Distinct responses of late Miocene eolian and lacustrine systems to astronomical forcing in NE Tibet

East Asian summer monsoon (EASM) and winter monsoon (EAWM) variability on orbital time scales during the late Miocene remains poorly constrained. Climate models reveal variable responses of the EASM and EAWM to astronomical forcing, but there is a lack of empirical evidence from the geological record to validate these results. In this study, we used time series analysis to reconstruct climatic changes and orbital forcing from eolian and lacustrine sediment archives from NE Tibet during the late Miocene. Analysis of magnetic susceptibility data demonstrates that lacustrine sediments in the Tianshui Basin (Yaodian section) show dominant ∼100 k.y. eccentricity forcing in the late Miocene (ca. 10.25−8 Ma). In contrast, eolian deposits in the Jianzha Basin (Jiarang section) show significant 405 k.y. eccentricity and 41 k.y. obliquity forcing over the same interval with weak evidence for ∼100 k.y. eccentricity cycles. Grain size data from the Yaodian section support a lacustrine origin of these sediments and also support previous work demonstrating aridification in NE Tibet after ca. 8.6 Ma. Taken together, our analyses highlight markedly different orbital forcing responses of lacustrine and eolian sedimentary systems during the late Miocene. We suggest that the dominant ∼100 k.y. lacustrine cycles in the Yaodian section, which were mainly controlled by EASM variability, may have been linked to Antarctic ice sheet and/or low-latitude insolation modulations related to precession amplitude modulation by eccentricity. In contrast, the orbital signature of eolian sediments in the Jiarang section was significantly influenced by the EAWM and can be linked to variability in meridional temperature/pressure gradients.

Astronomical forcing and climate : insights from ice core records

<p>Ice cores provide a wealth of insights into past changes in climate and atmospheric composition.</p><p>Obtaining information on past polar temperature changes is important to document climate variations beyond instrumental records, and to test our understanding of past climate variations, including the Earth system response to astronomical forcing.</p><p>Since the 1960s, major breakthrough in ice core science have delivered a matrix of quantitative Greenland and Antarctic ice core records.</p><p>Temperature reconstructions from polar ice cores document past polar amplification, and provide quantitative constraints to test climate models.</p><p>Climate information from the air and ice preserved in deep ice cores has been crucial to unveil the tight coupling between the carbon cycle and climate and the role of past changes in atmospheric greenhouse gas composition in the Earth system response to astronomical forcing.</p><p>Ice core constraints on past changes in ice sheet topography are also key to characterize the contribution of the Greenland and Antarctic ice sheets to past sea level changes.</p><p>The construction of a common chronological framework for Greenland and Antarctic ice core records has unveiled the bipolar sequence of events during the glacial-interglacial cycle, and the interplay between abrupt change and the response of the climate system to astronomical forcing.</p><p>International efforts have started to obtain the oldest ice cores (hopefully back to 1,5 million years) from Antarctica, in order to understand the reasons for the major shifts in the response of the climate system to astronomical forcing at that time, leading to more intense and longer glacial periods. </p>

Diverse Regional Sensitivity of Summer Precipitation in East Asia to Ice Volume, CO2 and Astronomical Forcing

<p>The East Asian summer monsoon (EASM) is an important component of the climate system and it influences about one-third of the world’s population. Numerous paleoclimate records and climate simulations have been used to study its long-term evolution and response to different forcings. The strong regional dependence of the EASM variation questions the relative role of ice sheets and insolation on the EASM precipitation in different sub-regions in East Asia. A Gaussian emulator, which was generated and calibrated by interpolating the outputs of 61 snapshot simulations performed with the model HadCM3, is used to quantitatively assess how astronomical forcing, CO<sub>2</sub> and northern hemisphere ice sheets affect the variation of the summer precipitation over the last 800 ky. Our results show that in the north of 25°N of the EASM domain, the variation of the summer precipitation is dominated by precession and insolation. This leads to strong 23-ky cycles in the summer precipitation. However, in the southern part (south of 25°N), the impact of ice volume becomes more important, leading to strong 100-ky cycles. Ice sheets influence the summer precipitation in the south mainly through its control on the location of the Intertropical Convergence Zone (ITCZ) which is very sensitive to ice volume. ITCZ is shifted significantly to the south under large ice sheets conditions. Therefore, the region under control of the ITCZ is more sensitive to the influence of ice volume than other regions. Our results also show that obliquity and CO<sub>2</sub> have relatively small effect on the summer precipitation as compared to precession and ice sheets.</p>

Supplemental Material: Distinct responses of late Miocene eolian and lacustrine systems to astronomical forcing in NE Tibet

The pre-treatment and testing process for the grain size of the bulk sample in the Yaodian profile, and provide grain size and Mn-Fe data in the Yaodian profile.