the hypothesis that the polar oceans stratified when Northern Hemisphere glaciation intensified in association with global cooling. We show that phytoplankton export production permanently declined 2.7 Ma ago. In combination with a general increase in nutrient consumption, these observations strengthen the case that the gross nutrient supply to the surface ocean decreased after 2.7 Ma, resulting from the development of a perennial halocline in the subarctic Pacific at that time.
From the Pacific sector of the Antarctic, we report high-resolution d15Ndb data showing the strongest correlation to date between nitrate consumption in the Antarctic and global climate across the last two glacial cycles. We show that Antarctic surface nitrate concentration declined in two steps across the last glacial inception, coeval with Antarctic cooling and associated atmospheric CO2 decline.
Altogether, the results of this dissertation contribute to advance our understanding on the role of the polar oceans in modulating atmospheric CO2 on various timescales.