Current Research Projects:

  • Evolution of Late-Quaternary Tropical Climate including the El Niño-Southern Oscillation, Indian Monsoon,  South American Monsoon and West African Aridity
  • Paleoaltimetry of the Tibetan Plateau
  • Frictional heating on faults evaluated with the thermal alteration of organic molecules
  • Vegetation and hydroclimate reconstruction from paleosols
  • Neogene expansion of C4 grasslands in India, Africa and North America
  • Biological and environmental controls on the hydrogen isotopic composition of organic molecules
  • picoCSIA - New instrumentation for molecular isotopic analyses on picomolar quantitities of carbon
  • Alkenone-based records of atmospheric CO2 concentrations

Evolution of Late-Quaternary Tropical Climate
The tropical atmospheric overturning circulation determines the distribution of rainfall for a large fraction of the human population.  Interannual, decadal and centennial variations in this circulation can have profound consequences for agriculture, water resources and habitat, yet the instrumental record is too short to evaluate most such variations.  Three research projects address this theme.

Tropical Pacific Ocean.  The dominant mode of interannual climate variability in the earth system is the coupled ocean-atmosphere interaction in the tropical Pacific (the El Niño-Southern Oscillation, ENSO).  Variations in ENSO affect the distribution of precipitation and extreme events around the globe yet the sensitivity of ENSO to future climate change remains highly uncertain.  I was co-chief scientist on a May, 2012 NSF-funded research cruise to the central equatorial Pacific to collect new sediment cores for studying the Holocene and Pleistocene evolution of ENSO variability.  I am using the intrasample distribution of paired oxygen isotope and Mg/Ca values from individual planktonic foraminifera to document changes in the variability of the surface ocean through time.  Individual foraminifera provide a ‘snapshot’ of the ocean over their 2-4 week life; the distribution of these ‘snapshots’ reflects the variability of the ocean over the time a sediment sample was deposited.  Our preliminary data demonstrate reduced ENSO but enhanced seasonal variability during the last glacial maximum (Ford, Polissar and Ravelo, 2015) and a funded NSF proposal is supporting continued work on this project with post-doc Gerald Rustic.  Published papers and reports describe sedimentation patterns (Lyle et al., 2016), new sediment cores (Lynch-Stieglitz et al., 2015), seawater oxygen isotopes (Conroy et al., 2014) and the Line Islands cruise (MGL-1208, cruise report) to the central Pacific Ocean.

South America.  I have several projects studying terrestrial climate in South America to understand the evolution, spatial pattern and causes of climate change (Polissar et al., 2006a, 2006b, 2013; Stansell et al., 2005, 2007). This work complements my investigations into the history of ENSO and provides a perspective on the atmospheric teleconnections to the tropical Pacific.  A significant recent finding has been synchronous evolution of Holocene climate in the northern and southern hemispheres that is likely linked to evolution of the equatorial Pacific Ocean rather than local insolation forcing as previously thought (Polissar et al., 2013).  New work in Peru, Colombia and Venezuela seeks to understand the spatial patterns and mechanisms of climate change.  The primary goal is to distinguish the mechanism of precessional forcing of South American climate through influences from the tropical Pacific, Atlantic and Caribbean Oceans. 

Indian Summer Monsoon (Tibet).  The Indian Summer Monsoon (ISM) feeds rivers and glaciers that sustain significant human populations.  Failure of a monsoon season can be devastating for these populations, highlighting their potential vulnerability to global warming.  I have begun a collaborative project with Broxton Bird (IUPUI) to better understand the sensitivity of hydroclimate on Earth’s “3rd Pole” to climate forcing.  We are using sedimentological and molecular hydrogen isotope measurements on Tibetan lake sediments to examine decadal-centennial modulation of the Indian Summer Monsoon over the Holocene. Preliminary results suggest a profound shift in ISM behavior during the middle Holocene (Bird et al., 2014) and additional changes during the Little Ice Age, potentially forced from Indo-Pacific sea surface temperatures.    A recently-funded NSF project will allow us to continue this research, expanding the preliminary dataset to include multiple lakes from different regions analyed at higher resolution.

West African Aridity.  The Sahel region of North Africa experienced dramatic swings in precipitation patterns over the latter half of the 20th century, with devastating impacts on food security during the droughts of the 1970s and 1980s. Looking ahead, projections of future climate change in the Sahel show little agreement, with some climate models predicting significantly wetter conditions in the 21st century and others suggesting drying, leaving great uncertainty as to the impacts of climate change on this sensitive region. This project aims to improve understanding of controls on North African climate by reconstructing regional climate changes over the last 1 million years using deep-sea sediments located off the west African coast. This long-term sampling of climate variability will reveal the region’s response during multiple ice age cycles, including periods when the high latitudes of the Northern Hemisphere were substantially warmer and colder than at present and periods when North Africa was much wetter and drier than at present. By systematically documenting North Africa’s response during this range of climatic conditions, these results will provide important new opportunities to test climate models’ ability to accurately represent past climate variability in the region. The project will employ recently developed tools to reconstruct windblown dust emissions from the Sahara desert (a tracer of winds and aridity), the carbon isotopic composition of fossil leaf molecules in the sediment (a tracer of vegetation on the continent) and the hydrogen isotopic composition of the same leaf molecules (a tracer of monsoon strength.) This dataset will enable a better understanding of what has driven climate change in the past, and what this will mean for future change in North Africa. The project will also include public outreach projects to offer education about the impact of climate change in North Africa.  The project is a collaboration between David McGee (MIT), Gisela Winckler (LDEO) and Pratigya Polissar (LDEO).

Paleoaltimetry of the Tibetan Plateau
The relationships of climate, erosion and tectonics to the Cenozoic uplift of the Himalaya are topics of intense interest.  Fundamental to this debate is the timing of climate and vegetation changes in this region and their links to uplift of the Tibetan plateau.  I developed a new approach to paleoaltimetry using the molecular hydrogen isotope signatures preserved in Eocene-Recent sediments on the Tibetan plateau.  Results to date suggest the presence of an extended region of high elevation as early as the late Eocene (Polissar et al., 2009; Currie et al., in press) and apparent northward growth of the plateau since then.  Current efforts are focused on understanding the Paleocene-Eocene elevation history and investigating the possibility for a decrease in elevation since the Miocene.  These data offer insight into the history of the Asian monsoon and the influence of Himalayan tectonics on climate.

Frictional heating on faults evaluated with the thermal alteration of organic molecules

How much heat is generated during earthquakes?  We expect to see a thermal signature from earthquakes because of the energy that presumably goes into frictional heating.  If the amount of heating on a fault can be measured, the energy budget during earthquakes could be constrained.

Heather Savage (LDEO) and I developed a new measurement for detecting frictional heating on faults using the thermal maturity of extractable organic molecules (Polissar et al., 2009; Savage et al., 2014).  We have verified that this measurement is sensitive to frictional heating through studies of faults that contain pseudotachlyte (frictional melt) and thus were unequivocally heated during an earthquake (Savage et al., 2014).  We are funded by NSF to develop a laboratory apparatus for short, high-temperature hydrous pyrolysis experiments to measure the kinetics of biomarker reactions at fault-relevant timescales (Sheppard et al., 2015).  Our results have constrained the maximum earthquake and possible ranges of fault width and friction on an ancient strand of the San Andreas Fault (Polissar et al., 2009; Sheppard et al., 2015), the energy budget for a large subduction zone paleoearthquake (Savage et al., 2014) and the presence of multiple earthquake-hosting strands in the shallow wedge sediments ruptured during the tsunami-generating 2011 Tohoku earthquake (funded by NSF, Rabinowitz et al., 2015).  Hannah Rabinowitz (Ph.D. student) is working on temperature estimates from the Tohoku earthquake and incoming student Genevieve Coffey (Ph.D. student) is working on samples from the San Andreas Fault Observatory at Depth (SAFOD) to investigate the seismic potential of the creeping section of the San Andreas Fault.

Vegetation and Hydroclimate Reconstruction from Paleosols
Unlike most other depositional environments, fossil soils (paleosols) are ‘in-place’ on the landscape and provide a picture of the mosaic of real-world ecosystems.  This perspective complements long-lived depositional environments such as lake basins that have continuous accumulation but integrate material from the surrounding watershed and within the lake.  Two current projects in collaboration with post-doc Kevin Uno are investigating the preservation of organic geochemical signatures for vegetation, fire and hydroclimate in paleosols and applying them to the late Cenozoic evolution of East Africa and the central U.S.  A recently funded NSF project will allow us to investigate the Plio-Pleistocene climate history of the central Great Plains with a highly collaborative group of paleontologists, geochemists, paleopedologists and global climate modelers.    

Neogene expansion of C4 grasslands in India, Africa and North America
Plants with the C4 photosystem such as tropical warm-season grasses have competitive advantages over plants with the C3 photosystem such as trees and shrubs under low atmospheric carbon dioxide concentrations, high growing-season temperatures and water stress.  Atmospheric carbon dioxide likely reached a low level that favored C4 photosystems around the Eocene-Oligocene boundary (34 Ma ago).  However, C4 ecosystems did not expand significantly until later in the Cenozoic and this expansion did not occur at the same time everywhere.  It therefore appears that regional factors ultimately determined when and where C4 ecosystems expand.  These factors could include changing aridity and seasonality of rainfall, and disturbances such as grazing and fire.  Working with post-doc Kevin Uno, Ph.D. student Sam Phelps, Professor Peter deMenocal and former graduate student Caussandra Rose we are investigating the establishment of C4 ecosystems in India, Africa and North America.  Investigations include the molecular isotopic record of C4 ecosystems, hydroclimate changes reconstructed from molecular hydrogen isotope ratios and the molecular record of fire.  These investigations include studies of paleosol sequences and deep-sea sediment cores (Uno et al., 2016a; Uno et al., 2016b; Rose et al., 2016).

 picoCSIA - New instrumentation for molecular isotopic analyses on picomolar quantitities of carbon
My research continues to focus on new analytical approaches that provide unique perspectives on the geologic record.  I am currently funded by NASA to reduce the analytical limits for compound-specific carbon isotope analysis 100-fold to the picomolar level.  Advances from this “pico-CSIA” project in collaboration with Kate Freeman (Penn State), Roger Summons (MIT) and Thermo Scientific will allow detection of isotopic biosignatures for early life on Earth as well as routine analysis of new geologic archives. Advances from this project should be transferrable to molecular D/H measurements and I also have funding to explore this application.

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