Our ongoing projects include studies of

1. •global distributions of subseafloor activity and biomass,
   

2. •the energetics and composition of marine microbial communities,
   

3. •limits to life,
   

4. •estuarine pH and anthropogenic acidification.
   

Past projects have focused on

1. •the structure and dynamics of ancient ecosystems (including causes and consequences of mass extinction at the end of the Cretaceous),
   

2. •the global distribution of marine plankton,
   

3. •the biological role of alkenones and their saturation in Emiliania huxleyi,
   

4. •the structure of the ocean-climate system in the “greenhouse” late Cretaceous (67 million years ago and 83 million years ago),
   

5. •the evolution of the ocean-climate system in the “icehouse” Pleistocene (0-3 million years ago),
   

6. •major impact events of the last 65 million years.
   

Many of these projects utilize the URI Geobiology Field Laboratory. All of them involve collaborations with talented scientists, including faculty, students, staff and postdocs at URI and other institutions around the world.

Ongoing Projects

Global distributions of subseafloor activity and biomass

We are developing and testing models for global distributions of subseafloor activity (organic-fueled respiration and fermentation) and biomass. Our results provide a firm basis for quantifying (1) the impact of subseafloor activity on global biogeochemical cycles and (2) Earth’s total biomass. Results to date demonstrate that organic-fueled respiration and biomass in subseafloor sediment vary by orders of magnitude from one oceanic region to another, in concert with oceanographic properties (e.g., distance from shore, organic flux to the seafloor) [D’Hondt et al., 2009, EOS abstract; Kallmeyer et al., 2009, EOS abstract]. Much of the fundamental data used in these models comes from our field expeditions (D’Hondt et al., 2004, Science; D’Hondt et al., 2009, PNAS). Our 2009 expedition to the equatorial and North Pacific ocean principally focused on generating data to refine these models. To further refine the models, graduate students Schrum and Walsh participated in IODP expedition 323 (Bering Sea, 2009).

Microbial energetics

These studies principally focus on subsurface communities in anoxic sediment of the Peru Margin, the Indian Margin and the equatorial Pacific, oxic sediment of the South Pacific gyre and the North Pacific gyre, and anoxic sediment of Narragansett Bay (Rhode Island). Results to date demonstrate that deep subseafloor communities are metabolically complex (D’Hondt et al., 2004, Science; Jørgensen et al., 2006, ODP Scientific Results 201). Their activities appear to include previously unknown processes, such as biological production of ethane and propane (Hinrichs et al., 2006, PNAS) and sulfate-reducing ammonium oxidation (Schrum et al., 2009, Geology). Mutualistic interactions sustain these communities for millions of years with extremely little ongoing input of organic matter (Wang et al., 2010, GCA). Microbial communities in anoxic subseafloor sediment are sustained by electron acceptor fluxes and energy fluxes per cell that are orders of magnitude lower than fluxes previously believed necessary to sustain life (D’Hondt et al., 2002, Science; Price and Sowers, 2004, PNAS; Spivack et al., in preparation). Although these fluxes are very low, Gibbs energy yields per subseafloor reaction are roughly equal to yields for the same reactions in surface environments (Wang et al., 2010, GCA).

Radiolysis and microbial sustenance

Hydrogen from splitting of water by radioactivity has been proposed as a possible electron donor (food) in organic-poor marine sediment (Morita and Zobell, 1955, Deep-Sea Research) and the principal electron donor in deep continental hard-rock aquifers (Pedersen, 1997, FEMS Microbiol. Rev.; Lin et al., 2005, GCA) . The process of radiolytic hydrogen generation is even more effective in fine-grained marine sediment than in deep subsurface fractures (Blair et al., 2007, Astrobiology). In the most organic-poor marine sediment, it appears to be the principal electron donor (D’Hondt et al., 2009, PNAS). In short, the primary electron donor used by microbial communities just a few meters below the seafloor in some regions may be independent from photosynthesis. This hypothesis has obvious implications for  the nature of microbial communities in organic-poor sediment and the habitability of wet sediment and rock on other worlds (such as Mars and Europa). Results from our most recent expedition to the South Pacific Gyre (Integrated Ocean Drilling Program Expedition 329) will test this hypothesis.

Subseafloor community composition

Recent studies demonstrate that deep subseafloor communities are phylogenetically diverse (D’Hondt et al., 2004, Science; Inagaki et al., 2006, PNAS) and vary compositionally from one distinct subseafloor environment to another (Inagaki et al., 2006, PNAS). These communities contain abundant active bacteria (Schippers et al., 2005, Nature) and archaea (Biddle et al., 2006, PNAS and 2008, PNAS; Sørensen and Teske, 2006, Geobiology). However, much remains to be done. We are collaborating to solve a range of problems, including (1) the composition of very low-biomass communities in sediment far from shore, (2) the taxonomic richness of sedimentary communities and (3) their relationship to communities in the overlying ocean.

Limits to life

We are very interested in testing physical and chemical limits to life. Subseafloor gradients in environmental conditions and biological states occur over distances that permit highly resolved sampling.

High temperature provides an obvious example of a limit to life and habitability that can be effectively studied through subseafloor exploration. A commonly accepted upper temperature limit for life is 121°C (Kashefi and Lovley, 2003, Science). This temperature is reached in marine sediments at depths as shallow as 0 meters below seafloor and as great as kilometers below seafloor (click here for our subseafloor isotherm maps). Higher temperature limits have been inferred by a small number of studies. Drilling into subseafloor sediments, igneous rocks, and hydrothermal deposits with temperatures that span the range of 100° to 250°C could be used to determine upper temperature limits for life.

Availability of energy (electron donors and electron acceptors) also undoubtedly limits the occurrence of life. In some subseafloor environments, energy availability may be too low to maintain life. In others, life-sustaining fluxes of electron donors may be independent of photosynthesis and the surface world. In order to determine the energetic limits to life and the organisms that live on the energetic edge, these chemical constraints should be studied in a broad range of environments, including, but not limited to, sediment with very low organic content, deep crustal rocks, and subseafloor regions of active serpentinization. Results from our 2010 expedition to core the entire sediment column and underlying crustal rock in the central South Pacific (IODP Expedition 329) will allow us to test the extent to which life is limited by very low organic content in sediment and the extent to which it is limited by very low availability of inorganic electron donors and/or electron acceptors in ancient (84- to 120-million-year-old) subseafloor basalt.

Estuarine pH and anthropogenic acidification

Estuaries are environments of special biological and human importance. Although they constitute a small fraction of the total ocean, they shelter a wide range of marine organisms. For example, nearly 90% of Middle Atlantic Bight fish species inhabit estuaries at some point during their life cycle (Able, 2005, Estuarine, Coastal and Shelf Science) . Marine commercial and recreational activities are most concentrated in estuaries. Sixty-eight percent (by monetary value) of the fish and shellfish landed by commercial fisheries in the U.S. is estuarine in origin (Lellis-Dibble et al., 2008, NOAA Tech Memo). Two-thirds of the world’s largest cities are located on estuaries. Estuaries have been estimated to have the highest financial value per area of any ecosystem on Earth (Costanza et al., 1997, Nature).

To assess the extent of present and future acidification of estuaries by anthropogenic CO2, we examined pH in three geographically distinct estuaries (D’Hondt et al., in preparation). In each estuary, pH varies by up to 1.5 units over a year and up to one unit over individual days. Estuarine pH variation is predictable on annual and diel/subdiel timescales. This variation is primarily driven by photosynthesis and respiration. The mean pH of each example estuary is well below that of the surface open ocean. Comparison to a previous dissolved oxygen study (Caffrey, 2004, Estuaries) suggests that the mean pH of estuaries throughout the United States is consistently below that of the surface open ocean. Mean pH in western Atlantic estuaries may already hinder larval survival and growth of some organisms (Talmage and Gobler, 2009, Limnol. Oceanogr.; Miller et al, 2009, PLoS ONE). If estuarine photosynthesis and respiration operate similarly in the future, as atmospheric CO2 rises, estuarine ecosystems will be chronically challenged by far lower pH than open ocean ecosystems.

Past projects

Dynamics of ancient ecosystems (including consequences of end-Cretaceous mass extinction)

Members of the D’Hondt laboratory have extensively studied the influence of major biological events (extinctions and radiations) on ecosystem structure and the relevance of metabolic strategies for major evolutionary radiations and survival of major extinctions.

Our earliest studies in this area principally relied on fossil data. One of these studies showed that open-ocean planktonic foraminiferal populations in the Atlantic, Pacific and Tethyan oceans change completely at the stratigraphic level of the end-Cretaceous impact debris (D’Hondt and Keller, 1991, Marine Micropaleontology). Another study confirmed that planktonic foraminifera with widely divergent morphologies evolved from a common ancestor very quickly after the end-Cretaceous mass extinction (D’Hondt, 1991, Journal of Foraminiferal Research).

We used isotopic and sedimentological data to show that the marine ecosystem and the biogeochemical cycling of carbon did not fully recover from the impact and mass extinction for more than three million years [D’Hondt et al., 1996, GSA Special Paper 307 and 1998, Science; Adams et al., 2005, Paleoceanography; D’Hondt, 2005, Annual Review of Ecology, Evolution and Systematics (AREES); Coxall et al, 2006, Geology]. We hypothesized that the long delay in recovery of carbon cycling resulted from altered ecological structure in the post-extinction ocean. Final recovery of planktonic foraminiferal diversity immediately followed final recovery of the carbon cycle (Coxall et al., 2006). This diversification may have resulted from the reappearance of oligotrophic oceans as the organic flux from the surface ocean to deep water fully recovered from the mass extinction. Other studies used Milankovitch-scale sedimentary cycles to show that sedimentation rates declined drastically at the time of impact (Herbert and D’Hondt, 1990, EPSL) and that the oceanic response to Milankovitch-scale climate forcing was altered for a million years after the impact (D’Hondt et al., 1996, Geology).

Carbon isotopic differences between planktonic and benthic microfossils at an Atlantic site (A) and a Pacific site (B) from 69 to 55 Mya (D’Hondt, 2005). These differences result from the organic carbon flux from surface ocean to deep water. Arrows mark (i) flux reduction at the time of mass extinction (bottom arrow), (ii) initial flux recovery (middle arrow), (iii) final flux recovery (top arrow). Initial planktonic diversification occurred during the first recovery and final diversification closely followed final recovery (Coxall et al., 2006). Other types of data show parallel patterns of collapse and recovery (D’Hondt, 2005).

In other studies, we used stable isotopes of carbon and oxygen to document (1) the occurrence of photosymbiosis among Cretaceous and Paleocene species of planktic foraminifera (D’Hondt and Zachos, 1993, Paleoceanography; D’Hondt et al., 1994, Paleobiology; D’Hondt and Zachos, 1998, Paleobiology) and (2) the role of photosymbiosis in the early Cenozoic radiation of planktonic foraminifera (D’Hondt, 2005; Coxall et al., 2006).

Collaborators Rebecca Robinson and David Fastovsky are now working to develop a molecular nitrogen isotopic technique for determining the trophic structure of terrestrial ecosystems. We intend to use this technique to test hypotheses of trophic collapse and trophic radiation during key intervals of life history.

Global distribution of marine plankton

We used the Brown University Foraminiferal Data Base and oceanographic data to map the geographic distribution of planktonic foraminiferal species and to quantitatively test hypotheses of the factors that control variation in species richness (Rutherford et al., 1999, Nature). Our analysis showed that sea surface temperature measured by satellite explains nearly 90 percent of the geographic variation in planktonic foraminiferal diversity throughout the Atlantic Ocean. Temperatures at standard water depths (50, 100, and 150 meters) explain the diversity pattern nearly as well. These findings indicate that geographic variation in zooplankton diversity may be directly controlled by the physical structure of the near-surface ocean. Our results further showed that planktonic foraminiferal diversity does not strictly adhere to the traditional paradigm of continually decreasing diversity from equator to pole. Instead, it peaks in the middle latitudes in all oceans. 

Relationship of planktonic foraminiferal species richness to temperature (Rutherford et al., 1999): (a) actual species richness, (b) species richness vs sea surfate temperature, (c) species richness predicted from b.

Biological role of alkenones and their saturation variability in Emiliania huxleyi

The ratio of 37-carbon diunsaturated to diunsaturated and triunsaturated alkenones (UK'37) produced by some haptophytes is widely used as a proxy for past sea surface temperatures.  However, our isothermal culturing experiments with Emiliania huxleyi  show that UK'37 values vary with nutrient availability and cell division rate (Epstein et al., 1998, Paleoceanography). These results provide a reasonable explanation for large isothermal variation in UK'37 values of single coccolithophorid strains grown in culture. They also suggest that alkenone-based estimates of past sea surface temperatures may have been influenced by dissolved nutrient concentrations as well as by temperature.

The biological function of C37 alkenones and, consequently, the cause of their temperature-dependent saturation, was previously unknown. In order to assess their cellular role, we cultured strains of E. huxleyi at 12:12 and 0:24 light/dark cycles (Epstein et al., 2001, Organic Geochemistry). Alkenone concentrations generally increased through both logarithmic and stationary phase in cultures with 12:12 light/dark cycles. They decreased when E. huxleyi was energy-deprived (in continuous darkness). These patterns of increasing concentration when light is available and decreasing concentration in darkness are typical of metabolic storage molecules in cultures of other marine phytoplankton. These results suggest that Emiliania huxleyi  uses alkenones for metabolic storage. If C37 alkenones are primarily used for metabolic storage, the temperature dependence of their unsaturation may result from differences in melting point, density, or enzymatic optima of biochemical pathways of the differently saturated alkenones. 

Causes of end-Cretaceous mass extinction

Laboratories throughout the world have accumulated a tremendous amount of evidence that the end-Cretaceous extinction was ultimately caused by the impact of a large extraterrestrial object. The first compelling evidence for this extraordinary hypothesis was provided by Alvarez et al. (1980, Science). Many proximate causes have since been proposed to link the impact to the extinction, including global darkness, acid rain and worldwide wildfire.

For brief reviews of the evidence for the end-Cretaceous impact, its relation to mass extinction, and proximate causes of extinction, see D’Hondt (1994, in Extinction and the Fossil Record) and D’Hondt (2005, AREES).

Structure and evolution of the ocean-climate system

Our studies of the past ocean-climate system principally focused on two topics. The first topic is orbital modulation of the ocean and climate. These studies include the first demonstration of semi-precessional modulation of the ocean-climate system (Park et al., 1993, Science), the above-mentioned recognition that Milankovitch-scale climate forcing was altered for a million years after the end-Cretaceous impact (D’Hondt et al., 1996, Geology), and the inference that strengthening of the semi-precessional cycle in the northern hemisphere triggered the transition to sustained 100,000-year glacial cycles 1.5 million years ago (Rutherford and D’Hondt, 2000, Nature).

The second topic is the physical structure of the late Cretaceous (~67 Ma) ocean. In these studies, we used oxygen isotopic data to infer that (1) the equator to pole sea-surface temperature gradient was lower than that it is today (D’Hondt and Arthur, 1996, Science), (2) the tropical sea surface may have been cooler then than it is today and was probably no warmer (D’Hondt and Arthur, 1996, Science), (3) there were at least three deep watermasses at that time, ranging in temperature between 5-7°C and 13-15°C (D’Hondt and Arthur, 2002, Paleoceanography), and (4) these watermasses originated principally in the Southern Ocean and the North Atlantic, much like the cooler deep waters of today (D’Hondt and Arthur, 2002, Paleoceanography). The second of these four results was challenged by Pearson et al. (2001, Nature), who inferred that the late Maastrichtian tropical sea surface was as warm as that of today or warmer.

Large body impacts

Our studies of large body impacts include the studies of the end-Cretaceous event mentioned above. They also include our earliest published studies, which inferred two or more large late Eocene impacts from the stratigraphic occurrences and geochemistry of microtektites (Keller et al., 1983, Science and 1984, Science; Keller and D’Hondt, 1987, Meteoritics; D’Hondt et al., 1987, Meteoritics). Before these studies, only one late Eocene event was recognized. Two major late Eocene impacts are now widely accepted; the Chesapeake Bay event and a much larger event that deposited impact debris throughout the equatorial IndoPacific, the Caribbean and the southern ocean.

D’Hondt (1999, Earth-Sciences History) reviewed the history of impact theories of mass extinction from the mid-18th century to the present.