Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany.
MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany.
Nature. 2021 Feb;590(7844):97-102. doi: 10.1038/s41586-021-03186-y. Epub 2021 Feb 3.
Following early hypotheses about the possible existence of Arctic ice shelves in the past, the observation of specific erosional features as deep as 1,000 metres below the current sea level confirmed the presence of a thick layer of ice on the Lomonosov Ridge in the central Arctic Ocean and elsewhere. Recent modelling studies have addressed how an ice shelf may have built up in glacial periods, covering most of the Arctic Ocean. So far, however, there is no irrefutable marine-sediment characterization of such an extensive ice shelf in the Arctic, raising doubt about the impact of glacial conditions on the Arctic Ocean. Here we provide evidence for at least two episodes during which the Arctic Ocean and the adjacent Nordic seas were not only covered by an extensive ice shelf, but also filled entirely with fresh water, causing a widespread absence of thorium-230 in marine sediments. We propose that these Arctic freshwater intervals occurred 70,000-62,000 years before present and approximately 150,000-131,000 years before present, corresponding to portions of marine isotope stages 4 and 6. Alternative interpretations of the first occurrence of the calcareous nannofossil Emiliania huxleyi in Arctic sedimentary records would suggest younger ages for the older interval. Our approach explains the unexpected minima in Arctic thorium-230 records that have led to divergent interpretations of sedimentation rates and hampered their use for dating purposes. About nine million cubic kilometres of fresh water is required to explain our isotopic interpretation, a calculation that we support with estimates of hydrological fluxes and altered boundary conditions. A freshwater mass of this size-stored in oceans, rather than land-suggests that a revision of sea-level reconstructions based on freshwater-sensitive stable oxygen isotopes may be required, and that large masses of fresh water could be delivered to the north Atlantic Ocean on very short timescales.
过去曾有早期假说认为北极地区过去可能存在冰架，而目前海平面以下深达 1000 米处存在的特殊侵蚀特征的观测结果，证实了在北极中央的罗蒙诺索夫海岭（Lomonosov Ridge）和其他地区存在一层厚厚的冰层。最近的建模研究探讨了在冰川时期冰架是如何形成的，以及它如何覆盖了大部分的北极海域。然而，到目前为止，在北极还没有确凿的海洋沉积物特征可以证明存在如此广泛的冰架，这让人对冰川条件对北极海洋的影响产生了怀疑。在这里，我们提供了至少两个北极海域和毗邻的北欧海域不仅被广泛的冰架覆盖，而且完全充满淡水的时期的证据，这导致海洋沉积物中钍 230 广泛缺失。我们提出，这些北极淡水期发生在距今 7 万至 6.2 万年前，大约在距今 15 万至 13.1 万年前，与海洋同位素阶段 4 和 6 的部分时期相对应。替代性解释是认为，钙质超微化石颗石藻（Emiliania huxleyi）在北极沉积物记录中的首次出现的年龄要年轻一些。我们的方法解释了北极钍 230 记录中出乎意料的最小值，这些最小值导致了对沉积速率的不同解释，并阻碍了它们在年代测定方面的应用。要解释我们的同位素解释，需要大约 900 立方公里的淡水，这一计算我们通过对水文通量和边界条件改变的估计来支持。如此大规模的淡水储存在海洋中，而不是陆地中，这表明可能需要根据对淡水敏感的稳定氧同位素来修正海平面重建，并且大量淡水可以在很短的时间内输送到北大西洋。
