Research

Research Focus

As an aquatic organic geochemist/biogeochemist, my primary strengths and expertise are in the biogeochemistry and processing of carbon, nitrogen and phosphorus in coastal, oceanic, estuarine, and riverine environments. My formal research training was in both ecosystem ecology and geochemistry; this has led to active and on-going research projects that are highly interdisciplinary, at and across interfaces between different aquatic systems. Much of my current work focuses on the carbon sequestration, sources, and burial of organic carbon and its major component associated elements (N and P) along a continuum from land to coastal ocean and between the coastal and open oceans.

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Fig 1. The global carbon cycle (units are in PgC or Pg C•y-1). Sources of inventories and fluxes are refs. 2-8, 48, 59, 61; diagram modified from Drenzek (114).

Many of the central issues in research concerning global climate change involve understanding the exchange and transport of organic and inorganic pools of carbon – in the context of the global carbon budget. If we are to successfully balance and model global carbon fluxes, it is important to understand the dynamics of carbon cycling in the most productive environments. In general, the most productive environments are located in land-margin ecosystems such as continental margins, and freshwater and marine wetlands. During the past few years my research has centered on organic carbon cycling in coastal and wetland environments. I have used state-of-the-art techniques to determine the role of terrestrial versus aquatic carbon sources in the overall carbon cycles of these ecosystems.

The primary focus of my biogeochemical research has involved the use of specific molecular biomarkers as tracers of organic carbon inputs to land-margin ecosystems. For example, I have utilized plant pigments (i.e., chlorophylls and carotenoids) and lignin-phenols (i.e., lignin) as tracers of organic carbon from planktonic versus terrestrial sources, respectively. We also routinely measure fatty acids, amino acids, glycerol diakyl glycerol tetraethers (GDGTs) in aquatic sediments and dissolved and particulate organic carbon (DOC and POC). Many of the techniques utilized in these biomarker analyses have required the use of high performance liquid chromatography (HPLC), HPLC mass spectrometry (LC-MS-MS), gas chromatography (GC), 13C nuclear magnetic resonance spectrometry (NMR), isotope ratio mass spectrometry (IRMS) (i.e., 15N and 13C), and gas chromatography – mass spectrometry (GC-MS-MS). Collaborative work using radionuclides (i.e., 210Pb and 137Cs) were also used to determine sedimentation and burial rates of carbon in these land-margin ecosystems. We have also recently set out lab up to perform compound-specific stable isotope (CSIA) and radiocarbon (CSRA) analyses. In addition to studying processes in modern environments, I have used chemical biomarkers as paleo-indicators of carbon cycling in past environments. This paleo-approach allows for analysis of long-term changes that may be associated with climate change. In fact, a considerable fraction of research current research involves paleo-reconstruction of carbon cycling in coastal environments using chemical biomarker techniques.

Current Research Projects:

1. Developing a high-resolution late Holocene sediment record of rapid Arctic climate change from the Beaufort Sea coastal zone. Arctic Sciences NSF. 

Funds are requested for an 36-month program to develop a new, high-resolution (annual to sub-decadal) paleoclimate record (0-1,000 y) from sediment cores taken Arctic lagoonal settings adjacent to rivers emptying into the Beaufort Sea. The present submission is a follow-up to an 18 mo OPP/ARC EAGER project (095336) that allowed us to collect an initial core and seismic dataset in Simpson Lagoon near the Colville River delta in August 2010. Our initial results outlined in the project description suggest that these sediment cores contain a valuable history of system response to climate change on the adjacent continent (e.g., river drainage basins) and terrestrial-marine linkages.  3
We propose herein to 1) carry out a detailed analytical study of the Simpson-Colville cores collected in the 2010 field study and integrate these results with other proxy records from   the Arctic, and 2) collect an additional core and seismic dataset in the Maguire Islands-Canning River lagoonal region to aid interpretation of local versus regional and pan-arctic trends recorded in the sediment record. All cores will be analyzed for stratigraphy (x-radiography), bulk organic and mineral content, granolometry, and geochronology (210Pb/137Cs). Detailed analysis for paleoclimate proxies will be carried out on selected cores: the age-depth relationship for the deeper part of these cores will be determined using radiocarbon. Climate indicators that will be applied are organic biomarkers (lignin-phenols, cutins, plant pigments, and δ13C) and mineral tracers (clay mineralogy, heavy mineral assemblages, granulometry, event layer stratigraphy). This work is in collaboration with Mead Allison at the River Institute.

2. The Role of Priming in Microbial Utilization of Terrestrially-Derived Dissolved Organic Carbon: A Proof of Concept. NSF, Low Temperature Geochemistry and Geobiology 

Global estimates of riverine flux of dissolved organic carbon (DOC) to the oceans range from about 0.25 to 0.36 Pg y-1. Interestingly, one of the major conundrums in chemical and biological oceanography over the past few decades has been that while the amount of DOC discharged by rivers can account for the renewal of DOC in the global ocean (every ~ 4000-6000 yr), riverine DOC, which is largely believed to be composed of terrestrially-derived dissolved organic carbon TDOC, is recognized as accounting for only a small fraction of oceanic DOC. So, the key remaining question is:  how is this large flux of terrestrially-derived DOM (TDOM) processed and “removed” in the coastal ocean? A process known as the “priming” of organic matter, discovered in 1926, revealed that rates of soil humus mineralization were enhanced by the addition of fresh organic residues. While the importance of loss rates by photodegradation and bacterial consumption of TDOC have been widely investigated in an attempt to answer the aforementioned conundrum in coastal waters, the role of priming processes has been totally ignored. This has also been ignored in open ocean studies – where “radiocarbon old DOC” from deep waters is consumed after it comes in contact with fresh marine DOC (MDOC) surface waters during mixing events.

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Fig 4. Sources of possible priming substrates in different terrestrial and aquatic environments. Modified from Raymond et al (114).

A recent paper by Bianchi (2011) in PNAS suggests that the priming is likely important in understanding global carbon in many aquatic ecosystems. As Bianchi points out, there has been a surge in the interest of priming studies in last decade in the soil literature, unfortunately, the aquatic community (particularly in oceanography) has lagged behind considerably. While mention of this priming or in some cases “co-metabolism” effects can be found in the oceanographic literature, it has largely been supported by superficial or equivocal evidence.  We posit that to begin answering this question you must begin the laboratory, where conditions can be closely monitored and controlled, in what we call a “proof of concept” approach. 

3. The Ecological Drill Hypothesis: Biotic Control on Carbonate Dissolution in a Low Relief Patterned Landscape. NSF, DEB Ecosystem Studies 

This study tests the hypothesis that ecosystem processes control the rate and geometry of limestone dissolution in the low-relief karst landscape of Big Cypress National Preserve (BICY; S. Florida), creating a strikingly regular spatial arrangement of isolated wetlands in an upland matrix. Regular patterning elsewhere arises from coupled biotic and abiotic processes operating at different spatial scales where organisms create favorable conditions (e.g., hydroperiod, sediment stability) for local expansion but as a result inhibit expansion at greater distance. In this case, we posit an “ecological drill” mechanism where a positive feedback is established between water storage in landscape depressions and aquatic ecosystem metabolism. Enhanced water storage limits dry season water stress, and deeper soils enhance phosphorus (P) availability and root development. Prolonged inundation also regulates ecosystem metabolism, particularly aquatic respiration, which controls water column acidity via both porewater CO2 enrichment and organic acid production. Elevated water column acidity, in turn, accelerates limestone dissolution as groundwater exchanges vertically and laterally between wetlands and the surrounding surficial aquifer, a feedback likely amplified by geologic P released during dissolution. Aquatic primary production can also induce carbonate precipitation as calcareous mud, which affects the direction and magnitude of groundwater exchanges, and thus dissolution geometry. Wetland expansion, which occurs at the expense of the upland catchment, is eventually inhibited when the catchment is too small to provide sufficient water subsidy to sustain prolonged inundation, creating strikingly regular pattern in wetland size and separation. We propose 7 research elements to address this hypothesis: 1) landscape pattern analysis using LiDAR surface and canopy data; 2) bedrock elevation, and soil surveys (mineralogy, P and OM content, age); 3) organic acid production and controls on carbonate dissolution; 4) detailed wetland hydrologic characterization; 5) spatial and temporal variation in porewater geochemistry (calcite dissolution and P mobilization); 6) metabolism (aquatic primary production, litterfall, respiration); and 7) simulation models to synthesize field observations, evaluate proposed mechanisms, explore controls on long term geomorphic trajectories, and extrapolate to other karst settings.

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This interdisciplinary work explores the surprisingly understudied coupling between surface organic carbon and sub-surface inorganic carbon dynamics, which are of critical importance for carbonate dissolution and karst evolution even where regular patterning does not occur (i.e., emergent patterns are more familiar fractal conduit networks and voids). This organic-inorganic C coupling has been invoked as important to transient behavior of the global C cycle. The study expands the constellation of examples illustrating the formative role ecosystem processes play in shaping the earth’s surface, with an emphasis on extremely long-lived biogeomorphic structures. Finally, it explores the links between ecosystem processes and landform evolution in response to modern, and currently accelerating, sea level rise. This work is in collaboration with lead PI Matt Cohen, UF, Jon Martin, UF, and others.

4. Flooding the Colorado River Delta: Impacts of Flow Restoration on River-Carbon Composition and Fluxes. NSF Hydrology. 

A new treaty between the United States and Mexico (Minute 319) was signed in late 2012, as part of a modification of the 1944 U.S.-Mexico Water Treaty (International Boundary and Water Commission, 2012) that will allow for greater sharing of water from the Colorado River (Flessa et al., 2013).  6
This treaty in part has also allowed for a planned flooding event, from March 2014 to May 2014, whereby 130 m3of water will be released into the dry Colorado River channel in Mexico. While only small amounts of water have entered the delta since 2000, with some moderate flooding events that occurred in the late 1990s, this region has not experienced “normal” flow rates since prior to dam building in the 1930s. While there have been a few pulse release experiments of this type (e.g., Rio Grande and Truckee Rivers) much of the previous post-pulse research to date has focused on sediment transport and the response of riparian zone communities.  7

In this project we examine how rapid mobilization of carbon in in newly flooded sediments and soils, affect river-carbon composition and fluxes (land/ocean and land/atmosphere), after being isolated from an active floodplain. We posit that during a flow restoration pulse, dissolved organic and inorganic carbon (DOC/DIC), previously stored as inactive pools in the dry floodplain will enter the modern carbon cycle. This is particularly important in light of possible increases in the occurrence of natural flooding events associated with climate change. This work is in collaboration with David Butman, Univ. of Washington, Peter Raymond, Yale University, and Karl Flessa University of Arizona.

5. Linking carbon exchange between coastal wetland and shelf environments: a case study in Barataria Bay, northern Gulf of Mexico. NASA Research Announcement NNH13ZDA001N-CARBON. 

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The aim of this proposal is to develop and optimize algorithms that integrate optical and chemical information of terrestrial DOM based on proxies for the prediction of its flux from marshes to coastal waters through estuaries. One key objective of the proposed study is to calibrate existing remote sensing algorithms for CDOM based on the SeaWiFS and MODIS sensors to the newly-launched and operating VIIRS sensor. A second key objective of the proposed study is to modify these algorithms for the discrete prediction of terrestrial DOM flux using its optical (absorption, fluorescence) and chemical properties (dissolved lignin, stable isotopes). The region of study is Barataria Bay, in coastal Louisiana, a site chosen because of prior work there by this res earch team and a site manageable to study in the 18-month time framefor the A.459 CMS program.We propose a targeted sampling of the shallow surface waters of the marshes lining Barataria Bay, the Bay Proper, and a short transect into Louisiana shelf waters, to quantify the optical and chemical signatures as representative of terrestrial fluxes from coastal wetlands to shelf waters. This region encompasses a critical US ecosystem undergoing stress from land use and climate; therefore the targeted study we propose will develop RS products that will be useful for understanding how the land-ocean linkage is responding to such stressors with 3Calibrating Remote Sensing Observations to Terrestrial Organic Matter Biomarkers: New Algorithms for VIIRS and Future Carbon Sensors respect to DOM fluxes. This work is in collaboration with Chris Osburn, North Carolina State University, Eurico D’Sa, Louisiana State University, and Dong Ko, Naval Research Lab.

6. The carbon budget of tidal wetlands and estuaries of the contiguous United States: a synthesis approach. NASA Research Announcement NNH13ZDA001N-CARBON. 

The main objective of the research proposed here is to develop a carbon budget for tidal wetlands and estuaries of the contiguous US using existing field observations, remote sensing products (MODIS, SeaWiFS, MERIS, and Landsat), and statistical models. While there is much literature on carbon fluxes and related data at specific sites, knowledge is fragmented and has not been analyzed at the continental spatial scale. It is only through comprehensive analysis and synthesis that the carbon budget for US coastal environments can be addressed and key questions can be answered, including: (1) Is the oxidation of organic carbon in estuaries largely supported by wetland production and export [28] or by the respiration of riverine organic carbon [12]? (2) How do flux processes vary between climatic zones, and between passive margins (Atlantic, Gulf) and active margins (Pacific)? (3) What estuaries have the highest potential for blue carbon projects to conserve net CO2 uptake by tidal wetlands? (4) Is the combined system (tidal wetlands and estuaries) a source or sink of CO2 to the atmosphere? What roles do riverine carbon input, burial, NEP, and carbon export to the coastal ocean play in determining this?

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The proposed research addresses aspects of the global carbon cycle that heretofore have received little attention, in contrast to the relatively well-studied terrestrial and open-ocean carbon cycles. The research is societally relevant because carbon management, which is an emerging need for climate policy, must be based on a rigorous assessment of the current carbon budget. The research directly addresses Theme 6 of this solicitation: “Carbon Cycle Science Synthesis Research,” and is responsive to NASA, DOE, and USDA’s respective goals to “improve understanding of the global carbon cycle and quantify changes in terrestrial and aquatic carbon storage,” “develop process-level understanding of terrestrial systems,” and apply improved understanding of carbon cycling to “improved management of land, ecosystem, and water resources to mitigate carbon and greenhouse emissions.” This work is in collaboration with the lead PI Ray Nijar, Penn State University and others.

7. Paleo-Reconstruction of Terrestrial Organic Matter Inputs New Zealand Fjords and Mechanisms of Organic Matter Preservation 

The coastal carbon cycle is a dynamic component of the global carbon system, but its diverse sources, degradation pathways, and burial rates, are still poorly understood [Bauer et al., 2013]. In general, active margins differ from passive coastal margins by their frequent seismic activities, large sedimentation rates, and efficient carbon burial. As such, small mountain rivers on active margins correspond to roughly the same particulate organic carbon (POC) discharge by large rivers from passive margins [Milliman and Syvitski, 1992]. 11While passive margins allow for a greater residence time of carbon in the system before burial, active margins can rapidly transport carbon within the coastal zone with more efficient and rapid burial of carbon. With relatively rapid recycling, active margins have a moderating role over longer periods [Blair and Aller, 2012]. Recent work in the Southern Alps of New Zealand (Fiordland) has shown efficient burial of organic carbon (OC, up to 12%) in the fiords [Richard W Smith et al., 2010]. Mass wasting of mountain-sides from high rainfall (ca. 7 m yr-1) and stratified waters in this active margin further enhance the efficiency of carbon burial in these fjord sediments. Moreover, there is a steep gradient in the topography that decreases with fjord in the southern direction; thus, much of the mass-wasting is located in the more northern fjords of Fiordland. [Larsen et al., 2014]. While it is well-known that fjords in the Northern Hemisphere are highly stratified and sites of high carbon burial – fjords in the Southern Hemisphere have been largely ignored to date. Thus, here we posit that Fiordland, with its high organic carbon content in sediments, stratified waters and high rates of mass-wasting inputs, provide an excellent natural laboratory for examining controls on carbon burial rates and testing for new chemical biomarker proxies with relatively high resolution.

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In this study, we plan to collect surface sediments, and short- and long- sediment cores from different fjords along a north-south gradient in Fiordland. Down-core shifts in bulk proxies, chemical biomarkers and isotope compositions will be used to examine carbon burial and to reconstruct past changes in organic carbon inputs, as they relate to climate change over the ~10,000 years. More specifically, we will use bulk sediment composition, carbon isotopic values, combined with state-of-the-art chemical biomarker/isotopic analyses such as stable compound-specific isotopic analysis, and CSRA, as well as using GC-MS-MS and LC-MS-MS techniques.  This work is in collaboration with Candida Savage, Otago University, and Mead Allison, The River Institute.