Geologic deposits in the world's lakes and oceans preserve unique records of past climate and ecosystem conditions. We pursue integrative approaches to investigating sedimentary deposits, including studies of modern environmental processes, the development of new techniques for decoding paleoenvironmental signals in lake and ocean sediments, and the generation and interpretation of new paleoclimate and paleoenvironmental datasets from key terrestrial and marine (Mesozoic and Cenozoic Oceans) archives.
The Terrestrial Paleoenvironments group, within the the Earth Systems History area, investigates signals in lake sediments to reconstruct global climate and environmental change. These include seasonal to orbitally-driven climate changes, annual- to millennial-scale climate variability, and massive environmental perturbations in the geologic record. We pursue an integrative approach that stresses understanding the geology, chemisty, physics, and biology of modern lake systems, and applying this knowledge to interpreting paleodata from lacustrine sediments and sedimentary rocks. We employ a huge array of analytical techniques including organic and inorganic geochemical methods, sedimentology, stable isotopes, and microfossils. Members of the terrestrial paleoenvironments group are investigating lakes and lake basins in North America, Europe, Africa, Asia, and Indonesia.
- Paleoceanography of Mesozoic and Cenozoic Oceans
- Millennial-Scale Climate Variability
- ENSO History and Dynamics
- High-Frequency Climate Dynamics
- North American Paleoclimatology
- Tropical Mountain Climates and Glacier Dynamics
Paleoenvironments and Ecosystem Dynamics:
- Continental Ecosystem Dynamics
- North American Vegetation History
- Mesozoic and Cenozoic Oceans
- Molecular Isotopic Proxy Development
- Estuarine and Environmental Processes and the Insomniacs
- Modern and Paleoecology of Plankton
- Stable Isotope Hydrology
We seek to understand how orbital forcing influences the land, ocean, and atmosphere in times of changing and minimal ice volume. We also seek to identify and understand the non-stationary responses of the Earth’s climate system (i.e., the change or decoupling of responses relative to each other and the orbital forcing over time). We do so by studying biotic, sedimentological, and geochemical variations at the scale of ancient orbital cycles, and by developing our understanding of which climate mechanisms could be responsible for producing cyclic sedimentation.
Our understanding of the role orbital forcing plays in Plio-Pleistocene climate change focuses on the roles of solar radiation and ice. Changes in ice volume provide most of the signal in d18O curves, provide one clear positive feedback loop (ice-albedo feedback), and explain repetitive stratigraphic packages of onlap and offlap. However, for much of the Cenozoic and Mesozoic, ice was confined to either the Antarctic region, or not present in any large amounts anywhere on the globe. Likewise, large changes in tropical climates reflect changes in temperature, precipitation, and winds rather than the direct effects of ice. Our department maintains a strong research interest in the history of orbital forcing of climate both before and after the development of large northern hemisphere glaciation (Professors Herbert, Matthews, and Prell). We seek to extend the orbital theory of climate to warmer worlds of the past, to study both stratigraphic and paleoclimatic applications of the orbital model. The implications of orbital forcing are integrated into many of our other research themes.
Stratigraphic applications of the orbital model include improving the Geomagnetic Polarity Timescale by tuning magnetozones and/or biozones with the orbital chronometer, and developing quantitative models of cyclic sedimentary fluxes that match geological observations. Much of our research effort requires that we develop techniques to rapidly acquire time series data, and statistical approaches to interpreting time series. Successful examples include the use of optical densitometry on color slides, of visible reflectance on core surfaces, and a new application of infrared spectroscopy to core surfaces.
Orbital forcing has played an important role in climate changes even during the relatively short time-period of the Holocene. For instance, precessional forcing of the African monsoon drove massive changes in continental hydrology, including the well-known "greening of the Sahara." This wet interval in northern and equatorial Africa during the early Holocene, known as the African Humid Period, varies in its amplitude, rates of change, and timing in different parts of the African continent due to regional land-surface and sea-surface temperature feedbacks. Professor Russell is working at several sites across the African continent to examine how feedbacks from vegetation and SSTs affected the rate and timing of the demise of the African Humid Period, and how these dynamics may differ from monsoonal systems in South America and Asia.
Monsoon circulation is a primary component of the tropical and global climate system and our ability to understand and model its short and long-term variability and evolution is of interest to both climate modelers and paleoclimatologists. The modern summer monsoon is driven by two primary heat sources: sensible heating of the Asian land mass and condensational (latent) heating within the troposphere over the Asian Plateau. Latent heat from moisture collected over the southern subtropical Indian Ocean istransported across the equator and released during precipitation over Asia and Africa. Both sensible and latent heat mechanisms contribute to the land-sea temperature and pressure differences that ultimately drive summer monsoon circulation.
Brown faculty investigate marine and lacustrine sediments that record monsoon variability on annual to million-year time-scales. Since modern interannual insolation is relatively constant, interannual variability in monsoon strength derives mainly from changes in the latent heat source or other internal feedbacks. However, over orbital time scales, insolation gradients change on the order of ±12%. Monsoon variability at the orbital time scale is thus linked to changes in both the sensible and latent heat sources as well as internal feedback processes which impact them [Clemens and Prell, 1991b; Clemens et al., 1991; Clemens et al., 1996].
At longer timescales, monsoon strength is sensitive to the Plio-Pleistocene evolution of the northern hemisphere ice sheets as well as tectonic development of the Asian Plateau, two internal climate factors which may be causally linked as suggested by the climate-uplift hypothesis. For the Plio-Pleistocene, monsoon sensitivity to orbital insolation and the long-term evolution of glacial boundary conditions yields a nonstationary phase response over the interval 0 to 3.5 Ma [Clemens et al., 1996]. We anticipate that similar non-stationary responses will occur in the Neogene due to sensitivity of the monsoon to internal feedback processes involving large-scale uplift, CO2, and vegetation change in Africa and Southern Asia [Kutzbach et al., 1997; Prell and Kutzbach, 1997].
Our paleoceanographic research has involved a series of integrated field and lab projects designed to identify and understand the evolution of the Asian Monsoon over seasonal to tectonic time scales. We have initiated or served as co-chief scientist on a number of monsoon-related expeditions, including: 1) Cruise RC27-04 to the Oman Margin to identify ODP drilling targets and collect cores for study of late Quaternary monsoon dynamics; 2) ODP Leg 117 coring on the Oman Margin for study of Neogene-scale monsoon dynamics and evolution; 3) JGOFS Arabian Sea Process Study (Cruises TN041 and TN047) to acquire sediment cores and sediment trap samples across the monsoon upwelling productivity zone, and 4) ODP Leg 184 to the South China Sea to core sediments to compare the variability and evolution of the East Asian Monsoon with the Arabian Sea Monsoon. These studies will range from sub-Milankovitch to Neogene-scale in scope. The combined results of these studies represent a progression from documentation of the stratigraphic integrity of recovered sections and the reliability of proxy climate indicators, to detailed interpretation of climate change and data/model comparisons based on a multiple high-resolution climate records over a variety of time scales.
One current project is focused on the South China Sea and how its monsoonal response compares on orbital and tectonic timescales to the Arabian Sea Indian Monsoon history. This work will emphasize development and analysis of multiple independent tracers of monsoon strength and regional runoff and aridity. Because all monsoon-strength proxies have some drawbacks, we emphasize the analysis and comparison of multiple indicators from continuous, high-resolution records to reliably identify monsoon responses. Our ongoing and anticipated projects will cover the Seasonal and Interannual timescales, utilizing our JGOFS sediment trap and surface-sediment data [Murray and Prell, 1996; Prell et al., 1996; Clemens, 1998], through Orbital timescales, which uses a variety of core types to identify how the timing of strong monsoons is related to sensible heating over the Asian plateau, latent heat availability from the southern subtropical Indian Ocean, and evolving glacial boundary conditions. We plan to test our observation that the timing of the monsoon response is nonstationary (i.e., that the monsoon phase systematically changed relative to the phase of global ice-volume and regional aridity) by developing new records in the South China Sea and other monsoon regions. At Tectonic timescales we continue to explore the intensification of modern monsoonal circulation at ~ 8 Ma, which is indicated by the increased abundance of endemic upwelling species, the distribution of C3-C4 vegetation, the onset of eolian sedimentation in China, and the uplift of the Himalayan-Tibetan complex. Our goal is to Climate model simulations indicate that such intensification may be a threshold response to the development of Tibetan orography at least 1/2 that of the present elevation [Prell and Kutzbach, 1992; Kutzbach et al., 1997; Prell and Kutzbach, 1997]. We continue to test the hypothesis that coeval long-term changes in ice volume, the terrestrial paleosol record, and monsoon nonstationarity are all linked through the effects of plateau uplift on global climate (e.g. [Raymo and Ruddiman, 1992; Kutzbach et al., 1997; Ruddiman et al., 1997a; Ruddiman and Prell, 1997]).
The see-sawing of atmospheric mass and surface ocean waters across the tropical Pacific Ocean known as the El Niño Southern Oscillation (ENSO) has the potential to affect the lives of billions of people via its impacts on Pacific fisheries and global rainfall and temperatures, yet we continue to lack the ability to predict future ENSO variability in response to changes in radiative forcing from greenhouse gases. Brown faculty study Pacific climate using marine and lake sediments, providing key new insights into the dynamics of ENSO on annual to Plio-Pleistocene timescales.
Indonesian lake sediments can provide important insights into high-resolution records of ENSO variability, but an a very underutilized archive. Recent research by Jim Russell analyzing the minerology and stable isotopic composition of carbonate minerals in laminated sediments from a maar crater lake in Eastern Java (Crausbay et al., 2006) indicates that the frequency and severity of El Niño events has varied dramatically during the past 800 years, with intervals of strong El Niño events between about 1450 and 1700 AD, and weak El Niño prior to that time. These changes may result from the combined radiative effects of changes in solar output, volcanic eruptions, and trace gases, indicating a strong sensitivity of the ENSO system to natural (and perhaps anthropogenic!) radiative forcing. Current research seeks to extend these results using additional crater lakes in Eastern Java, and to investigate long-term ENSO dynamics using sediments from large, tectonic lakes on the island of Sulawesi.
Current research by Tim Herbert on the Peru margin is breaking new ground in an area that has been very difficult to work in. Deposition of marine sediments along the continental margin is frequently interrupted by hiatuses (marked by phosphorite layers) and erosion. Furthermore, the sediments lack carbonate microfossils for 14C dating and conventional paleoceanographic measurements. We have been using organic proxies to monitor past sea surface temperature and productivity, and isotopes of Nitrogen from the organic fraction to monitor the sub-surface geochemistry. The Peru margin today is home of the world’s strongest open-ocean oxygen minimum zone. An important consequence of anoxia is denitrification, which leaves a strong isotopic imprint on the Nitrogen upwelled to surface plankton. Our reconstruction (see below) indicates that oceanography and ecology along the Peru margin have changed dramatically over the past 20,000 years. The modern association of relatively cool waters, high productivity, and intense dentrification, appear to be features of only the past 6,000 years.
Coastal watersheds and estuaries are complex, highly interlinked, dynamic ecosystems. The great temporal and spatial variability of the processes in these systems makes them ideal for field studies designed to explore the patterns and causes of natural environmental change.
This variability also complicates the identification and quantification of societal impacts on these systems. Time scales can vary: tidal flushing (hourly), storm events (hours to weeks), spring/neap tidal responses (weeks to months), seasonal and interannual responses. Remote sensing and hydrologic studies of the watersheds and estuaries can be coordinated to build an integrated understanding of estuarine systems. Research is also underway to use sediment cores from coastal marshes and inland lakes to delineate hurricane overwash deposits and forest damage in southern New England during the last 350 years. Pollen and sedimentary evidence are being used to help identify hurricanes in prehistoric times. Increased tidal flooding which occurred between 100-150 years ago indicates an acceleration of the rate of sea level rise in southeastern New England during that period.
Check here for a description of recent work by the Insomniacs investigating environmental dynamics of the Narragansett Bay.