Testing Southern Ocean Control of Atmospheric CO2 using Diatom-based Paleo-proxies

该奖项由极地项目办公室的南极地质和地球物理项目提供，支持一个研究冰期-间冰期循环期间南大洋在调节大气二氧化碳（CO2）中的作用的项目。极地南大洋是深海与大气交换二氧化碳（CO2）的主要区域，因为极冷的海水向近表层浅滩流动。因此，在众多解释海洋控制大气二氧化碳的理论中，大多数仍然可行的理论都指出海洋在其中起着主导作用。因此，增加上涌的二氧化碳和营养物的消耗（例如增加出口产量）或减少向地表供应的速率（例如增加分层或海冰覆盖）可能是冰川极大期大气二氧化碳减少的原因。由于目前在南大洋表层水中只有一小部分可用的大量营养素（硝酸盐和磷酸盐）被消耗，因此任何一种情况都会使相对养分利用率大幅增加，从而使其成为一个主要变量。南大洋表面养分利用的历史将在跨越极地锋的三个海洋钻探计划（ODP）站点重建过去6万年。这些引人注目的大西洋板块遗址在整个研究期间具有连续的高沉积物堆积率（每千年15厘米）。它们在南大洋中也是独一无二的，因为它们处于高生物成因蛋白石含量的现代带内，并且有足够的有孔虫丰度用于氧同位素年代地层学。将在过去6万年中实现百年分辨率，以确定500年内气候和大气二氧化碳变化的阶段性，重点是2万至6万年前的千年尺度事件以及最后一次冰川终止。由于海冰已被认为是一种驱动机制，因此还将对来自ODP站点的现有海冰代理数据以及Vostok冰芯气候和二氧化碳记录进行详细的比较。氮同位素比率将是养分利用的主要指标，这已通过现代南大洋的研究得到了详细的验证。为了避免与有机质成岩作用相关的伪影，我们将主要对硅藻-结壳结合的有机质进行分析。这三个ODP地点在研究期间具有相对较高的生物蛋白石含量。虽然并非完全明确，但不需要额外费用获得的硅藻碳同位素数据将有助于限制可能的情景，特别是因为它将与其他示踪剂数据一起综合获得。它将主要反映地表二氧化碳和/或硅藻生长速率的变化。与Christina De La Rocha博士（剑桥大学）合作，硅同位素将在分析氮和碳同位素比的样品的一个子集上进行测量。为了获得过去4万年中营养物质利用的最佳年表，我们将使用伍兹霍尔海洋研究所（WHOI）加速器质谱（AMS）设施的约翰·海斯（John Hayes）小组最近开发的制备方法，探索硅藻壳结合有机质的放射性碳定年法。分析氮、碳和硅同位素比的样品子集将发送给WHOI进行样品制备和AMS放射性碳分析。这种方法不仅允许在没有足够的有孔虫材料进行放射性碳定年的关键时间间隔进行定年，而且还确保所获得的日期与我们的营养利用记录直接相关。对这种新方法的验证将在每个地点进行，方法是对来自同一沉积物深度（后者足够丰富）的硅藻体和有孔虫进行测年。以下主要假设将被检验：#1南大洋表面相对养分利用是过去大气二氧化碳变化的主要驱动因素，因此是全球气候变化的主要驱动因素。季节性海冰覆盖是南大洋相对养分利用和CO2的重要调节器，在南极和全球气候之间提供正反馈。#3冰川降低了硅酸盐对硝酸盐的利用率，导致亚南极表层水硅酸盐升高，富蛋白石沉积物向北移动，并改变了极地锋区形成的中间水的营养成分。

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the role of the Southern Ocean in regulating atmospheric carbon dioxide (CO2) during glacial-interglacial cycles. The polar Southern Ocean is the primary region where the deep-sea exchanges carbon dioxide (CO2) with the atmosphere due to shoaling of very-cold seawater to the near-surface layer. Consequently, of the numerous theories that have been advanced to explain oceanic control of atmospheric CO2, most of those that remain viable point to a dominant role for this region. Increasing consumption of upwelled CO2 and nutrients (e.g. increasing export production) or decreasing rates of supply to the surface (e.g. increasing stratification or sea-ice cover) may thus be responsible for reduced atmospheric CO2 during glacial maxima. Since at present only a small fraction of available macronutrients (nitrates and phosphates) are consumed in Southern Ocean surface waters, either scenario would produce a substantial increase in relative nutrient utilization, thus making it a master variable.The history of Southern Ocean surface nutrient utilization will be reconstructed for the last 60,000 years at three Ocean Drilling Program (ODP) sites that span the polar front. These remarkable Atlantic sector sites have continuous, high sediment accumulation rates (15 centimeters per thousand years (cm/kyr)) for the entire period to be studied. They are also unique for the Southern Ocean in being within the modern zone of high biogenic opal content as well as having sufficient foraminiferal abundance for oxygen isotope chronostratigraphy. Centennial resolution will be achieved over the past 60,000 years to determine phasing with changes in climate and atmospheric CO2 to within 500 years with emphasis on millennial-scale events between 20,000 and 60,000 years ago as well as the last glacial termination. Since sea-ice has been implicated as a driving mechanism, detailed comparisons will also be made for existing sea-ice proxy data from the ODP sites as well as the Vostok ice core climate and CO2 record. Nitrogen isotopic ratio will be the primary proxy for nutrient utilization, which has been verified in detail through studies of the modern Southern Ocean. To avoid artifacts associated with organic matter diagenesis, we will conduct analyses primarily on diatom-frustule bound organic matter. The three ODP sites have relatively high biogenic opal content over the time period of interest. Though not completely unambiguous, diatom carbon isotopic data obtained at no extra cost will be useful for constraining possible scenarios particularly since it will be obtained synoptically with the other tracer data. It will reflect primarily changes in surface CO2 and/or diatom growth rate. In collaboration with Dr. Christina De La Rocha (Cambridge U.), silicon isotopes will be measured on a subset of the samples analyzed for nitrogen and carbon isotopic ratio. To achieve the best possible chronology for nutrient utilization for the last 40,000 years, we will explore radiocarbon dating of diatom frustule-bound organic matter using preparation methodology recently developed by John Hayes' group at the Woods Hole Oceanographic Institution (WHOI) Accelerator Mass Spectrometry (AMS) facility. Subsets of samples analyzed for nitrogen, carbon, and silicon isotopic ratio will be sent to WHOI for sample preparation and AMS radiocarbon analysis. This approach not only permits dating of critical intervals where there is insufficient foram material for radiocarbon dating but also ensures that the dates obtained pertain directly to our nutrient utilization record. Verification of this new method will be made at each site by dating diatom frustules and forams from the same sediment depths where the latter are sufficiently abundant.The following major hypotheses will be tested:#1 Southern Ocean surface relative nutrient utilization is the primary driver of past changes in atmospheric CO2 and hence global climate change.#2 Seasonal sea ice cover is an important regulator of Southern Ocean relative nutrient utilization and CO2 providing a positive feedback between Antarctic and global climate.#3 Glacial reduction of the silicate to nitrate utilization ratio resulted in elevated silicate in Subantarctic surface waters, northward movement of opal-rich sediments, and changes in the nutrient composition of intermediate waters formed in polar frontal region.