Changes of lake organic carbon sinks from closed basins since the Last Glacial Maximum and quantitative evaluation of human impacts

Background Closed basins occupy 21% of the world’s land area and can substantially affect global carbon budgets. Conventional understanding suggests that the terminal areas of closed basins collect water and carbon from throughout the entire basin, and changes in lake organic carbon sinks are indicative of basin-wide organic carbon storages. However, this hypothesis lacks regional and global validation. Here, we first validate the depositional process of organic carbon in a typical closed-basin region of northwest China using organic geochemical proxies of both soil and lake sediments. Then we estimate the organic carbon sinks and human impacts in extant closed-basin lakes since the Last Glacial Maximum (LGM). Results Results show that 80.56 Pg organic carbon is stored in extant closed-basin lakes mainly found in the northern mid-latitudes. Carbon accumulation rates vary from 17.54 g C m−2 yr−1 during modern times, 6.36 g C m−2 yr−1 during the mid-Holocene and 2.25 g C m−2 yr−1 during the LGM. Then, we evaluated the influence by human activities during the late Holocene (in the past three thousand years). The ratio of human impacts on lake organic carbon storage in above closed basins is estimated to be 22.79%, and human-induced soil organic carbon emissions in the past three thousand years amounted to 207 Pg. Conclusions While the magnitude of carbon storage is not comparable to those in peatland, vegetation and soil, lake organic carbon sinks from closed basins are significant to long-term terrestrial carbon budget and contain information of climate change and human impact from the whole basins. These observations improve our understanding of carbon sinks in closed basins at various time scales, and provide a basis for the future mitigation policies to global climate change.

Evolution Of The Westerly Winds Belt In The Middle Latitudes Of The Southern Hemisphere Since The Last Glacial Maximum

<p><b>The Southern Westerly Winds (SWW) are a symmetric component of the global climate system that govern the modern climate of all Southern Hemisphere landmasses south of ~30°S. Changes in the strength and latitudinal position of the SWW influence the precipitation patterns in the southern mid-latitudes, and have been postulated as fundamental drivers of ocean-atmospheric CO2 exchange since the Last Glacial Maximum (LGM: ~34.0-18.0 ka). Despite their role in modern and past climatic dynamics, the evolution of the SWW at locations within their zone of influence is still uncertain; this is largely because of the paucity of paleoclimate records with well constrained chronology, adequate sampling resolution and an appropriate depositional setting. Resolving these issues will help understand the behaviour of the SWW in the past at different spatial (regional and hemisphere) and temporal (centennial to multi-millennial) scales. Here I present new paleoclimate data based on the examination of detailed chronologies of fossil pollen, charcoal and chironomids preserved in lake sediments from western Patagonia: Lago Emerenciana (43°S) and Lago Pintito (52°S) and New Zealand’s southwestern South Island: Lake Von (45°S). These data, spanning a broad range of the SWW zone of influence, provide insights into the role of shifting SWW in environmental and climate dynamics of the middle latitudes of the Southern Hemisphere spanning the last ~24,000 years.</b></p> <p>In the first study site, I performed detailed fossil pollen and charcoal analyses from sediment cores collected from Lago Emerenciana, a relatively small closed-basin lake located in northwestern Patagonian (43°S), to examine past vegetation, fire regime and climate change during the last ~24,000 years. I detect very low temperature and increased precipitation between ~24.0 and ~17.0 ka, followed by a warming trend and reduced precipitation between ~17.0 and ~14.3 ka. A cold reversal and increased precipitation regime occurred between ~14.3 and ~12.4 ka, followed by a return to warming and a slight decline in precipitation between ~12.4 and ~11.0 ka. I identify warmer temperatures and a major decline in precipitation at the beginning of the Holocene between ~11.0 and ~9.0 ka, conditions that persisted until ~6.2 ka. Centennial to millennial precipitation variability occurred during the last ~6200 years. </p> <p>In the second study site, I developed high resolution fossil pollen and charcoal records, along with an exploratory chironomid record from sediment cores obtained from Lake Von, a small closed-basin lake located in the southwestern sector of the South Island of New Zealand (45°S), to examine vegetation, fire and climate trends spanning the last ~18,000 years. I observe a trend toward warming and relatively dry conditions between ~18.0 and ~14.8 ka with relatively wet conditions between ~18.0 and ~16.7 ka, increased precipitation between ~16.7 and ~14.8 ka, and cooling conditions and enhanced precipitation between ~14.8 and ~12.8 ka, followed by a marked drop in precipitation between ~12.6 and ~11.2 ka. I detect warmer and diminished precipitation between ~10.8 and ~7.2 ka, followed by lower temperature and enhanced precipitation between ~7.2 and ~3.7 ka. The mid-late Holocene is also characterised by alternating dry and wet oscillations of millennial- and centennial-scale phases with low precipitation between ~6.0 and ~5.2, ~4.4 and ~4.1, ~3.7 and ~2.9, and ~1.9 and ~0.56 ka, and increased precipitation in the intervening intervals. In the third study site, I produced high resolution fossil pollen and charcoal records from sediment cores I collected from Lago Pintito, a small and shallow closed-basin lake located in southwestern Patagonia (52°S). This record allows the detection of past vegetation, fire and hydroclimatic shifts at millennial and centennial scales over the last ~17,000 years. From these data, I identify cold and dry conditions between ~17.0 and ~16.4 ka, increased precipitation between ~16.4 and ~14.2 ka and ~12.5 and ~11.4 ka, and intense precipitation but lower in magnitude than the neighbouring intervals between ~14.2 and~12.5 ka. I detect a major decline in precipitation at the beginning of the Holocene between ~11.4 and ~6.8 ka, followed by centennial-scale changes in precipitation until the present. </p> <p>The comparison between precipitation variability reconstructed from the records from western Patagonia (Lago Emerenciana and Lago Pintito) and New Zealand’s southwestern South Island (Lake Von) allows the inference of SWW changes at a hemispheric scale during and since the LGM, based on the premise that there is a strong and positive correlation between zonal wind speeds and local precipitation in these regions. The results of this thesis suggest: i) strong SWW influence at 43°S between ~24.0 and ~17.5 ka, ii) a southward shift of the SWW between ~17.5 and ~16.5 ka and reduced SWW influence north of 52°S, iii) strengthening and/or a northward shift of the SWW between ~16.5 and ~ 14.5 ka, with strong SWW influence between 52°S and 43°S, iv) a northward shift of the SWW between ~14.5 and ~12.6 ka which resulted in stronger SWW influence between 43°S and 46° S and weaker SWW influence at 52°S, v) a southward shift of the SWW between ~12.6 and ~11.2 ka leading to weaker SWW influence between 43°S and 46°S and stronger SWW influence at 52°S, vi) a generalized multi-millennial decline in the strength of the SWW between ~11.2 and ~7.2 ka, and vii) high variability in the SWW in Western Patagonian and New Zealand’s southwestern South Island during the last ~7200 years. Based on these findings, I postulate that hemisphere-wide changes in the position and/or strength of the SWW have modulated the atmospheric CO2 concentration through wind-driven upwelling of CO2-rich deep waters in the high southern latitudes during and since the LGM.</p>

Evolution Of The Westerly Winds Belt In The Middle Latitudes Of The Southern Hemisphere Since The Last Glacial Maximum

<p><b>The Southern Westerly Winds (SWW) are a symmetric component of the global climate system that govern the modern climate of all Southern Hemisphere landmasses south of ~30°S. Changes in the strength and latitudinal position of the SWW influence the precipitation patterns in the southern mid-latitudes, and have been postulated as fundamental drivers of ocean-atmospheric CO2 exchange since the Last Glacial Maximum (LGM: ~34.0-18.0 ka). Despite their role in modern and past climatic dynamics, the evolution of the SWW at locations within their zone of influence is still uncertain; this is largely because of the paucity of paleoclimate records with well constrained chronology, adequate sampling resolution and an appropriate depositional setting. Resolving these issues will help understand the behaviour of the SWW in the past at different spatial (regional and hemisphere) and temporal (centennial to multi-millennial) scales. Here I present new paleoclimate data based on the examination of detailed chronologies of fossil pollen, charcoal and chironomids preserved in lake sediments from western Patagonia: Lago Emerenciana (43°S) and Lago Pintito (52°S) and New Zealand’s southwestern South Island: Lake Von (45°S). These data, spanning a broad range of the SWW zone of influence, provide insights into the role of shifting SWW in environmental and climate dynamics of the middle latitudes of the Southern Hemisphere spanning the last ~24,000 years.</b></p> <p>In the first study site, I performed detailed fossil pollen and charcoal analyses from sediment cores collected from Lago Emerenciana, a relatively small closed-basin lake located in northwestern Patagonian (43°S), to examine past vegetation, fire regime and climate change during the last ~24,000 years. I detect very low temperature and increased precipitation between ~24.0 and ~17.0 ka, followed by a warming trend and reduced precipitation between ~17.0 and ~14.3 ka. A cold reversal and increased precipitation regime occurred between ~14.3 and ~12.4 ka, followed by a return to warming and a slight decline in precipitation between ~12.4 and ~11.0 ka. I identify warmer temperatures and a major decline in precipitation at the beginning of the Holocene between ~11.0 and ~9.0 ka, conditions that persisted until ~6.2 ka. Centennial to millennial precipitation variability occurred during the last ~6200 years. </p> <p>In the second study site, I developed high resolution fossil pollen and charcoal records, along with an exploratory chironomid record from sediment cores obtained from Lake Von, a small closed-basin lake located in the southwestern sector of the South Island of New Zealand (45°S), to examine vegetation, fire and climate trends spanning the last ~18,000 years. I observe a trend toward warming and relatively dry conditions between ~18.0 and ~14.8 ka with relatively wet conditions between ~18.0 and ~16.7 ka, increased precipitation between ~16.7 and ~14.8 ka, and cooling conditions and enhanced precipitation between ~14.8 and ~12.8 ka, followed by a marked drop in precipitation between ~12.6 and ~11.2 ka. I detect warmer and diminished precipitation between ~10.8 and ~7.2 ka, followed by lower temperature and enhanced precipitation between ~7.2 and ~3.7 ka. The mid-late Holocene is also characterised by alternating dry and wet oscillations of millennial- and centennial-scale phases with low precipitation between ~6.0 and ~5.2, ~4.4 and ~4.1, ~3.7 and ~2.9, and ~1.9 and ~0.56 ka, and increased precipitation in the intervening intervals. In the third study site, I produced high resolution fossil pollen and charcoal records from sediment cores I collected from Lago Pintito, a small and shallow closed-basin lake located in southwestern Patagonia (52°S). This record allows the detection of past vegetation, fire and hydroclimatic shifts at millennial and centennial scales over the last ~17,000 years. From these data, I identify cold and dry conditions between ~17.0 and ~16.4 ka, increased precipitation between ~16.4 and ~14.2 ka and ~12.5 and ~11.4 ka, and intense precipitation but lower in magnitude than the neighbouring intervals between ~14.2 and~12.5 ka. I detect a major decline in precipitation at the beginning of the Holocene between ~11.4 and ~6.8 ka, followed by centennial-scale changes in precipitation until the present. </p> <p>The comparison between precipitation variability reconstructed from the records from western Patagonia (Lago Emerenciana and Lago Pintito) and New Zealand’s southwestern South Island (Lake Von) allows the inference of SWW changes at a hemispheric scale during and since the LGM, based on the premise that there is a strong and positive correlation between zonal wind speeds and local precipitation in these regions. The results of this thesis suggest: i) strong SWW influence at 43°S between ~24.0 and ~17.5 ka, ii) a southward shift of the SWW between ~17.5 and ~16.5 ka and reduced SWW influence north of 52°S, iii) strengthening and/or a northward shift of the SWW between ~16.5 and ~ 14.5 ka, with strong SWW influence between 52°S and 43°S, iv) a northward shift of the SWW between ~14.5 and ~12.6 ka which resulted in stronger SWW influence between 43°S and 46° S and weaker SWW influence at 52°S, v) a southward shift of the SWW between ~12.6 and ~11.2 ka leading to weaker SWW influence between 43°S and 46°S and stronger SWW influence at 52°S, vi) a generalized multi-millennial decline in the strength of the SWW between ~11.2 and ~7.2 ka, and vii) high variability in the SWW in Western Patagonian and New Zealand’s southwestern South Island during the last ~7200 years. Based on these findings, I postulate that hemisphere-wide changes in the position and/or strength of the SWW have modulated the atmospheric CO2 concentration through wind-driven upwelling of CO2-rich deep waters in the high southern latitudes during and since the LGM.</p>

Holocene Lake Evolution and Glacial Fluctuations Indicated by Carbonate Minerals and Their Isotopic Compositions in the Sediments of a Glacial Melt Recharge Lake on the Northwestern Tibetan Plateau

Lakes and glaciers are widely distributed on the Tibetan Plateau and are linked via hydrological processes. They are experiencing rapid changes due to global warming, but their relationships during the Holocene are less well known due to limited coupled geological records. Here, we analyzed the δ13C-VPDB and δ18O-VPDB values and ion content of calcite and aragonite in a 407-cm-long sediment core from Guozha Co, a closed basin on the northwestern Tibetan Plateau supplied by glacial meltwater, in order to understand how the lake responded to glacier changes during the Holocene. Our results indicate that the glacial meltwater lowered the lake’s temperature and the δ18Olake water and δ18Oendogenic + authigenic carbonate values and diluted the ion concentrations in the lake water. Three stages of evolution, 8.7–4.0, 4.0–1.5, and 1.5 kyr BP to present, are distinguished based on the decrease in glacial meltwater recharge. Guozha Co has been a closed basin since at least 8.7 kyr BP, and it has changed from a fresh water lake during 8.7–1.5 kyr BP to a brackish lake from 1.5 kyr BP to present due to several climate events. The famous 4.2 kyr BP cold event was identified in the core at 4.0 kyr BP, while warm events occurred at 6.2, 3.9, 2.2, 0.9, and 0.4 kyr BP. Both glaciers and lakes in this area are controlled by climate, but they exhibit opposite changes, that is, glaciers retreat and lakes expand, and vice versa. Our results provide an accurate interpretation of the cold events based on carbonate minerals and carbon–oxygen isotopes in glacial meltwater–recharged lake sediments.

Evidence for high-elevation salar recharge and interbasin groundwater flow in the Western Cordillera of the Peruvian Andes

Improving our understanding of hydrogeological processes on the western flank of the central Andes is critical to communities living in this arid region. Groundwater emerging as springs at low elevations provides water for drinking, agriculture, and baseflow. Some springs also have recreation or religious significance. However, the high elevation sources of recharge and specific groundwater flowpaths that support these springs and convey groundwater to lower elevations in southern Peru remain poorly quantified in this geologically complex environment. The objectives of this study were to identify recharge zones and groundwater flowpaths supporting natural springs east of the city of Arequipa in the volcanic mountain terrain, particularly, the potential for recharge within the high-elevation closed-basin Lagunas Salinas salar. We used geochemical and isotopic tracers in springs, surface waters (rivers and lakes), and precipitation (rain and snow) sampled from March 2019 through February 2020. We obtained monthly samples from six springs, bimonthly samples from four rivers, and various samples from high-elevation springs during the dry season. We analyzed stable water isotopes (δ18O and δ2H) and general chemistry of springs, rivers, local rainfall, and snow from Pichu Pichu volcano. The monthly isotopic composition of spring water was invariable over time, suggesting that the springs receive a stable source of groundwater recharge and are not supported by relatively short groundwater flowpaths. The chemistry of springs in the low- and mid-elevations (2500 to 2900 masl) point towards a mix of recharge from the salar (4300 masl) and mountain-block recharge (MBR) in or above a queñuales forest ecosystem at ~4000 masl on the adjacent Pichu Pichu volcano. Springs at higher elevation closer to the salar and in a region with a high degree of faulting had higher chloride concentrations indicating higher proportions of interbasin groundwater flow from the salar. We conclude that while the salar is a closed basin, surface water from the salar recharges through the lacustrine sediments, mixes with mountain-block groundwater, and is incorporated into the regional groundwater flow system. Groundwater flow in the mountain block and the subsequent interbasin groundwater flow is accommodated through extensive faulting and fracturing. Our findings provide valuable information on the flowpaths and zones of recharge that support low-elevation springs in this arid region. In this study, high-elevation forests and a closed-basin salar are important sources of recharge. These features should be carefully managed to prevent impacts to the down-valley springs and streams.