This study presents new stable carbon and oxygen isotope data from Lower Cretaceous lacustrine carbonate rock samples recovered from a well drilled in the Santos Basin, offshore southeast Brazil. These samples represent a record of a continental environment just prior to the opening of the South Atlantic Ocean and the ultimate break-up of Gondwanaland. The geochemical data, along with carbonate mineralogy, indicate repeated cycles of lake level variation that could be attributed to climatic oscillations. Despite the absence of correlations between δ¹³C and δ¹⁸O values, facies analysis and the isotopic and mineralogical data suggest that lake hydrology was essentially closed for most of the depositional interval studied here. The existence of persisting trends of nearly constant δ¹³C values with a spread in δ¹⁸O values though, suggests long water residence times in the palaeolake, equilibrium between atmosphere and lake water CO₂, as well as significant evaporation of water. The overall geological model that emerges unveils a more comprehensive picture of the depositional conditions that favoured the continuity of a significant carbonate factory in the middle of the Gondwanan continent, corroborating previous studies that suggested the lasting existence of a large and somewhat shallow endorheic lake in the area during the Early Cretaceous. As a result of this recorded trend strongly suggesting equilibrium between lake waters DIC (dissolved inorganic carbon) reservoir and atmospheric CO₂, the data are most consistent with lacustrine deposition rather than precipitation of travertine, contrasting with some suggestions for the genesis of the carbonates of the Barra Velha Formation. Finally, this apparent equilibrium with the atmosphere likely left a preserved record in the continental carbonates of the final stages that preceded a major global environmental disturbance associated with an increase in atmospheric CO₂, known for this time as the Oceanic Anoxic Event (OAE) 1a. If this is correct, it also helps to put further time constraints on this studied interval, which should not be younger than Barremian age, and to provide a regional continental perspective on a global event.

这项研究提供了从巴西东南沿海桑托斯盆地钻探的一口井中回收的下白垩统湖相碳酸盐岩岩石样品的新的稳定碳和氧同位素数据。这些样本代表了南大西洋开放和冈瓦纳大陆最终解体之前的大陆环境记录。地球化学数据以及碳酸盐矿物学表明，湖水位变化的重复周期可能归因于气候振荡。尽管不存在之间的相关性δ的¹³ C和δ ^18个O值，相分析和同位素和矿物学的数据表明，湖泊水文学基本上关闭大多数这里研究沉积间隔的。δ几乎恒定的持续趋势的存在^13个C值与δ的扩展^18个O值虽然，建议在palaeolake长水的停留时间，平衡气氛和湖水CO之间₂，以及水显著蒸发。出现的整体地质模型揭示了沉积条件的更全面图景，这些沉积条件有利于冈瓦纳大陆中部一个重要的碳酸盐工厂的连续性，这证实了先前的研究表明在该湖中长期存在着一个较大且较浅的背胶湖。在白垩纪早期。由于这种趋势的记录，强烈暗示了湖水DIC（溶解的无机碳）储层与大气CO ₂之间的平衡，该数据与湖相沉积而不是钙华沉积最吻合，这与对巴拉维哈组碳酸盐岩成因的一些建议形成了对比。最后，这种与大气的明显平衡很可能在最后阶段的大陆碳酸盐中保留了记录，该记录发生在与大气CO ₂增加相关的重大全球环境扰动之前，这次被称为海洋缺氧事件（OAE）1a 。如果这是正确的话，这还有助于在这个研究的时间间隔上增加时间限制，该时间间隔不应小于巴雷米亚年龄，并为全球性事件提供区域性的大陆视角。