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首页> 外文学位 >Testing the silicic acid leakage hypothesis: Records of opal burial from the equatorial Atlantic, equatorial Pacific and Southern Oceans over the past 30ka.
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Testing the silicic acid leakage hypothesis: Records of opal burial from the equatorial Atlantic, equatorial Pacific and Southern Oceans over the past 30ka.

机译：测试硅酸泄漏假说：过去30ka来自赤道大西洋，赤道太平洋和南大洋的蛋白石埋葬记录。

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The Silicic Acid Leakage Hypothesis (SALH) suggests that during the Last Glacial Maximum (LGM) unused silicic acid escaped the Southern Ocean through intermediate and mode waters, and was transported to the equatorial oceans. Previous work indicates that in the presence of sufficient silicic acid, diatoms are better able to utilize nitrate and other nutrients than cocolithophorids, giving diatoms a competitive advantage. The potential implications of the SALH therefore include an ecological shift in tropical regions from cocolithophorids to diatoms during the LGM and, more importantly, a drawdown of CO2 from the atmosphere due to calcium carbonate compensation. The SALH also makes some testable predictions, namely that equatorial diatom productivity was increased during the LGM (relative to the Holocene), while Southern Ocean diatom productivity was reduced. This should be reflected in the sedimentary record as a change in biogenic opal burial, which has been shown in previous studies to be reliable proxy for diatom productivity. Downcore records of opal burial were measured in the equatorial Atlantic and Pacific Oceans, and in the Pacific sector of the Southern Ocean to test the SALH. In addition, 231Pa/230Th ratios measured on the same samples were used to discriminate between changes in productivity and changes in opal preservation.;The equatorial Pacific data do not support the SALH, as there is only evidence for greater opal burial during the LGM in the two westernmost cores, while the remaining eight cores show lower LGM opal burial, confirmed by 231Pa/230Th ratios. The data support results from previous studies that invoke increased El Nino-like conditions during the LGM. Increased western Pacific opal flux and decreased opal flux in the EEP cold tongue during the LGM are both consistent with this idea. The equatorial Atlantic data are consistent with the SALH as all cores show increased LGM opal burial. Downcore records of 231Pa/230Th ratios are highly consistent with downcore opal records, implying that changes in opal burial reflect productivity and not dissolution. Cores from both basins show deglacial peaks in opal burial.;Box estimates were used to compare the magnitude of the LGM-Holocene change in opal burial in the equatorial Atlantic and Pacific Oceans. In the equatorial Atlantic Ocean, glacial opal fluxes exceeded Holocene fluxes by approximately 1.8Gt opal/ka. Applying the same sized box to the eastern equatorial Pacific (the region of highest productivity) shows that Holocene fluxes there exceeded glacial fluxes by 1.8Gt opal/ka. While these are only first-order constraints on the silica budget, these results illustrate that the net glacial decrease in equatorial Pacific opal burial is of roughly equal magnitude to the increase in Atlantic opal burial during the same time.;Total opal burial in the Southern Ocean was decreased (29%) during the LGM as compared to the Holocene. The spatial pattern of opal burial also differed between the two time periods; Holocene opal burial is greatest south of the Antarctic Polar Front (APF), while LGM opal burial was shifted north of the APF. However, the increase in glacial opal burial north of the APF was insufficient to offset decreased fluxes to the south, resulting in the observed net deficit in glacial opal burial in the Pacific SO. When averages of our fluxes (weighted by the area represented) were applied over the 60°W-140°E (the range of opal accumulation found in our samples), this yielded Holocene and LGM burial rates of 53.0 and 37.8 Gt opal/kyr, respectively. The difference, 15.2 Gt opal/kyr, is larger than the burial of opal in the equatorial Atlantic and Pacific oceans combined, and is an order of magnitude larger than the observed glacial-interglacial changes in equatorial opal burial.;Our data show a 29% decrease in SO opal burial during the last glacial period, which satisfies the primary requirement of the SALH. However, evidence from the equatorial oceans does not support the subsequent predictions of the SALH, namely that either opal burial or the Corg:CaCO 3 ratio should have increased in the glacial tropics. The magnitude of changes in equatorial opal burial are an order of magnitude smaller than the projected total export of Si from the SO, and are offsetting between the Atlantic and Pacific basins. This suggests that other factors, perhaps changes in upwelling or intermediate water circulation, are likely responsible for observed changes in equatorial opal burial during the LGP. Furthermore, our data suggest that there exists an unidentified glacial sink for Si, possibly along the continental margins. The existence of this sink, and its possible effects on glacial CO2, remain an interesting topic for future investigation. While the absence of evidence in support of the SALH does not constitute 'evidence of absence', the sum of paleo evidence to date from the tropical and Southern oceans does not support the predictions of this hypothesis.
机译：硅酸泄漏假说（SALH）表明，在最后一次冰期最大值（LGM）期间，未使用的硅酸通过中度和中等水位逃逸到南大洋，并被运输到赤道海洋。先前的工作表明，在硅酸含量充足的情况下，硅藻比椰脂石更能利用硝酸盐和其他养分，从而使硅藻具有竞争优势。因此，SALH的潜在影响包括在LGM期间热带地区从椰脂类生物转变为硅藻类，更重要的是，由于碳酸钙补偿，CO 2从大气中吸出。 SALH还做出了一些可检验的预测，即在LGM期间（相对于全新世），赤道硅藻的生产率增加了，而南大洋硅藻的生产率却下降了。这应该在沉积记录中反映为生物蛋白石埋葬的变化，这在先前的研究中已经证明是硅藻生产率的可靠替代。在赤道大西洋和太平洋以及南大洋的太平洋地区测量了蛋白石掩埋的下层记录，以测试SALH。另外，在相同样品上测得的231Pa / 230Th比值被用来区分生产力的变化和蛋白石保存的变化。赤道太平洋的数据不支持SALH，因为只有证据表明在LGM期间有更大的蛋白石埋葬。最西部的两个岩心，其余八个岩心的LGM蛋白石埋藏量较低，这由231Pa / 230Th比率证实。数据支持来自以前的研究，这些研究在LGM期间引发了类似El Nino的疾病。在LGM期间，西太平洋蛋白石通量的增加和EEP冷舌的蛋白石通量的降低均与该观点一致。赤道大西洋的数据与SALH一致，因为所有岩心都显示出LGM蛋白石埋藏的增加。 231Pa / 230Th比率的下层记录与下层蛋白石记录高度一致，这表明蛋白石埋藏量的变化反映了生产力而不是溶解。两个盆地的岩心均显示出欧泊埋藏中的冰期峰。框估计用于比较赤道大西洋和太平洋欧泊埋藏中LGM-全新世变化的幅度。在赤道大西洋，冰川蛋白石通量超过全新世通量约1.8Gt蛋白石/ ka。将相同大小的盒子应用到赤道东太平洋（生产力最高的地区）表明，那里的全新世通量比冰川通量高出1.8Gt蛋白石/ ka。虽然这些只是对二氧化硅预算的一阶约束，但这些结果表明，赤道太平洋蛋白石埋葬的净冰川减少量与同期大西洋蛋白石埋葬的增加幅度大致相等。与全新世相比，在LGM期间海洋减少了（29％）。蛋白石埋藏的空间格局在两个时期之间也有所不同。全新世蛋白石的埋葬是南极极地带（APF）南部最大的，而LGM蛋白石的埋葬则被转移到了APF的北部。但是，APF以北的冰冻蛋白石掩埋物的增加不足以抵消向南的通量的减少，从而导致太平洋SO中观测到的冰冻蛋白石掩埋物的净短缺。当在60°W-140°E（样本中发现的蛋白石积聚范围）上应用平均通量（按表示的面积加权）时，新石和LGM的埋藏率为53.0和37.8 Gt蛋白石/基石， 分别。 15.2 Gt蛋白石/基里尔的差异大于赤道大西洋和太平洋海洋中的蛋白石的总和，并且比观察到的赤道蛋白石埋藏的冰间冰期变化大一个数量级。我们的数据显示，有29在上一个冰期，SO蛋白石埋葬量减少了％，这满足了SALH的主要要求。但是，来自赤道海洋的证据不支持对SALH的后续预测，即在热带地区，蛋白石埋藏或Corg：CaCO 3的比例均应增加。赤道蛋白石埋葬的变化幅度比预计的SO中硅的总出口量小一个数量级，并且在大西洋和太平洋盆地之间相互抵消。这表明LGP期间观察到的赤道蛋白石埋藏变化可能是其他因素，可能是上升流或中间水循环的变化。此外，我们的数据表明，可能沿大陆边缘存在着一个未确定的硅冰槽。该汇的存在及其对冰川CO2的可能影响，仍然是未来研究的有趣话题。尽管缺少支持SALH的证据并不构成“缺席证据”，迄今为止来自热带和南部海洋的古证据的总和并不支持这一假设的预测。

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