This EAGER grant investigates Indian summer monsoon dynamics during the last 1500 years using lake sediments from the Tibetan Plateau. Specifically, they examine if and how radiative variability impacted the monsoon during the following target events: the Medieval Climate Anomaly (AD 900 to 1300), Little Ice Age (AD 1400 to 1800), and current warm period. This research targets small alpine (~400 m diameter) lakes in the Nyainqentanglha Mountains, which are located at the southeastern edge of the Tibetan Plateau. This region is ideal because monsoon moisture is transported over the Nyainqentanglha Mountains from the Bay of Bengal before entering the Tibetan Plateau. As such, the hydrologic balance of lakes in this region should be highly sensitive to changes in the strength of the monsoon. This project provides valuable information about monsoon variability that can be used to explore how and why this system changes through time. The results of this study will aid efforts to understand how the monsoon, which sustains sensitive ecosystems and over 1 billion people, may respond to future warming trends driven by anthropogenic increases in radiative forcing from greenhouse gases (i.e. CO2). The analytical efforts will be focused on the highest quality sediment, and reconstructs monsoon rainfall using oxygen isotopes (d18O) measured on authigenic or biogenic CaCO3 where possible, or geochemical indicators of erosion and sediment flux calculations if the bedrock geology precludes the preservation of CaCO3 in lake sediments. The results will be synthesized with ice core and tree ring records from the Tibetan Plateau and Himalaya and speleothem records from western China in order to a develop regional perspective monsoon variability and assess how it compares with East Asian monsoon variability during the targeted events. Despite the potential for significant discovery, this research faces many field-based and analytical challenges that make it high risk. Most importantly, age models of sufficient temporal resolution that are needed to make detailed statements about monsoon variability are notoriously difficult in this region. However, the principal investigators have considerable experience working with similar lake systems in comparable environments in the high Andes of tropical South America. From this work, they have successfully developed techniques sediment dating.
Intellectual Merit of the Proposed Activity This research directly addresses the P2C2 research objectives to understand the response of monsoon systems to climate change and to determine the sensitivity of these systems to abrupt changes in radiative forcing, particularly during past warm intervals. This research provides sub-decadally resolved records of monsoon variability that allow for a detailed analysis of how this monsoon system responded to changes in radiative forcing during the past 1500 years, which will improve forecasts of monsoon variability in response to warming trends.
Broader Impacts of the Proposed Activity This research will benefit the more than 1 billion people who depend on the monsoon for their livelihood. The information about monsoon dynamics and their relationship with radiative forcing will be beneficial for forecasting and preparing for future variability in this system as a result of rising global temperatures from anthropogenic increases in greenhouse gases. With an improved understanding of monsoon variability, effective water management strategies can be developed to cope with potential future water shortages and/or increased monsoonal variability.
Outcomes and Findings This work focused on developing new Holocene length records of Indian summer monsoon (ISM) variability from the southeastern Tibetan Plateau, a region that is a major center of action for this central component of the global climate system, but from which there are few decadally resolved paleoclimate records. Fieldwork was conducted in April and May of 2011. During this expedition, sediment cores were retrieved from 6 lakes on the southeastern Tibetan Plateau. Paleoclimate records have been developed from two of these lakes, specifically Paru Co and Badi Nam Co. The Paru Co record spans the last 11,000 years and provides a detailed history of changes in the ISM system at millennial to multi-decadal timescales. Century-scale hydrogen isotopes measured on organic n-alkanes (δDwax) provide a synoptic view of ISM variability while decadally resolved grain size measurements provide a multi-decadal view of local ISM rainfall and lake level changes over the Paru Co watershed. Together, these results provide a picture of how the ISM changed during the last 10,000 years. When compared to existing paleoclimate records of terrestrial climate changes from the Tibetan Plateau and ocean-atmosphere variability in the Indo-Pacific region, our work provides new insights into the spatiotemporal structure of ISM variability during the Holocene and the potential mechanisms that were responsible for this variability. Grain size and δDwax were also measured on the Badi Nam Co core, and while results are still being processed, they initially appear to support the Paru Co results. The major findings of this work are as follows: 1) Reliable age models not requiring reservoir effect corrections can be established for lake sediment archives from small alpine lakes on the southeastern Tibetan Plateau. This finding is important because many paleoclimate investigations using lake sediments from the Tibetan Plateau have relied on age models that required large corrections for "reservoir effects" that were assumed to have remained constant throughout the Holocene. As a result, a larger degree of confidence can be place on the temporal significance of the findings based on the Paru Co and Badi Nam Co records. 2) There is a strong link between orbital forcing and the strength of the ISM where increased insolation during the early Holocene contributed to an enhanced ISM and decreasing insolation weakened the ISM during the middle and late Holocene. The influence of insolation on the ISM appears to have been through its affect on the position of the Intertropical Convergence Zone whereby stronger insolation in the Northern Hemisphere during the early Holocene results in a more northerly position of the ITCZ and hence a strengthened ISM. Reductions in Northern Hemisphere insolation and concomitant strengthening of insolation in the Southern Hemisphere lead to a more southerly position of the ITCZ and thus weakened the ISM. 3) Superimposed on these millennial scale trends are centennial scale pluvial (wet) and drought (dry) phases, particularly during the early Holocene. These variations appear to be in phase with sea surface temperature reconstructions from the Indo-Pacific such that warmer ocean temperatures coincide with enhanced rainfall and vice versa. Also noted is the occurrence of a century scale lake high-stand at Paru Co during the middle Holocene that coincides with the 5.3 ka climate event. Because this event registers as a global cooling in many other regions of the world, we suggest that enhanced winter snowfall and cooler summers (resulting in lower evaporation from Paru Co) may account for this event. Intellectual Merit This research significantly advanced our knowledge of a key component of the global climate system by developing well-dated records of ISM rainfall that (i) document past temporal and spatial variability in this system at decadal timescales, (ii) document the rates of change associated with Holocene ISM variability, (iii) help determine the sensitivity of the ISM system to long-term and abrupt climate forcing mechanisms and (iv) provide quantitative estimates of Holocene ISM variability, including the isotopic composition of precipitation, that can be used to test predictions from numerical climate models. Broader Impacts The improved understanding of ISM variability that results from this research is directly relevant to the ~20% of the world’s population that depend on it for their way of life. This research also provides policy makers with a scientific foundation upon which to formulate policy and take action to mitigate and/or prevent adverse consequences from future climate change. Furthermore, our results benefit the broader scientific community, including biologists and ecologists investigating Tibetan ecosystem evolution and social scientists exploring climate-society relationships. This research provided postdoctoral training for Dr. Bird, an early career scientist, and research training for three undergraduate students at Indiana University-Purdue University, Indianapolis, a diverse urban campus in Indianapolis, Indiana.This research also contributed to scientific collaboration with China through our continuing partnership with the Institute of Tibetan Plateau Research of the Chinese Academy of Science.
