Cadmium (Cd) isotope signatures (δCd) deduced from modern and ancient marine deposits are often used as a proxy for bioproductivity and for micronutrient cycling, whereas chromium (Cr) isotope signatures (δCr) are used as a potential proxy for oxygenation. The Cretaceous—Paleogene (K/Pg) boundary records one of Earth's five big mass extinctions, which contains geochemical fingerprints of the Chixculub asteroid impact and is coeval with the Deccan Trap volcanism. Here we present for the first time a combined record of Cd and Cr isotopes from the Danish Basin to interrogate the effects on bioproductivity across the K/Pg boundary. The δCd values range between −0.28‰ and 0.27‰ defining an average of 0.05‰ ± 0.24 (2σ, = 87). Authigenic δCr values range between 0.06‰ and 1.06‰ and define an average of 0.59‰ ± 0.51 (2σ, = 81). An abrupt jump, across the K/Pg boundary layer, from consistently positively fractionated Cd and Cr isotope signatures recorded in Late Maastrichtian white chalks towards less positively fractionated δCd values and heterogeneous δCr values in the Early Danian carbonates, likely indicates a decrease in surface water bioproductivity. This decrease is accompanied by redox fluctuations in the water column as a consequence of environmental changes either related to the Chixculub impact event and/or to the Deccan volcanic eruptions coeval with this boundary. Assuming an efficient removal of Cd and Cr through consumption and/or adsorption by primary producers, or by incorporation into the skeleton/shells of calcifiers, and using the fractionation factor of Cd into carbonates, we reconstruct surface water conditions during the Late Masstrichtian—Early Danian that are compatible with those of modern oceans, with δCd varying from 0.17 to 0.72‰. Overall, the combined Cr-Cd isotope fluctuations over the K/Pg boundary are consistent with a scenario characterized by climate-induced changes of nutrient availability and concomitant responses of primary production levels, ultimately related to the asteroid impact and to the proceeding volcanic activities that influenced the ocean's chemistry. We anticipate our results to point for further studies of other stratigraphic sections worldwide to reveal an extensive Cd-Cr record for the Cretaceous—Paleogene boundary and its corresponding collapse of bioproductivity and decreasing ocean oxygenation.

中文翻译：

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镉和铬同位素组合记录了丹麦盆地白垩纪-古近纪边界生物生产力的崩溃

从现代和古代海洋沉积物推导出来的镉 (Cd) 同位素特征 (δCd) 通常用作生物生产力和微量营养素循环的代表，而铬 (Cr) 同位素特征 (δCr) 则用作氧合的潜在代表。白垩纪-古近纪（K/Pg）边界记录了地球五次大灭绝之一，其中包含奇克斯库鲁布小行星撞击的地球化学指纹，并且与德干地盾火山活动同时发生。在这里，我们首次展示丹麦盆地的镉和铬同位素的组合记录，以探讨跨 K/Pg 边界对生物生产力的影响。 δCd 值范围在 -0.28‰ 和 0.27‰ 之间，定义平均值为 0.05‰ ± 0.24 (2σ, = 87)。自生 δCr 值范围在 0.06‰ 到 1.06‰ 之间，定义平均值为 0.59‰ ± 0.51 (2σ, = 81)。跨越 K/Pg 边界层，从马斯特里赫特晚期白色白垩记录的持续正分馏的 Cd 和 Cr 同位素特征突然跳跃到早期达尼安碳酸盐岩中较不正分馏的 δCd 值和异质 δCr 值，这可能表明地表水的减少生物生产力。这种减少伴随着水柱中的氧化还原波动，这是由于与奇克斯库鲁布撞击事件和/或与该边界同时发生的德干火山喷发有关的环境变化造成的。假设通过初级生产者的消耗和/或吸附，或通过纳入钙化剂的骨架/壳中，并使用镉在碳酸盐中的分馏因子，有效去除镉和铬，我们重建了马斯特里赫特阶晚期-早期的地表水条件与现代海洋的大年相一致，δCd 变化范围为 0.17 至 0.72 ‰。总体而言，K/Pg 边界上的 Cr-Cd 同位素组合波动与气候引起的养分有效性变化和初级生产水平的相应响应的情景一致，最终与小行星撞击和正在进行的火山活动有关。影响了海洋的化学。我们期望我们的结果能够为全球其他地层剖面的进一步研究指明方向，以揭示白垩纪-古近纪边界的广泛镉-铬记录及其相应的生物生产力崩溃和海洋氧合作用下降。