New constraints on equatorial temperatures during a Late Neoproterozoic snowball Earth glaciation
New constraints on equatorial temperatures during a Late Neoproterozoic snowball Earth glaciation
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DOI：
10.1016/j.epsl.2014.09.017
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发表时间：
2014-11
影响因子：
5.3
通讯作者：
R. Ewing;I. Eisenman;M. Lamb;Laura N. Poppick;A. Maloof;W. Fischer
中科院分区：
文献类型：
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作者：
R. Ewing;I. Eisenman;M. Lamb;Laura N. Poppick;A. Maloof;W. Fischer
在低温纪末期（约6.35亿年前），强烈的冰川作用标志着地球历史上最冷的气候状态-冰川沉积物在低的热带古纬度积累的时期。解释这些沉积物的主要思想是雪球地球假说，它预测了全球冻结的表面条件和低于冰点的温度，全球气候模型将热带地区的表面温度置于-20 °C至-60 °C之间。然而，精确的古地表温度的基础上的地质限制仍然难以捉摸的全球冰川的严重性尚未确定。在这里，我们作出新的地质观察热带冰缘，风成和河流沉积结构形成的结束，在南澳的Marinoan冰川作用，这些意见使我们能够约束古代的表面温度。我们发现冰缘砂楔和相关的变形表明，地面温度足够温暖，允许韧性变形的桑迪风化层。广泛的变形结构可能表明存在一个渗透到地表以下2-4米的古活动层。这些观测结果，再加上一个由太阳辐射引起的地面温度模型，将当地年平均地表温度限制在几度的冰点之内。这种温度限制与我们对河流沉积物的观察非常匹配，河流沉积物需要足够温暖的温度才能产生地表径流。虽然这一估计与地球历史上最冷的近海平面热带温度之一相吻合，但如果这些结构代表了马里纳翁冰川的峰值条件，它们就不支持雪球地球假说的持续深冻。相反，接近0 °C的表面温度允许季节性表面融化，大气-海洋耦合和早期后生动物可能的热带避难所。如果这些结构是在冰川开始或冰川消退期间形成的，那么它们对进入或退出雪球状态的时间尺度和特征有影响。
Intense glaciation during the end of Cryogenian time (∼635 million years ago) marks the coldest climate state in Earth history – a time when glacial deposits accumulated at low, tropical paleolatitudes. The leading idea to explain these deposits, the snowball Earth hypothesis, predicts globally frozen surface conditions and subfreezing temperatures, with global climate models placing surface temperatures in the tropics between −20 °C and −60 °C. However, precise paleosurface temperatures based upon geologic constraints have remained elusive and the global severity of the glaciation undetermined. Here we make new geologic observations of tropical periglacial, aeolian and fluvial sedimentary structures formed during the end-Cryogenian, Marinoan glaciation in South Australia; these observations allow us to constrain ancient surface temperatures. We find periglacial sand wedges and associated deformation suggest that ground temperatures were sufficiently warm to allow for ductile deformation of a sandy regolith. The wide range of deformation structures likely indicate the presence of a paleoactive layer that penetrated 2–4 m below the ground surface. These observations, paired with a model of ground temperature forced by solar insolation, constrain the local mean annual surface temperature to within a few degrees of freezing. This temperature constraint matches well with our observations of fluvial deposits, which require temperatures sufficiently warm for surface runoff. Although this estimate coincides with one of the coldest near sea-level tropical temperatures in Earth history, if these structures represent peak Marinaon glacial conditions, they do not support the persistent deep freeze of the snowball Earth hypothesis. Rather, surface temperatures near 0 °C allow for regions of seasonal surface melting, atmosphere–ocean coupling and possible tropical refugia for early metazoans. If instead these structures formed during glacial onset or deglaciation, then they have implications for the timescale and character for the transition into or out of a snowball state.