Mid-Pleistocene links between Asian dust, Tibetan glaciers, and Pacific iron fertilization.

Increasing Asian dust fluxes, associated with late Cenozoic cooling and intensified glaciations, are conventionally thought to drive iron fertilization of phytoplankton productivity in the North Pacific, contributing to ocean carbon storage and drawdown of atmospheric CO2. During the early Pleistocene glaciations, however, productivity remained low despite higher Asian dust fluxes, only displaying glacial stage increases after the mid-Pleistocene climate transition (~800 ka B.P.). We solve this paradox by analyzing an Asian dust sequence, spanning the last 3.6 My, from the Tarim Basin, identifying a major switch in the iron composition of the dust at ~800 ka, associated with expansion of Tibetan glaciers and enhanced production of freshly ground rock minerals. This compositional shift in the Asian dust was recorded synchronously in the downwind, deep sea sediments of the central North Pacific. The switch from desert dust, containing stable, highly oxidized iron, to glacial dust, richer in reactive reduced iron, coincided with increased populations of silica-producing phytoplankton in the equatorial North Pacific and increased primary productivity in more northerly locations, such as the South China Sea. We calculate that potentially bioavailable Fe2+ flux to the North Pacific was more than doubled after the switch to glacially- sourced dust. These findings indicate a positive feedback between Tibetan glaciations, glaciogenic production of dust with enhanced iron bioavailability, and changes in North Pacific iron fertilization. Notably, this strengthened link between climate and eolian dust coincided with the mid-Pleistocene transition to increased storage of C in the glacial North Pacific and more intense northern hemisphere glaciations.

The 3.6-Ma aridity and westerlies history over midlatitude Asia linked with global climatic cooling.

Midlatitude Asia (MLA), strongly influenced by westerlies-controlled climate, is a key source of global atmospheric dust, and plays a significant role in Earth's climate system . However, it remains unclear how the westerlies, MLA aridity, and dust flux from this region evolved over time. Here, we report a unique high-resolution eolian dust record covering the past 3.6 Ma, retrieved from the thickest loess borehole sequence (671 m) recovered to date, at the southern margin of the Taklimakan desert in the MLA interior. The results show that eolian dust accumulation, which is closely related to aridity and the westerlies, indicates existence of a dry climate, desert area, and stable land surface, promoting continuous loess deposition since at least ∼3.6 Ma. This region experienced long-term stepwise drying at ∼2.7, 1.1, and 0.5 Ma, coeval with a dominant periodicity shift from 41-ka cyclicity to 100-ka cyclicity between 1.1 Ma and 0.5 Ma. These features match well with global ice volume variability both in the time and frequency domains (including the Mid-Pleistocene Transition), highlighting global cooling-forced aridity and westerlies climate changes on these timescales. Numerical modeling demonstrates that global cooling can dry MLA and intensify the westerlies, which facilitates dust emission and transport, providing an interpretive framework. Increased dust may have promoted positive feedbacks (e.g., decreasing atmospheric CO2 concentrations and modulating radiation budgets), contributing to further cooling. Unraveling the long-term evolution of MLA aridity and westerlies climate is an indispensable component of the unfolding mystery of global climate change.

Paleogeography control of Indian monsoon intensification and expansion at 41 Ma.

As a crucial part of the Asian monsoon stretching from tropical India to temperate East Asia, the Indian monsoon (IM) contributes predominant precipitation over Asian continent. However, our understanding of IM's onset, development and the underlying driving mechanisms is limited. Increasing evidence indicates that the IM began in the Eocene or even the Paleocene and was unexceptionally linked to the early rise of the Tibetan Plateau (TP). These were challenged by the heterogeneous and diachronous uplift of the TP and all the reported records were confined to tropical zone under tropical monsoon driven by the Intertropical Convergence Zone (ITCZ) that is irrelevant to the TP. Therefore, reliable paleoclimatic records from the extra-tropical IM region is crucial to reveal how the tropical IM expanded to subtropical and temperate zones and what driving factors might be related to it. Here we present robust Eocene paleoenvironmental records from central Yunnan (~26°N) in subtropical East Asia. The multiproxy results of two sites demonstrate a consistent sudden switch from a dry environment in the early Eocene to a seasonally wet one at 41 Ma, suggesting a jump of the tropical IM to the southern subtropical zone at 41 Ma. The full collision of India with Asia, and the resulting changes in paleogeography at 41 Ma (closure of the Neotethys sea, retreat of the Paratethys seas, fast northward movement of the southern margin of the TP and rise of the central TP), aided by synchronous Antarctic cooling, might have worked together to drive the IM enhancement and northward expansion.

Oscillation of mineral compositions in Core SG-1b, western Qaidam Basin, NE Tibetan Plateau.

Uplift of the Tibetan Plateau since the Late Miocene has greatly affected the nature of sediments deposited in the Qaidam Basin. However, due to the scarcity of continuously dated sediment records, we know little about how minerals responded to this uplift. In order to understand this response, we here present results from the high-resolution mineral profile from a borehole (7.3-1.6 Ma) in the Basin, which shows systematic oscillations of various evaporite and clay minerals that can be linked to the variation of regional climate and tectonic history. In particular, x-ray diffraction (XRD) analyses show that carbonate minerals consist mainly of calcite and aragonite, with minor ankerite and dolomite. Evaporates consist of gypsum, celesite and halite. Clay minerals are principally Fe-Mg illite, mixed layers of illite/smectite and chlorite, with minor kaolinite and smectite. Following implications can be drawn from the oscillations of these minerals phases: (a) the paleolake was brackish with high salinity after 7.3 Ma, while an abrupt change in the chemical composition of paleolake water (e.g. Mg/Ca ratio, SO4(2-) concentration, salinity) occurred at 3.3 Ma; (b) the three changes at ~6.0 Ma, 4.5-4.1 Ma and 3.3 Ma were in response to rapid erosions/uplift of the basin;