The regolith hypothesis attributes the inferred growth of a thicker and more voluminous Laurentide Ice Sheet (LIS) across the mid‐Pleistocene transition (MPT) to the removal of softer subglacial substrates of either regolith or sedimentary cover and subsequent exposure of crystalline bedrock with higher basal friction at the base of the LIS. This hypothesis predicts that sediment derived from the LIS was dominated by weathered sources prior to the MPT and then transitioned to fresh Canadian Shield sources during and after the MPT. Here, we present clay mineralogy, modified indices of chemical alteration, K/Ar ages and radiogenic isotopes (Sr, Nd, Pb) of Pleistocene clay detrital fractions from Ocean Drilling Program Site 625 in northeastern Gulf of Mexico to evaluate changes on glacial‐interglacial and longer‐term timescales in sediment composition during the Pleistocene and test this prediction. Mineralogical and geochemical proxies show a predominance of chemically weathered clays (kaolinite and smectite with clay ratios ≥3.0) sourced from the Appalachians as determined by radiogenic isotopes and K/Ar ages throughout the site's three million‐year sediment record. This is consistent with extensive pre‐Pleistocene weathering of Caledonian‐Appalachian (∼450 Ma) material that covered North American cratonic sources within the Mississippi drainage basin. However, following the MPT, we observe a unique addition of physically weathered clays (chlorite and illite with clay ratios <3.0) with radiogenic isotopes and K/Ar ages indicating contributions from more ancient geological sources, most simply explained as sediment derived from physical erosion from the crystalline Canadian Shield. These observations are generally consistent with predictions made by the regolith hypothesis.

中文翻译：

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墨西哥北部湾沉积物中加拿大结晶质盾构源晚更新世的出现

go石假说归因于，在更新世中期（MPT）期间推断出较厚且体积更大的洛朗德冰盖（LIS）的增长归因于去除了lith石或沉积覆盖层的较软的冰川下基质，并随后暴露了具有较高基础含量的结晶基岩LIS底部的摩擦。该假设预测，在MPT之前，来自LIS的沉积物主要由风化源控制，然后在MPT期间和之后过渡到新鲜的加拿大盾构源。在这里，我们介绍了粘土矿物学，化学变化的改良指数，K / Ar年龄和放射性同位素（Sr，Nd，来自墨西哥湾东北部海洋钻探计划站点625的更新世粘土碎屑级分（Pb），以评估更新世期间冰川间冰期和长期沉积物组成的时间尺度的变化，并检验这一预测。矿物学和地球化学代理显示，在整个站点300万年的沉积记录中，由放射状同位素和K / Ar年龄确定，主要来自阿巴拉契亚山脉的化学风化粘土（高岭石和蒙脱石，粘土比率≥3.0）。这与覆盖密西西比流域内北美克拉通源的加里东-阿巴拉契亚（〜450 Ma）材料的大更新世前风化是一致的。然而，遵循MPT，我们观察到独特的物理风化粘土（粘土比率<3的亚氯酸盐和伊利石）的添加。0）的放射性同位素和K / Ar年龄表明来自更古老的地质来源的贡献，最简单地解释为沉积物是来自加拿大盾构结晶的物理侵蚀而来的。这些观察结果通常与雷戈利斯假说的预测一致。