Holocene paleoenvironmental change inferred from two sediment cores collected in the Tibetan lake Taro Co

The Tibetan Plateau, also known as the “Water Tower of Asia” because of its function as a water storage and supply region, responds dramatically to modern climate changes. Paleoecological shifts inferred from lake sediment archives provide essential insights into past climate changes, and the processes that drove those shifts. This is especially true for studies of lakes in endorheic basins on the Tibetan Plateau, where lake level is regulated predominantly by Monsoon intensity. Such water bodies provide excellent opportunities to reconstruct past changes in humidity. Most paleolimnological investigations of lakes on the Tibetan Plateau, however, have involved the study of a single sediment core, making it difficult to discern between changes caused by local events and those caused by lake-wide or regional processes. Here we present results from a paleolimnological study of Lake Taro Co, a currently closed-basin lake in Central Tibet. We compared a sediment record from the central part of the lake to a record from the near-shore area, and present results of sedimentological and bioindicator (chironomid, diatom, pollen) analyses from both records. Results show three periods of lake-wide ecosystem change (> ca. 5250, 5250–2250 and < since about 2250 cal year BP), which reflect a continuous drying trend throughout the Middle and Late Holocene. In addition to this lake-wide trend, we identified two local events in the sediment core from the southeastern, nearshore site. These include (1) a hiatus between 12,400 and 5400 cal year BP and (2) an 1800-year period of distinct paleoenvironmental conditions (5400–3600 cal year BP). We hypothesize that both events were caused by relocation of a river in the southeast sector of the lake’s catchment. We propose that the first relocation caused an erosion event that removed sediment, thereby producing the hiatus. During the following 1800 years, the core site may have been located on the river delta, before another river relocation at 3600 cal year BP established the modern prodelta situation. Our study demonstrates the value of using multiple sediment cores from a lake, to better identify processes that control widespread versus local events.

A Moments View of Climatology and Variability of the Asian Summer Monsoon Anticyclone

A comprehensive investigation of the climatology of and interannual variability and trends in the Asian summer monsoon anticyclone (ASMA) is presented, based on a novel area and moments analysis. Moments include centroid location, aspect ratio, angle, and “excess kurtosis” (measuring how far the shape is from elliptical) for an equivalent ellipse with the same area as the ASMA. Key results are robust among the three modern reanalyses studied. The climatological ASMA is nearly elliptical, with its major axis aligned along its centroid latitude and a typical aspect ratio of ~5–8. The ASMA centroid shifts northward with height, northward and westward during development, and in the opposite direction as it weakens. New evidence finding no obvious climatological bimodality in the ASMA reinforces similar suggestions from previous studies using modern reanalyses. Most trends in ASMA moments are not statistically significant. ASMA area and duration, however, increased significantly during 1979–2018; the 1958–2018 record analyzed for one reanalysis suggests that these trends may have accelerated in recent decades. ASMA centroid latitude is significantly positively (negatively) correlated with subtropical jet core latitude (altitude), and significantly negatively correlated with concurrent ENSO; these results are consistent with and extend previous work relating monsoon intensity, ENSO, and jet shifts. ASMA area is significantly positively correlated with the MEI ENSO index two months previously. These results improve our understanding of the ASMA using consistently defined diagnostics of its size, geometry, interannual variability, and trends that have not previously been analyzed.

Tales from the Underground: Speleothem Records of Past Hydroclimate

Geochemical records from speleothems have significantly advanced our understanding of natural climate variability over the last ~600,000 years. Speleothems are sensitive recorders of past changes in hydroclimate because they can be precisely dated and contain multiple hydrologically sensitive geochemical proxies. Oxygen isotope records from speleothems tell us about the timing and mechanisms of past changes in precipitation amount, temperature, atmospheric circulation, and/or global monsoon intensity. Variations in speleothem carbon isotope ratios or trace element concentrations reflect changes in local water balance, vegetation, and karst hydrology. Speleothem paleoclimate records represent a window into the past that can provide crucial information for understanding how anthropogenic climate change and natural climate variability will impact future water resources on Earth.

Paleofire, Vegetation, and Climate Reconstructions of the Middle to Late Holocene From Lacustrine Sediments of the Toushe Basin, Taiwan

&lt;p&gt;We identified four climatic stages between 6.2 and 1.3 cal kyr before present (BP) based on pollen and charcoal concentrations by high&amp;#8208;resolution Accelerated mass spectrometer (AMS) &lt;sup&gt;14&lt;/sup&gt; C&amp;#8208;dated sediment profile from Taiwan's Toushe Basin. From 6.2 to 4.6 cal kyr BP, the region was warm&amp;#8208;wet with infrequent wildfires and dominant subtropical evergreen broad&amp;#8208;leaved forests. The climate was cooler&amp;#8208;drier from 4.6 to 3.0 cal kyr BP, with a decline in forest and increased fire frequency. From 3.0 to 2.1 cal kyr BP,climate further cooled and dried, with the development of alpine meadows and higher fire frequency. The region became warmer and wetter from 2.1 to 1.3 cal kyr BP, accompanied by forest recovery. Climatic changes were linked to changes in East Asia Summer Monsoon intensity,which is mainly controlled by solar radiation. Wildfires were likely controlled by precipitation variability that is influenced by East Asia Summer Monsoon and El Ni&amp;#241;o&amp;#8211;Southern Oscillation. Toushe Basin experienced drought conditions and frequent wildfires during the El Ni&amp;#241;o years.&lt;/p&gt;

Response to Comments by Daniel Gebregiorgis et al. “A Brief Commentary on the Interpretation of Chinese Speleothem δ18O Records as Summer Monsoon Intensity Tracers”. Quaternary 2020, 3, 7

We would like to thank Gebregiorgis et al [...]

A Brief Commentary on the Interpretation of Chinese Speleothem δ18O Records as Summer Monsoon Intensity Tracers

Zhang et al [...]