RESEARCH ACTIVITIES

Thomas S. Bianchi, 

Institute for Earth and Ecosystem Sciences, Department of Geology, Tulane University, New Orleans, Louisiana 70118 

I.  General Overview

Research Interests

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, we need to first 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, salt marshes, mangroves, and freshwater 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.  Many of the techniques utilized in these biomarker analyses have required the use of high performance liquid chromatography (HPLC), HPLC mass spectrometry (LC-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).  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.  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.

Common pigments used in biomarkers studies.

 River-Dominated Margins Research

I have also been involved in two large research endeavors (funded by federal agencies), focused on river-dominated margins, that are scheduled to begin within the next three years.  These two programs will be seeking interdisciplinary research teams that have the expertise, experience and sampling/analytical capabilities necessary to propose and carry out cutting-edge research in the field of river-ocean interactions.  The most immediate of these programs is the NSF-CoOP (Coastal Ocean Processes) Program.  This is a long-standing program that funds basic (process-level) research in coastal environments.  Every 4 years a new focus (type of coastal environment) is chosen.  The next round will focus on river-dominated coastal environments.  The Announcement of Opportunity for this program is expected Winter 2001-2002 with a start date during 2003 projected.  Funding for CoOP projects is typically on a level of $10-15 M per year for 5 year---with only one proposal funded each round.  The two recognized contenders for this round are the Mississippi River system and the Gulf of Alaska.  Brent McKee (Geology Dept.) and myself were chosen to participate in a Fall 1998 workshop that began defining the upcoming program; subsequently IEES researchers (McKee, Dagg, Bianchi, Allison) have been funded by ONR and the CoOP program to support the preparation of CoOP review papers.   The reviews are intended to clarify the current state of understanding of biogeochemical and transport processes in buoyancy-driven systems and to provide background for the future Announcement of Opportunity.  Another, and perhaps the most prestigious of the upcoming opportunities is RiOMar (River-dominated Ocean Margins) which currently is one of four scientific community initiatives in the multi-agency Global Change Program led by NSF.  RiOMar specifically addresses the impact of rivers, and their associated ocean margins, on the global carbon cycle; see our webpage on RiOMar (http://www.tulane.edu/~riomar/) for further information.  The Global Change Program is potentially one of the largest identifiable funding opportunities (for any discipline) that NSF will sponsor this decade.  In addition, many other agencies (DOD, NASA, USGS) will participate with contributing funds.  A potential RiOMar program will receive on the order of $150 M per year for at least an initial 10-year cycle of the program.  The timing for this is still being finalized at NSF but an expected beginning for open competition between the existing initiatives (with possible funding of all 4 at some level) is FY 2003.  The first NSF sponsored RiOMar meeting will be held on the Tulane uptown campus this Nov. 1-3; we have invited approximately 55 experts from around the world to discuss issues concerning biogeochemical cycling of river-dominated margins as they relate to global climate change.

 In the following sections I have listed specific projects, current and pending, that my research group and I will focus on over the next few years.  Although I have cited relevant literature in the text I have not included a references section.

 II. Specific Current and Pending Projects

River-Dominated Margins Research 

 Mississippi River- Louisiana Shelf

            a. Historical Changes in Carbon Burial on the Shelf - It has been estimated that 90% of the total organic matter in the oceans is buried in continental margin sediments - which represents only 10% of the global ocean sea floor (Berner, 1982).   While it is well known that many of these coastal regions of high primary productivity have characteristically high rates of sedimentary organic carbon accumulation - it remains uncertain how important oxygen minimum zones/anoxic sediments and benthic macrofauna (bioturbation) are during preservation (see review by Hedges and Keil, 1995).    Continental margins are dynamic regions that receive inputs of organic carbon derived from both terrestrial and marine sources.   Rivers such as the Mississippi (and associated deltaic environments) provide the major pathways for the input of terrestrial organic carbon to marine sediments; marine primary productivity in these deltaic margins is also high due to high nutrient inputs associated with riverine discharge (Turner and Rabalais, 1991; Redalje et al., 1994; Hedges and Keil, 1995).   In fact, it has been estimated that 80% of the total organic carbon preserved in marine sediments occurs in "terrigenous-deltaic" regions near river mouths (Romankevich, 1984; Berner, 1989).  Low oxygen conditions (hypoxia) may serve an important role in the enhancement of organic matter preservation in these highly productive margins that generally receive large inputs of riverine nutrients.

Past work has suggested that certain decay products of chlorophyll-a (phorbin-steryl esters [PSEs]) have a high potential as fossil indicators of paleoecological change in carbon burial in marine sediments (King and Repeta, 1991, 1994).   The Louisiana coast, which contains the most prolific deltaic regions in the U.S. (Atchafalaya and Mississippi rivers), provides an excellent field laboratory to determine long-tem changes in carbon burial - using stable chlorophyll derivatives and carotenoids, and state-of-the-art biomarker techniques (liquid chromatography/mass spectrometry (LC/MS).   

            Thus, the principal working hypotheses for a pending Petroleum Research Foundation (PRF) proposal are as follows:

 H¹: Phorbin steryl esters (PSEs) and chlorin carotenoid esters (CCEs) are stable paleotracers of historical change in carbon burial on the Louisiana shelf (covering the past ca. 3000 BP).

 H²: There will be higher concentrations of the grazing biomarkers (chlorophyllone a,  PSEs, and CCEs) in recent sediments due to increasing trends of eutrophication and carbon production on the Louisiana shelf.

 H³: Downcore sediment concentrations of PSEs and carotenoid biomarkers (renieratene and isorenieratene) of green sulfur bacteria (Clorobiaceae), should provide long-term historical information over the past ca. 3000 BP (prior to anthropogenic inputs) on the occurrence of hypoxic events on the Louisiana shelf.

 The following objectives will be used to test this hypothesis:

 1.      To determine downcore concentrations of chloropigments (PSEs, CCEs, chlorophyllone) and carotenoids (renieratene and isorenieratene) in deep cores collected from a hypoxic site on the Louisiana shelf over the past ca. 3000 BP.

 2.      To use radiocarbon and pigment data to determine long-term changes in carbon burial over ca. 3000 years BP on the Louisiana shelf.

 3.   To use radionuclide data (²¹⁰Pb, ²³⁴Th, and ⁷Be) to correlate changes in sedimentation and mixing rates with pigment data as well as determining the relative ages of sedimentary layers in the upper layers of deep cores over the past ca. 125 years.

 b. Inputs and Transport of Terrestrially-Derived Carbon across the Louisiana Shelf - Global geochemical budgets of organic carbon are based on the steady-state assumption that the flux of CO₂ out of the ocean is equal to the net organic carbon flux into the oceans from rivers (terrigenous carbon input minus burial in marine sediments).  Thus, the magnitude and nature of carbon storage in shelf sediments is an important constraint on the global CO₂ budget.  Deltaic shelves are particularly important since it has been estimated that 80% of the total organic carbon preserved in marine sediments occurs in "terrigenous-deltaic" regions near river mouths, such as the Mississippi River.  However, biogeochemical cycling of carbon in these river-dominated coastal regions remains largely understudied.  While there has been considerable interest in characterizing organic carbon inputs in river-dominated margins, previous studies have been based on data collected during one or two brief sampling periods.  As valuable as these studies are, they yield only a brief look at complex and dynamic systems.  We propose to incorporate the use of biomarkers (to ascertain organic carbon sources) into the framework of a particulate transport study to determine the spatial and temporal variability of marine and terrestrial carbons inputs to sediments across the margin.

Continental margins are dynamic regions that receive inputs of organic carbon derived from both terrestrial and marine sources.  Vascular plant-derived organic matter can potentially represent a significant fraction of the total organic carbon preserved in sediments on continental margins.  A major problem in the understanding of fates and sources of sedimentary organic carbon (SOC) is making accurate estimates of the relative contributions of terrestrial and marine materials.

 Terrestrial inputs of organic carbon to the continental margin waters of the northern Gulf of Mexico are high compared with other coastal margins of the U.S. because of significant discharges from one of the world’s largest river systems--the combined inputs of the Atchafalaya and Mississippi (Hedges and Parker, 1976; Eadie et al., 1994; Trefry et al., 1994).  While there has been some work on terrestrial sources in particulate, dissolved, and sedimentary organic carbon (POC, DOC, SOC) in the Mississippi River - extending to the deltaic shelf (using stable isotopes and lignin-phenols) (Redalje et al., 1992; Benner, 1994), no previous work has examined the flux and transport of organic carbon across the coastal margin from the river to the open Gulf of Mexico. 

 We will use lignin-phenols (using GC-MS) to examine for terrestrial source materials in SOC.  Over the two past decades lignin has proven to be a useful chemical biomarker for terrestrial inputs to continental margins ( Hedges and Ertel, 1982;  Goni and Hedges, 1992; Bianchi et al., 1997).  Lignin is a macromolecule found in the cell walls of vascular plants which upon oxidation  (via CuO oxidation) can yield eight dominant vanillyl, syringyl, and cinnamyl phenols (Hedges and Ertel, 1982).  These lignin oxidation products (LOP) can be used to distinguish between woody and non-woody tissues of gymnosperms and angiosperms (Hedges and Ertel, 1982).  Syringyl derivatives are unique to woody and non-woody angiosperms, while cinnamyl groups are common to non-woody angiosperms and gymnosperms (Hedges and Parker, 1976; Hedges and Mann, 1979).  Recently, it has been shown that the full suite of vascular plant biomarkers commonly used in the CuO oxidation procedure can be analyzed more rapidly using thermochemolysis techniques in the presence of tetramethylammonium hydroxide (TMAH) and then analyzed by GC-MS (Clifford et al., 1995; McKinney et al., 1995).   Flash pyrolysis and thermochemolysis techniques have been commonly used to breakdown large complex geoploymers commonly found in soils and sediments in preparation for GC-MS analysis.  More specifically, this technique involves thermochemolysis with tetramethylammonium hydroxide at sub-pyrolysis temperatures of 300°C.   Moreover, the TMAH procedure appears to be more sensitive for calculation of benzenecarboxylic acid/benzaldehyde ratios, displaying a larger dynamic range than previous methods.  Thus, the procedures used in this study will be the CuO oxidation and TMAH for comparative purposes.

Once introduced by rivers or fixed on shelves  POC  is carried along the Louisiana shelf, decomposed, or transported to deeper regions in the Gulf of Mexico.  Sediments buried on the shelf generally contain less organic carbon than incoming river particles (Trefry et al., 1994).  Based on coastal primary production estimates and riverine inputs it was recently estimated that only 20-50% of the organic carbon in coastal waters off the Mississippi is actually buried in Louisiana shelf sediments, and that <40% of the organic carbon buried is of terrestrial origin (Eadie et al., 1994; Trefry et al., 1994).  There are two possible explanations for this observation: (1) Large amounts of terrestrial and marine organic carbon are being remineralized in shelf waters and sediments.  Sediments buried on the shelf generally contain less organic carbon than incoming river particles (Trefry et al., 1994).  Based on bulk organic carbon budgets, stable carbon isotope and radioisotope studies, McKee and Twilley (in review) estimated that ca. 30% of the particulate organic carbon supplied by the Mississippi to the adjacent shelf (i.e., terrestrial organic carbon) was remineralized within 4 months and an additional 40% is remineralized on a decadal time scale.  Similar results were found for the deltaic environments associated with the Amazon, Changjiang, and Huanghe Rivers.  McKee and Twilley (submitted) estimate  the area-weighted organic carbon burial in deltaic-shelf sediments to be 75 gCm^-2 yr^-1, with less than 50% of the organic carbon buried being of terrestrial origin; (2) An alternative explanation for these observations is that POC is transported off the shelf (via the Benthic Boundary Layer [BBL]) and buried in slope and Mississippi Canyon sediments.  The northern Gulf of Mexico, (Mississippi delta to the Canyon), which largely remains understudied, provides an excellent field laboratory to determine the seasonal variability of river discharge and BBL transport on the abundance and composition of organic carbon inputs across a river-dominated shelf; in addition to using state-of-the-art biomarker techniques for determining the associated changes in the age and sources of SOC.

Thus, our principal working hypothesis and sub-hypotheses are as follows:

 H¹ - Seasonal changes in the composition and age of SOC across the Louisiana shelf/slope are largely determined by pulses in terrestrially- and marine-derived organic carbon which occur at distinct time periods controlled by changes in Mississippi River discharge and storm energy.

 Sub-H² - Much of the marine-derived SOC across the Louisiana shelf/slope, produced on the shelf during summer periods of high nutrient input and water column stratification,  is transported off the shelf when there is an increase in wind forcing (via storm events) and BBL transport in the Fall season.

 Sub-H³ - Much of the terrestrially-derived SOC across the Louisiana shelf/slope is  introduced to the shelf and transported during periods when high river discharge and BBL transport co-occur.

 The following objectives will be used to test this hypothesis:

1.  To use plant pigments, lignin-phenols, and stable isotopes to determine the major sources of bulk  SOC, upper water column POC, and POC within the BBL across the continental margin off the Mississippi River over three distinct hydrological periods with  varying river discharge and storm events.

 2. To determine the d¹³C of terrestrial and marine components in SOC and POC based on the isotopic signature of selected lipids and phenols derived from macromolecular complexes in SOC, across the continental margin off the Mississippi River over three distinct hydrological periods with varying river discharge and storm events.

 3. To use radiocarbon dating to determine the ¹⁴C activities of selected terrestrial and marine plant lipids in SOC and POC, across the continental margin off the Mississippi River over three distinct hydrological periods with varying river discharge and storm   events.

 4. To use naturally occurring radionuclides (²¹⁰Pb, ²³⁴Th, and ⁷Be) to determine sedimentation and mixing rates across the continental margin over three distinct hydrological periods with varying river discharge and storm events.

5.  To use Mississippi River discharge rates (from USGS), regional climatological data (from NASA Stennis Space Center), and current velocities in the BBL to document seasonal changes in the dominant physical transport mechanisms across the Louisiana shelf.  

 c. Colloidal Interactions with Trace Metals in Rivers-  Understanding what controls dissolved trace element concentrations in rivers is of substantial interest to researchers examining basic scientific questions related to geochemical weathering and transport of elements and to scientists involved in pollution control evaluation and monitoring of water quality for human health and biotoxicity purposes.  With the adoption of ultraclean sampling and analysis methods more workers are now producing reliable fluvial dissolved trace element data and progress has been made in outlining some of the controls on fluvial dissolved trace elements.  However, we are not yet to the point where one can use the hydrological and chemical characteristics of a river to make a reasonable prediction of how dissolved trace element concentrations will vary seasonally or even what levels of dissolved metals to expect. It is especially important to understand the processes affecting Fe and Mn because the oxides of these elements are important adsorbers of many metal ions.  Data from our own work (as well as the work of others) have led us to the overarching hypotheses of this project:

H¹: There is rapid cycling of Fe and Mn between dissolved and particulate forms in rivers. 

 H²: This cycling can be strongly temperature-dependent and hence seasonally variable.

 H³: Changes in this rapid cycling affect the dissolved-particulate partitioning of Fe and Mn as well as particle-reactive trace elements such Zn and Pb.  4) The cycling involves redox processes for Mn and possibly Fe, though organic complexation may be a more important factor for Fe.

 Objectives of our proposed work include understanding the extent, relevance

 and mode of microbial oxidation of Fe and Mn in rivers, elucidating the process(es) of reduction of Fe and Mn in rivers, understanding under what conditions photochemical processes are important for Fe and Mn cycling in rivers, and understanding the role of DOC in fluvial Fe and Mn cycling. Our approach involves studying the variability, controls, and rates of the various key processes (e.g., microbial oxidation, reduction by DOC, photochemistry) that transform Fe and Mn from one phase to another.  Our work will involve both field and laboratory studies.  The field work includes monthly sampling of river systems as well as detailed process-oriented studies of two or more of the systems having different hydrogeochemistries (Mississippi and Pearl Rivers).  Laboratory studies will be used to identify the most important processes at work as well as determine rate constants.  Field and laboratory work will also be done to characterize the fluvial organic matter and relate organic composition (e.g., functional groups related to complexation or reduction) to Fe and Mn cycling.  With rate and process information, models can be constructed to predict Fe and Mn concentrations in rivers as a function of time.  The benefits of this increased understanding of the controls on fluvial dissolved trace elements are several-fold.  First, it gives us a better capability to predict dissolved trace element concentrations, both temporally at times when samples from a given system are not taken and spatially in systems for which no data exist.  Second, information on how various processes can cause seasonal dissolved trace element variability is pertinent to the design of sampling and monitoring programs.  Finally, an understanding of what processes are important in the regulation of dissolved trace element concentrations gives us better insight into how human activities can affect fluvial trace elements in ways beyond direct metal contamination.  For example, the microbial connection suggests a means by which toxins could indirectly affect dissolved trace elements. The work will also provide basic information on fluvial hydrogen peroxide levels and its generation as well as results pertinent to the issue of the photooxidation and fate of chromophoric dissolved organic matter.

 Baltic Sea Research

Cyanobacterial Blooms

The Baltic Sea of today is one of the largest (400,000 km²) semi-enclosed brackish water bodies on Earth.  The basin has an average depth of 54 m but there are distinct areas with deeper basins.  For example, the Bornholm and Gotland Basins (BB and GB) are some of the deeper basins with water depths that reach 90 to 240 m, respectively.  The eutrophication of the Baltic Sea has been a topic of considerable interest for the past three decades (Fonselius 1972; Cederwall and Elmgren 1980; Larsson et al. 1985; HELCOM 1996).  In fact, the areal expanse of laminated sediments and increases in organic carbon (C) content of sediments since the early 1960's have been used as indicators of the recent eutrophication in the Baltic Sea (Jonsson et al. 1990).   In particular, the massive surface accumulations of nitrogen-fixing cyanobacteria (largely Nodularia spp.) regularly recorded in the Baltic Sea in summer are reported to have intensified since 1982, in part attributed to particularly warm weather (Kahru et al. 1994; Kahru 1997).  However, the detailed mechanisms causing such blooms remain largely unknown (Paerl 1996).  Surface cyanobacterial blooms are common in lakes, and in some tropical marine waters (Howarth et al. 1988; Paerl 1996).  The genus Aphanizomenon commonly dominates water column biomass during Baltic blooms (Wasmund 1997).  Because Aphanizomenon in the Baltic is not known to be toxic and seldom forms surface scums, it generally receives less attention than Nodularia, which has repeatedly caused mortality of wild and domesticated animals around the Baltic.  Large cyanobacterial blooms occurred in the Baltic proper already by the 19th century (Lindström 1855), well before major increases in anthropogenic nutrient inputs (Larsson et al. 1985).  Thus, the Baltic provides an excellent system for comparing ecosystem response to both recent anthropogenic and past climatic change.

            We recently documented, for the first time, an 8000-year record of fossil cyanobacterial pigments, diatom microfossil assemblages and d¹⁵N variations in sediment cores from the Baltic proper (Gotland Basin) Bianchi et al. (2000).  Using zeaxanthin, a particularly stable pigment biomarker for cyanobacteria, we show that nitrogen-fixing cyanobacterial blooms are as old as the present brackish-water phase of the Baltic Sea, starting as far back as c. 7000 years BP, soon after the former freshwater Ancylus Lake turned into the brackish Litorina Sea. This indicates that the presently predominating nitrogen (N) limitation of phytoplankton in the Baltic Sea proper is not man-induced, but a natural phenomenon, which has endured for some 7000 years.

 Future Baltic Research

Climate Change

There have been clear changes in the distribution and composition of plants in and around the Baltic Region in response to a general warming of climate over the last 10,000 years.  Linkages between exchange of water masses between the Baltic and North Seas, in response to climate change driven water circulation changes in the North Atlantic Gyre make the Baltic Sea a good integrator of regional response to global climate change.  Regional studies have shown that there are good correlations between oxygen isotope data from Greenland ice cores and changes in the biogeochemical dynamics of the Baltic Sea during different stages in the Holocene.  The well-preserved laminated sediments of the Holocene in the deep basins of the Baltic, as well as the long-term record of recent anthropogenic change in this system, offer a unique opportunity to examine both past and recent changes to climate change.   We seek to determine the response of land and aquatic plants to relatively abrupt climate shifts as apparently recorded by chemical biomarkers in the Baltic Sea and Lake Malaren sediments over the last 10,000 years.   The organic geochemical record of Baltic Sea sediments affords us with a nearly ideal opportunity to examine the botanical response to similar to climate change associated with the warming into the present interglacial world.

R/V Fyrbyggaren, vessel we used  used for cyanobacterial research in Baltic Sea.   Slicing laminated Baltic cores. Freeze-coring in Baltic sediments.

Laminated sediment core from the Baltic proper.