Decadal-centennial-scale solar-linked climate variations and millennial-scale internal oscillations during the Early Cretaceous.

Understanding climate variability and stability under extremely warm 'greenhouse' conditions in the past is essential for future climate predictions. However, information on millennial-scale (and shorter) climate variability during such periods is scarce, owing to a lack of suitable high-resolution, deep-time archives. Here we present a continuous record of decadal- to orbital-scale continental climate variability from annually laminated lacustrine deposits formed during the late Early Cretaceous (123-120 Ma: late Barremian-early Aptian) in southeastern Mongolia. Inter-annual changes in lake algal productivity for a 1091-year interval reveal a pronounced solar influence on decadal- to centennial-scale climatic variations (including the ~ 11-year Schwabe cycle). Decadally-resolved Ca/Ti ratios (proxy for evaporation/precipitation changes) for a ~ 355-kyr long interval further indicate millennial-scale (~ 1000-2000-yr) extreme drought events in inner-continental areas of mid-latitude palaeo-Asia during the Cretaceous. Millennial-scale oscillations in Ca/Ti ratio show distinct amplitude modulation (AM) induced by the precession, obliquity and short eccentricity cycles. Similar millennial-scale AM by Milankovitch cycle band was also previously observed in the abrupt climatic oscillations (known as Dansgaard-Oeschger events) in the 'intermediate glacial' state of the late Pleistocene, and in their potential analogues in the Jurassic 'greenhouse'. Our findings indicate that external solar activity forcing was effective on decadal-centennial timescales, whilst the millennial-scale variations were likely amplified by internal process such as changes in deep-water formation strength, even during the Cretaceous 'greenhouse' period.

Weakened pacific overturning circulation, winter monsoon dominance and tectonism re-organized Japan Sea paleoceanography during the Late Miocene global cooling.

The Late Miocene global cooling (LMGC; approximately 7.9-5.8 Ma) was associated with remarkable changes in monsoon dynamics, biogenic bloom in the global oceans, and the rise of modern ecosystems at the expense of old biota. However, the possible linkage between the environmental changes and ecosystem shifts during the LMGC is still debated. In this paper, we show the high-resolution changes in the fluxes of selected radiolarian species, suggesting a drastic reorganization in the paleoceanography and ecosystem in the Japan Sea during the LMGC. The endemic radiolarian Cycladophora nakasekoi dominated the Japan Sea until 7.4 Ma when the Japan Sea sediment changed from dark radiolarian-rich sediment to organic-poor diatom ooze. Changes in the fluxes of C. nakasekoi and Tricolocapsa papillosa, the latter related to changes in the Pacific central water (PCW), show 100, 200, and ~ 500 ka cycles with their high flux mostly within the darker sediment intervals during the low-eccentricity period until 7.4 Ma, suggesting that orbitally paced PCW inflow might have been the major nutrient source into the Japan Sea. At about 7.4 Ma, these species decreased at the expense of increased Larcopyle weddellium, a radiolarian related to the North Pacific intermediate water (NPIW), and Cycladophora sphaeris, a subarctic radiolarian species, implying a decrease in PCW inflow and an increase in the inflow of NPIW and subarctic shallow water. Such a change would have been related to the LMGC-induced weakening in the Pacific Meridional overturning circulation and the southward shift of the subarctic front due to intensified East Asian winter monsoon. Such a drastic reorganization in the hydrography in the Japan Sea probably caused changes in nutrient provenance from the PCW to the NPIW and resulted in faunal turnover, marked by the disappearance of the old regional and endemic faunal components, such as C. nakasekoi.

Astronomical pacing of the global silica cycle recorded in Mesozoic bedded cherts.

The global silica cycle is an important component of the long-term climate system, yet its controlling factors are largely uncertain due to poorly constrained proxy records. Here we present a ∼70 Myr-long record of early Mesozoic biogenic silica (BSi) flux from radiolarian chert in Japan. Average low-mid-latitude BSi burial flux in the superocean Panthalassa is ∼90% of that of the modern global ocean and relative amplitude varied by ∼20-50% over the 100 kyr to 30 Myr orbital cycles during the early Mesozoic. We hypothesize that BSi in chert was a major sink for oceanic dissolved silica (DSi), with fluctuations proportional to DSi input from chemical weathering on timescales longer than the residence time of DSi (<∼100 Kyr). Chemical weathering rates estimated by the GEOCARBSULFvolc model support these hypotheses, excluding the volcanism-driven oceanic anoxic events of the Early-Middle Triassic and Toarcian that exceed model limits. We propose that the Mega monsoon of the supercontinent Pangea nonlinearly amplified the orbitally paced chemical weathering that drove BSi burial during the early Mesozoic greenhouse world.

Late Oligocene-early Miocene birth of the Taklimakan Desert.

As the world's second largest sand sea and one of the most important dust sources to the global aerosol system, the formation of the Taklimakan Desert marks a major environmental event in central Asia during the Cenozoic. Determining when and how the desert formed holds the key to better understanding the tectonic-climatic linkage in this critical region. However, the age of the Taklimakan remains controversial, with the dominant view being from ∼ 3.4 Ma to ∼ 7 Ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert. In this study, we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy. We constrained the initial desertification to be late Oligocene to early Miocene, between ∼ 26.7 Ma and 22.6 Ma. We suggest that the Taklimakan Desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan-Pamir Plateau and Tian Shan, which had reached a climatically sensitive threshold at this time.

Pre-Miocene birth of the Yangtze River.

The development of fluvial systems in East Asia is closely linked to the evolving topography following India-Eurasia collision. Despite this, the age of the Yangtze River system has been strongly debated, with estimates ranging from 40 to 45 Ma, to a more recent initiation around 2 Ma. Here, we present (40)Ar/(39)Ar ages from basalts interbedded with fluvial sediments from the lower reaches of the Yangtze together with detrital zircon U-Pb ages from sand grains within these sediments. We show that a river containing sediments indistinguishable from the modern river was established before ~23 Ma. We argue that the connection through the Three Gorges must postdate 36.5 Ma because of evaporite and lacustrine sedimentation in the Jianghan Basin before that time. We propose that the present Yangtze River system formed in response to regional extension throughout eastern China, synchronous with the start of strike-slip tectonism and surface uplift in eastern Tibet and fed by strengthened rains caused by the newly intensified summer monsoon.

A complete terrestrial radiocarbon record for 11.2 to 52.8 kyr B.P.

Radiocarbon ((14)C) provides a way to date material that contains carbon with an age up to ~50,000 years and is also an important tracer of the global carbon cycle. However, the lack of a comprehensive record reflecting atmospheric (14)C prior to 12.5 thousand years before the present (kyr B.P.) has limited the application of radiocarbon dating of samples from the Last Glacial period. Here, we report (14)C results from Lake Suigetsu, Japan (35°35'N, 135°53'E), which provide a comprehensive record of terrestrial radiocarbon to the present limit of the (14)C method. The time scale we present in this work allows direct comparison of Lake Suigetsu paleoclimatic data with other terrestrial climatic records and gives information on the connection between global atmospheric and regional marine radiocarbon levels.

Osmium evidence for synchronicity between a rise in atmospheric oxygen and Palaeoproterozoic deglaciation.

Early Palaeoproterozoic (2.5-2.0 billion years ago) was a critical phase in Earth's history, characterized by multiple severe glaciations and a rise in atmospheric O(2) (the Great Oxidation Event). Although glaciations occurred at the time of O(2) increase, the relationship between climatic and atmospheric transitions remains poorly understood. Here we report high concentrations of the redox-sensitive element Os with high initial (187)Os/(188)Os values in a sandstone-siltstone interval that spans the transition from glacial diamictite to overlying carbonate in the Huronian Supergroup, Canada. Together with the results of Re, Mo and S analyses of the sediments, we suggest that immediately after the second Palaeoproterozoic glaciation, atmospheric O(2) levels became sufficiently high to deliver radiogenic continental Os to shallow-marine environments, indicating the synchronicity of an episode of increasing O(2) and deglaciation. This result supports the hypothesis that climatic recovery from the glaciations acted to accelerate the Great Oxidation Event.