CaCO3, foraminifer fragmentation, benthic carbon isotopes, coarse lithic counts and radiogenic isotopes (Sr, Nd, Pb) of the Pliocene and earliest Pleistocene terrigenous component (3.3-2.4 Ma) of IODP Site 306-U1313@en

We present Plio-Pleistocene records of sediment color, %CaCO3, foraminifer fragmentation, benthic carbon isotopes (d13C) and radiogenic isotopes (Sr, Nd, Pb) of the terrigenous component from IODP Site U1313, a reoccupation of benchmark subtropical North Atlantic Ocean DSDP Site 607. We show that (inter)glacial cycles in sediment color and %CaCO3 pre-date major northern hemisphere glaciation and are unambiguously and consistently correlated to benthic oxygen isotopes back to 3.3 million years ago (Ma) and intermittently so probably back to the Miocene/Pliocene boundary. We show these lithological cycles to be driven by enhanced glacial fluxes of terrigenous material (eolian dust), not carbonate dissolution (the classic interpretation). Our radiogenic isotope data indicate a North American source for this dust (~3.3-2.4 Ma) in keeping with the interpreted source of terrestrial plant wax-derived biomarkers deposited at Site U1313. Yet our data indicate a mid latitude provenance regardless of (inter)glacial state, a finding that is inconsistent with the biomarker-inferred importance of glaciogenic mechanisms of dust production and transport. Moreover, we find that the relation between the biomarker and lithogenic components of dust accumulation is distinctly non-linear. Both records show a jump in glacial rates of accumulation from Marine Isotope Stage, MIS, G6 (2.72 Ma) onwards but the amplitude of this signal is about 3-8 times greater for biomarkers than for dust and particularly extreme during MIS 100 (2.52 Ma). We conclude that North America shifted abruptly to a distinctly more arid glacial regime from MIS G6, but major shifts in glacial North American vegetation biomes and regional wind fields (exacerbated by the growth of a large Laurentide Ice Sheet during MIS 100) likely explain amplification of this signal in the biomarker records. Our findings are consistent with wetter-than-modern reconstructions of North American continental climate under the warm high CO2 conditions of the Early Pliocene but contrast with most model predictions for the response of the hydrological cycle to anthropogenic warming over the coming 50 years (poleward expansion of the subtropical dry zones).我们报道了综合大洋钻探计划（IODP）U1313站位的上新世-更新世（Plio-Pleistocene）记录，该站位是对北大西洋亚热带基准深海钻探计划（DSDP）607站位的重新钻探；记录内容包含沉积物颜色、碳酸钙（%CaCO₃）占比、有孔虫破碎率、底栖碳同位素（δ¹³C），以及陆源组分的放射成因同位素（锶Sr、钕Nd、铅Pb）数据。研究表明，沉积物颜色与碳酸钙占比所记录的冰期-间冰期循环早于北半球大规模冰川作用的发生，且可与330万年前（Ma）以来的底栖氧同位素序列明确且稳定地对应，其对应关系可能在中新世/上新世边界时期也断续存在。我们证实，这些岩性循环的驱动因素是冰期时期陆源物质（风尘）输入通量的增强，而非经典解释所认为的碳酸盐溶解作用。本研究的放射成因同位素数据显示，该风尘物质的源区为北美地区（约3.3~2.4 Ma），这与U1313站位沉积的陆生植物蜡源生物标志物的推断源区一致。但我们的数据显示，无论冰期-间冰期状态如何，该风尘均来自中纬度源区，这一发现与生物标志物研究推断的、由冰川作用机制主导风尘产生与搬运过程的重要性结论相悖。此外，我们发现风尘堆积的生物标志物组分与岩源组分之间的关系显著非线性。两类记录均显示，自海洋同位素阶段（MIS）G6（272万年前，2.72 Ma）起，冰期堆积速率出现跃升；但生物标志物记录的信号振幅约为风尘记录的3~8倍，且在MIS 100（252万年前，2.52 Ma）时期尤为显著。我们推断，自MIS G6时期起，北美地区突然进入了显著更为干旱的冰期气候状态；而北美冰期植被生物群区的重大转变与区域风场的变化（因MIS 100时期劳伦泰德冰盖（Laurentide Ice Sheet）大规模发育而进一步加剧），或可解释生物标志物记录中信号的增幅现象。我们的研究结果与早上新世时期高CO₂温暖背景下北美大陆气候较现代更为湿润的重建结果一致，但与多数模型预测的、未来50年人为变暖下水文循环的响应模式（亚热带干旱带向极地方向扩张）相悖。