Spatial heterogeneity in benthic foraminiferal assemblages track regional impacts of paleoenvironmental change across Cretaceous OAE2

The impact of global climate events on local ecosystems can vary spatially. Understanding this heterogeneity can illuminate which environments will be most impacted and the proximal drivers of ecosystem responses. Cenomanian-Turonian marine deposits of the Western Interior Seaway (WIS) record paleoceanographic changes associated with transgression and the onset of Oceanic Anoxic Event 2 (OAE2). They provide an ideal setting to study basinwide paleoecological responses during a global perturbation. Here, we integrate benthic foraminiferal assemblages from before, during, and after OAE2 via multivariate ordination analysis to examine spatial patterns in faunal responses across the western United States on a common scale and to interrogate a previously defined faunal marker used for basin-wide correlation, the Benthonic Zone (BZ). We identify oxygenation and organic matter quality as primary and secondary controls of variation across the 10 stratigraphic records and use this variation to infer paleoenvironmental changes. Stratigraphic trends reveal, in contrast to previous studies, deoxygenation at the onset of OAE2. They also reveal temporal patterns in oxygenation and productivity consistent with the gradual northward migration of a southern water mass into the WIS. This spatial heterogeneity hinders the use of the BZ in biostratigraphy because assemblages change in response to diachronous environmental change, and thus timing of the BZ with respect to OAE2 is not consistent across the basin. Our study demonstrates that regional processes can overshadow ecosystem responses to global events and underscores the importance of considering how changes in the position of water masses impact the expression of global biogeochemical perturbations.

全球气候事件对局地生态系统的影响存在空间异质性。阐明此种异质性，可明确受影响最为显著的生态环境，以及生态系统响应的近程驱动因子。北美西部内陆海道（Western Interior Seaway, WIS）的森诺曼期-土仑期海相沉积，记录了与海侵及大洋缺氧事件2（Oceanic Anoxic Event 2, OAE2）起始相关的古海洋学变化，为探究全球扰动期间全盆地的古生态响应提供了理想研究载体。本研究整合OAE2发生前、发生期间及发生后的底栖有孔虫组合数据，通过多元排序分析（multivariate ordination analysis），以统一标尺探究美国西部各区域的动物群响应空间模式，并对此前用于全盆地对比的生物地层学标志——底栖带（Benthonic Zone, BZ）开展检验。研究识别出，在10个地层记录中，水体氧化还原状态与有机质品质分别是群落变异的一级与二级控制因素，并借此推演古环境变化。与既往研究不同，地层序列趋势显示OAE2起始阶段发生了水体脱氧作用；同时还揭示出氧化还原状态与生产力的时间模式，与南部水团逐渐北移侵入WIS的过程相一致。此种空间异质性使得底栖带难以应用于生物地层学研究，因为动物群组合会响应穿时环境变化而发生改变，因此BZ相对于OAE2的出现时间在全盆地范围内并不一致。本研究证实，区域过程可掩盖生态系统对全球事件的响应，并凸显了考量水团位置变化如何影响全球生物地球化学扰动表现的重要性。