The overarching theme of our research is the use of sedimentary, microfossil and geochemical indicators to produce and synthesize records of present and past storms, floods and sea levels, and their extent of geological and ecological impacts. These records provide means to assess future risk, reveal the spatial and temporal variability of coastal inundation and decipher the relationship of these events to global climatic changes. Active research projects include:
GEOGRAPHIC VARIABILITY OF HOLOCENE SEA LEVEL (HOLSEA)
I lead an International Quaternary Association (INQUA) funded project, which aims to compile a global atlas of Holocene sea levels to inform projections of sea-level rise in response to future climate change by addressing two fundamental questions: 1) How did global mean sea level (GMSL) change since the last deglaciation and what were the contributions of temperature-driven variations and the Greenland and Antarctic ice sheets? 2) What were the trends, magnitudes and driving mechanisms of spatial variability in relative sea level (RSL) during this period? For more information, go to https://www.holsea.org/
EXTENDING HIGH-RESOLUTION RSL RECONSTRUCTIONS INTO THE MID TO EARLY HOLOCENE
By extending high-resolution (decimeter and century scale) sea-level reconstructions into the past 8,000 years in tropical locations, we can begin to explore climatic drivers of sea-level variability during a period of time when ice-sheets were in their current configuration. By expanding the geographic range of these reconstructions, we can begin to take advantage of the distinct spatial and temporal patterns of RSL variability to determine driving mechanisms (e.g., land ice contributes, static-equilibrium effects, atmosphere/ocean dynamics, glacial isostatic adjustment, sediment compaction). These sea-level records are integrated with advanced statistical and geophysical models to partition the influence of those drivers to past changes in sea level and project their relative contribution in the future. Active field sites include Belize and Florida and will be expanded in the near future to include coastlines of Australia, Southeast Asia, and China.
HOLOCENE CARBON STORAGE IN TROPICAL AND TEMPERATE ENVIRONMENTS
Coastal ecosystems are some of the most productive on Earth. One essential service that they provide is the sequestration and storage of "blue" carbon from the atmosphere and oceans, which provides a potential solution to global climate change. However, more must be know about long-term carbon dynamics and interaction with the human environment for this sink to be considered a viable option. We seek to measure the amount of carbon stored in freshwater and coastal wetlands (e.g. mangroves, salt marshes) in present-day and Holocene archives to understand how environmental and anthropogenic influences affect carbon dynamics, including organic matter alteration, stability and biodegradation.
QUANTIFYING PROCESSES OF COASTAL CHANGE
We examine sub-annual to millennial-scale sedimentation and biogeochemical processes in coastal wetland environments as a means to assess ecosystem resilience and vulnerability to future hazards. I am involved in a number of ongoing monitoring studies in New Jersey, Oregon, and the Gulf of Mexico from Mississippi to the Florida Keys. The objectives of these monitoring projects are to assess the sedimentological, ecological, and biogeochemical response of coastal systems to changes across salinity gradients, abrupt inundation events (e.g., tidal restoration), variations in rates of shoreline erosion, and degree of exposure to wave energy. Modern observations are used to inform core-based studies that examine sediment exchange and carbon burial on decadal to millennial timescales and to examine internal (e.g., biota, accommodation space) and external (e.g., sea-level rise, wave energy) drivers influencing these processes.
IMPACT OF EXTREME EVENTS ON COASTAL SYSTEMS AND EVOLUTION
We aim to understand how coastal systems are altered by extreme events by investigating the sedimentary fingerprint and impacts of modern floods and storms on coastal evolution and geomorphology. Observations of contemporary events are used to reconstruct their frequency, magnitude and impacts in the geologic record using sedimentary and/or paleontological proxies. Past records of extreme events extend the short-term instrumental record, and through this improved understanding of impacts and processes, enhance our ability to forecast how coastal systems will respond to extreme events in the future.
We produce new records of relative sea-level change in temperate and tropical settings using established (e.g., microfossil) and novel (e.g., stable carbon isotope geochemistry) proxies to address methodological limitations (e.g., Khan et al., 2015a,b). Developing quantitative metrics or proxies of past ecological conditions and environmental (e.g., shoreline position, wave energy) and sea-level change using transfer functions, combined with new analytical approaches (e.g., DNA analysis), will allow for fuller understanding of mechanisms of past coastal change and enable identification of ecological thresholds, which may feed into predictions of future responses to climatic change.