In modern ooid-forming environments in the Caribbean, aerobic respiration of organic matter below the sediment–water interface drives an increase in pCO2 and a corresponding decrease in carbonate saturation state (Ω) that creates shallow sediment porewater that is neutral or slightly caustic to carbonate. The locus of ooid growth, therefore, is presumed to be in the water column during suspension, where supersaturation with respect to calcium carbonate is the norm. In the past, however, during conditions of low aqueous O2, high Ω, or low organic-matter input, the shallow sub-sediment marine burial environment was conducive to carbonate precipitation.Here we present petrographic and electron probe microanalyzer (EPMA) data from exquisitely preserved oolites through time that suggests that some ancient ooids may have grown within the sediment pile. We propose that each increment of ooid cortical growth originated as incipient isopachous marine cement formed during shallow burial within migrating ooid dunes. After a period of burial (∼ weeks to months), ooids were remobilized and rounded during bedload transport.This “bedform model” for ooid growth explains: 1) why ancient ooids are not limited by the precipitation–abrasion balance that appears to prohibit modern tangential Caribbean ooids from achieving grain sizes larger than coarse sand, 2) the radial crystal fabric that defines the internal structure of many ancient ooids, and 3) the first-order correlation of the abundance of large and giant ooids in the rock record to periods with predicted high porewater Ω. This model implies that photosynthetic microbes were unimportant for growth of large and giant ooid but it remains agnostic to the effect of other microbes. The physical and chemical milieu of modern marine ooid-forming environments is perhaps not the best analogue for ancient ooid-forming environments; this should be considered when using ancient ooids to reconstruct secular trends in ocean chemistry.

中文翻译：

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沉积物中古卵石生长的一个例子

在加勒比海的现代形成卵石的环境中，沉积物-水界面以下的有机物需氧呼吸促使pCO2升高，碳酸盐饱和度（Ω）相应降低，从而形成浅水沉积物孔隙水，对碳酸盐呈中性或略带腐蚀性。因此，推测在悬浮过程中卵石的生长轨迹位于水柱中，其中相对于碳酸钙的过饱和是常态。但是，在过去，在低含水O2，高Ω或低有机物输入的条件下，浅浅的亚沉积海相埋藏环境有利于碳酸盐的沉淀。在此，我们提供了岩石学和电子探针显微分析仪（EPMA）的数据经过一段时间保存的精美的橄榄岩表明在沉积物中可能生长了一些古老的卵石。我们提出，卵石皮质生长的每一个增量都起源于在迁移的卵丘中浅埋期间形成的初始等渗海洋胶结物。经过一段时间的埋葬（约数周至数月）后，类固醇在床床运输过程中被移动并倒圆。这种用于类固醇生长的“床形模型”解释了：1）为什么古老的类固醇不受降水-磨蚀平衡的限制，似乎阻止了现代切向加勒比海长石，其晶粒尺寸大于粗砂； 2）径向晶体织物，它定义了许多古老长石的内部结构，以及3）岩石记录中大型和巨型长石的丰度与周期的一阶相关性预测的高孔隙水Ω。该模型表明，光合微生物对于大而巨大的卵石的生长并不重要，但对其他微生物的作用仍然不可知。现代海洋形成类固醇的环境的物理和化学环境可能不是古代形成类固醇的最佳模拟环境。当使用古代的类固醇来重建海洋化学的长期趋势时，应考虑这一点。