ALBERT

Description: During the Holocene there has been a gradual increase in the influence of humans on Earth systems. High-resolution sedimentary records can help us to assess how erosion and weathering have evolved in response to recent climatic and anthropogenic disturbances. Here we present data from a high-resolution (~75 cm/k.y.) sedimentary archive from the South China Sea. Provenance data indicate that the sediment was derived from the Red River, and can be used to reconstruct the erosion and/or weathering history in this river basin. Accelerator mass spectrometry 14 C dating provides direct age control and reveals coherent variations in clay mineralogy, geochemistry, and terrigenous flux, indicative of strong chemical weathering and physical erosion during the mid-Holocene warm period (6400–4000 cal [calibrated] yr B.P.), followed by weakening from ca. 4000–1800 cal yr B.P., and renewed intensification since 1800 cal yr B.P.. Comparison with climatic records from China indicates that precipitation and temperature controlled both physical erosion and chemical weathering intensity before 1800 cal yr B.P.. However, weathering proxies in the offshore sediment indicate recent increased soil erosion. We suggest that enhanced human activity (deforestation, cultivation, and mining) since the end of the Chinese Han Dynasty (220 CE) has overwhelmed the natural climatic controls on erosion in the Red River.

Description: 〈span〉〈div〉Abstract〈/div〉The Southern Hemisphere Westerly Winds (SHWW) exert important controls on regional and global climate. Instrumental and reanalysis records indicate strengthening and poleward contraction of the SHWW belt since the late twentieth century. Such changes also have implications for Southern Ocean upwelling and CO〈sub〉2〈/sub〉 degassing. Therefore, a better understanding of the long-term SHWW behaviors and dynamics beyond recent decades is critical for projecting future changes. Here, we applied isotope analysis of 〈span〉Sphagnum〈/span〉 moss cellulose from a peat bog in southernmost Patagonia (∼54°S) to reconstruct changes in oxygen isotope composition of precipitation (δ〈sup〉18〈/sup〉O〈sub〉p〈/sub〉) that could elucidate past shifts in moisture sources and trajectories. We interpreted the positive shifts in δ〈sup〉18〈/sup〉O〈sub〉p〈/sub〉 to indicate weaker SHWW and, importantly, more frequent easterly flows that enhance moisture supply sourced from the Atlantic Ocean. In contrast, negative shifts in δ〈sup〉18〈/sup〉O〈sub〉p〈/sub〉 indicate stronger SHWW and intensification of the Andean rain shadow. Our data, along with other evidence from southernmost Patagonia and the Antarctic Peninsula, suggest a coherent pattern of centennial-scale variability in SHWW strength on either side of the Drake Passage over the past two millennia, probably as a teleconnection response to El Niño–Southern Oscillation–like variability. Our study implies that investigations of past changes in the SHWW and associated teleconnection mechanisms should consider synoptic-scale atmospheric circulation patterns, rather than seeing SHWW as a simplistic west-to-east (zonal) wind-flow pattern, particularly on the time scales over which the SHWW express zonal asymmetry among different sectors of the Southern Ocean.〈/span〉

Description: Indian summer monsoon (ISM) variations have been linked to the orbital-scale boreal summer insolation and millennial-scale North Atlantic climates. Recent studies show the critical role of Indian Ocean sea-surface temperatures (SSTs) in affecting deglacial millennial-scale monsoon oscillations. However, it is unclear whether SSTs can affect monsoon rainfall and terrestrial hydroclimate during the Holocene. Here we report multiproxy evidence of hydroclimate changes in southwest China since the Last Glacial Maximum. Similar to the often-documented gradual decrease in Holocene monsoon rainfall with superimposed millennial-scale variations, our records particularly show pronounced hydroclimate fluctuations including wet conditions at ~5000-4000 yr ago, and perhaps over the past 1000 yr. We also find coherent variations between our records and sea-surface salinities in the eastern Indian Ocean, suggesting that terrestrial hydroclimate and resultant continental drainage have affected surface ocean conditions. These fluctuations are likely linked to changes in SSTs downstream of the monsoon source in the tropical western Indian Ocean, i.e., a warmer ocean and more monsoon rainfall. We conclude that the influence of both insolation and tropical SSTs on the ISM has persisted from the last deglaciation into Holocene.