Our efforts were put towards reconstructing the patterns of the Holocene Indian summer monsoon variability with decadally-resolved Tibetan lake sediments.
The ISM is a central feature of the Earth’s climate system that directly impacts over 1 billion people. Little is known about its log-term variability at human timescales, however, because few high-resolution terrestrial records of Holocene ISM rainfall are available from key centers of action. This work will develop four such records in order to (i) place modern ISM variability into a historical context, (ii) test hypotheses for the climate mechanisms controlling the ISM and (iii) provide data for validation of global climate models. By combining proxies that are sensitive to local and synoptic precipitation we will investigate ISM variability at multiple spatial and temporal scales, thereby providing new insight into how this system responds to abrupt and long-term changes in climate boundary conditions. Ultimately, our objective is to provide a paleoclimate context for ISM variability and empirical inputs for climate models that will allow for better predictions of future changes. Such information is essential for mitigating and/or adapting to ISM rainfall variability under global warming.
In collaboration with colleagues from the Institute of Tibetan Plateau Research -Chinese Academy of Sciences, we are investigating Indian summer monsoon (ISM) variability during the Holocene with decadally resolved sediment records from four alpine lakes on the southeastern Tibetan Plateau. This research builds on our previous NSF-supported work in the Nyainqentanglha Mountains, which are a major center of action in the ISM system and a primary gateway for moisture entering the Tibetan Plateau. With this work, we will reconstruct local ISM variability with physical sedimentology and geochemistry and synoptic variability with hydrogen isotope measurements on terrestrial leaf waxes. The results will be synthesized with paleoclimate records from the Tibetan Plateau and surrounding region, as well as with records from key regions in the global climate system to address central questions about Holocene ISM variability.
We are exploring the lake records of South American Summer Monsoon Variability from the Colombian Andes to better understand them.
Our ongoing research is investigating South American summer monsoon (SASM) variability during the last 10,000 years at decadal timescales using a series of alpine lake sediment archives collected along a north-south transect in the Colombian Andes between ~7.5º and 5.5ºN. Despite its vital importance for more than 350 million people, the long-term behavior of the SASM on human timescales is not well understood. This is especially true for the Northern Hemisphere component of the SASM where there are no decadally resolved terrestrial paleoclimate records.
Through collaboration with Dr. Escobar of the Universidad del Norte, this work will begin to address this gap in knowledge. Ultimately, this research will provide a paleoclimate context for the SASM mean state and its variability that will allow better predictions of future changes and help guide policy makers when formulating policy to take action to mitigate and/or prevent adverse consequences from future climate change.
Methodologically, this research pairs measurements of sedimentological paleoclimate indicators, including grain size, magnetic susceptibility, and organic matter content with measurements of leaf wax hydrogen isotopes and calcite oxygen isotopes. By combining these methods, this work will investigate local and large-scale changes in SASM precipitation over long periods of time at societally relevant timescales. This is important because it is difficult to distinguish between Andean and Amazonian SASM variability based on isotopic studies alone.
This investigation is testing three hypotheses: (i) Decadal to century-scale changes in SASM precipitation are synchronous between the Northern and Southern Hemisphere Andes. (ii) Variations in Holocene SASM precipitation were similar over the Amazon and Andes, although of greater magnitude over the Andes. (iii) Changes in the location of the Atlantic branch of the Intertropical Convergence Zone (ITCZ) are the primary driver of abrupt SASM variability at decadal to century timescales.
Quaternary SASM Variability at Laguna de Tota
With support from the NSF, an international team of US and Colombian geoscientists will characterize the bathymetry and sedimentary and tectonic structure of Laguna de Tota, Colombia.
The Tota survey will provide empirical constraints on the thickness of sediment sequences accumulated in Tota while helping to identify the lake’s depositional center(s) and the presence or absence of lake-level transgression-regression sequences. The data generated by this work will offer a new perspective of the structure and deposits contained in Colombian highland basins.
Like Lake Titicaca, these data may provide important paleoclimatic information about the northern Andean tropics in and of themselves through the identification of sedimentary structures that are indicative of past lake-level changes at Tota. The geophysical data are also an essential component of our ultimate goal to developing a Quaternary paleoclimate record from Tota’s sedimentary archive.
Ultimately a set of long, continuous sediment cores from Tota has the potential to help illuminate environmental and monsoon variability in the northern South American tropics, thereby allowing a more interhemispheric view of the South American climate, likely over multiple Pleistocene glacial-interglacial cycles. Such information will help refine climate models that simulate tropical South America’s climatic response to continued warming trends.
The Holocene Climate of the Central Peruvian Andes Inferred from High-Resolution Lacustrine Sedimentary, Stable Isotope, and Geochemical Indicators is the major study of our research in Peru.
Part of Dr. Bird’s doctoral research examined mean state changes in the South American summer monsoon (SASM) using physical sedimentology and oxygen isotope measurements of authigenic sedimentary calcite from Laguna Pumacocha, a varved alpine lake in the central Peruvian Andes. With this unique sediment record, he reconstructed SASM variability at annual and decadal resolution for the last 2300 years and 11,200 years, respectively.
Future research is expanding the existing network of high-resolution lake records of Holocene and Glacial climate to include other regions of Peru and Colombia. This work will integrate modeling techniques for constraining precipitation variability with isotopic records from closed and open lake systems and new methods to reconstruct air temperature with compound specific biomarkers. We are also developing geochemical records of metal pollution (e.g., 208Pb, 207Pb, Pb, Sb, As) from the Andes to investigate historic and prehistoric metallurgical practices.
This work showed that long-term and abrupt mean state changes in SASM rainfall during the MCA, LIA, and CWP were intimately linked to Intertropical Convergence Zone variability as mediated by Western Hemisphere tropical sea surface temperatures, particularly in the Atlantic. During the late Glacial and Holocene, millennial-scale strengthening of the SASM tacked increasing Southern Hemisphere summer insolation. Abrupt changes in SASM rainfall were largely contemporaneous between the Andes and Amazon, although with a greater magnitude over the Andes.