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

该奖项由极地方案办公室南极地质和地球物理方案提供，支持一个项目，该项目旨在调查南大洋在冰川-间冰期循环期间调节大气二氧化碳(CO2)的作用。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变浅。因此，在解释海洋对大气二氧化碳的控制的众多理论中，大多数仍然可行的理论指出，这一地区发挥了主导作用。因此，增加上升的二氧化碳和营养物质的消耗(例如增加出口生产)或减少对地表的供应(例如增加层化或海冰覆盖)可能是造成冰川盛期大气二氧化碳减少的原因。由于目前只有一小部分可利用的常量营养素(硝酸盐和磷酸盐)被消耗在南大洋表层水域，这两种情况都会导致相对营养物质利用率的大幅增加，从而使其成为主变量。这些引人注目的大西洋区域遗址在整个研究期间都有连续的高沉积速率(每千年15厘米(厘米/KYR))。它们对南大洋来说也是独一无二的，因为它们位于生物蛋白石含量较高的现代带内，并且有足够的有孔虫丰度，可以用于氧同位素年代地层学。将在过去60,000年内实现百年决议，以确定气候和大气二氧化碳变化的阶段性，以500年内为重点，重点是20,000至60,000年前的千年尺度事件以及最后一次冰川终止。由于海冰被认为是一种驱动机制，因此还将对来自各ODP站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比较。氮同位素比率将是营养物质利用的主要指标，这一点已通过对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有关的伪影，我们将主要对硅藻-小果体结合的有机质进行分析。在感兴趣的时间段内，这三个ODP点的生物蛋白石含量相对较高。硅藻碳同位素数据虽然不是完全明确的，但不需要额外费用就可用于限制可能的情况，特别是因为它将与其他示踪剂数据同义地获得。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，将在分析氮和碳同位素比率的样品子集上测量硅同位素。为了获得过去40,000年来最好的养分利用年表，我们将使用John Hayes团队最近在伍兹霍尔海洋研究所(WHOI)加速器质谱仪(AMS)设施开发的制备方法来探索硅藻结合有机质的放射性碳测年。对氮、碳和硅同位素比率进行分析的样品子集将被送往世界卫生组织进行样品制备和AMS放射性碳分析。这种方法不仅可以在没有足够的有孔材料进行放射性碳测年的关键时间间隔进行测年，而且还确保所获得的日期与我们的养分利用记录直接相关。这一新方法的验证将在每个地点进行，方法是从相同的沉积物深度测定硅藻树枝和有孔虫的年龄，后者足够丰富。将检验以下主要假设：#1南大洋表面相对营养物质利用是过去大气CO2变化和全球气候变化的主要驱动因素。#2季节性海冰覆盖是南大洋相对营养物质利用的重要调节器，CO2在南极和全球气候之间提供正反馈。#3冰川减少硅酸盐/硝酸盐利用率导致亚北极表层水域硅酸盐增加，富含蛋白石的沉积物北移，以及极地锋区形成的中间水营养成分的变化。

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.