Steven D'Hondt

Global distribution of microbial abundance and biomass in subseafloor sediment

The global geographic distribution of subseafloor sedimentary microbes and the cause(s) of that distribution are largely unexplored. Here, we show that total microbial cell abundance in subseafloor sediment varies between sites by ca. five orders of magnitude. This variation is strongly correlated with mean sedimentation rate and distance from land. Based on these correlations, we estimate global subseafloor sedimentary microbial abundance to be 2.9⋅1029 cells [corresponding to 4.1 petagram (Pg) C and ∼0.6% of Earth’s total living biomass]. This estimate of subseafloor sedimentary microbial abundance is roughly equal to previous estimates of total microbial abundance in seawater and total microbial abundance in soil. It is much lower than previous estimates of subseafloor sedimentary microbial abundance. In consequence, we estimate Earth’s total number of microbes and total living biomass to be, respectively, 50–78% and 10–45% lower than previous estimates.

Environmental controls on the geographic distribution of zooplankton diversity

Proposed explanations for the geographic distribution of zooplankton diversity include control of diversity by geographic variation in: physical and chemical properties of the near-surface ocean; the surface area of biotic provinces; energy availability; rates of evolution and extinction; and primary productivity. None of these explanations has been quantitatively tested on a basin-wide scale. Here we used assemblages of planktic foraminifera from surface sediments to test these hypotheses. Our analysis shows that sea-surface temperature measured by satellite explains nearly 90% of the geographic variation in planktic foraminiferal diversity throughout the Atlantic Ocean. Temperatures at depths of 50, 100 and 150 m (ref. 9) are highly correlated to sea-surface temperature and 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. Furthermore, our results show that planktic foraminiferal diversity does not strictly adhere to the model of continually decreasing diversity from equator to pole. Instead, planktic foraminiferal diversity peaks in the middle latitudes in all oceans.

Subseafloor sedimentary life in the South Pacific Gyre

The low-productivity South Pacific Gyre (SPG) is Earths largest oceanic province. Its sediment accumulates extraordinarily slowly (0.1–1 m per million years). This sediment contains a living community that is characterized by very low biomass and very low metabolic activity. At every depth in cored SPG sediment, mean cell abundances are 3 to 4 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each SPG site is 1 to 3 orders of magnitude lower than the rates at previously explored sites. Because of the low respiration rates and the thinness of the sediment, interstitial waters are oxic throughout the sediment column in most of this region. Consequently, the sedimentary community of the SPG is predominantly aerobic, unlike previously explored subseafloor communities. Generation of H2 by radiolysis of water is a significant electron-donor source for this community. The per-cell respiration rates of this community are about 2 orders of magnitude higher (in oxidation/reduction equivalents) than in previously explored anaerobic subseafloor communities. Respiration rates and cell concentrations in subseafloor sediment throughout almost half of the world ocean may approach those in SPG sediment.

An effect of dissolved nutrient concentrations on alkenone-based temperature estimates

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 clone CCMP372 show UK′37 values to also vary with nutrient availability and cell division rate. 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.

Early onset and tropical forcing of 100,000-year Pleistocene glacial cycles.

Between 1.5 and 0.6 Myr ago, the period of the Earths glacial cycles changed from 41 kyr, the period of the Earths obliquity cycles, to 100 kyr, the period of the Earths orbital eccentricity, which has a much smaller effect on global insolation. The timing of this transition and its causes pose one of the most perplexing problems in palaeoclimate research. Here we use complex demodulation to examine the phase evolution of precession and semiprecession cycles—the latter of which are phase-coupled to both precession and eccentricity—in the tropical and extra-tropical Atlantic Ocean. We find that about 1.5 Myr ago, tropical semiprecession cycles (with periods of about 11.5 kyr) started to propagate to higher latitudes, coincident with a growing amplitude envelope of the 100-kyr cycles. Evidence from numerical models suggests that cycles of about 10 kyr in length may be required to explain the high amplitude of the 100-kyr cycles. Combining our results with consideration of a modern analogue, we conclude that increased heat flow across the equator or from the tropics to higher latitudes around 1.5 Myr ago strengthened the semiprecession cycle in the Northern Hemisphere, and triggered the transition to sustained 100-kyr glacial cycles.

Heterotrophic organisms dominate nitrogen fixation in the South Pacific Gyre

Oceanic subtropical gyres are considered biological deserts because of the extremely low availability of nutrients and thus minimum productivities. The major source of nutrient nitrogen in these ecosystems is N2-fixation. The South Pacific Gyre (SPG) is the largest ocean gyre in the world, but measurements of N2-fixation therein, or identification of microorganisms involved, are scarce. In the 2006/2007 austral summer, we investigated nitrogen and carbon assimilation at 11 stations throughout the SPG. In the ultra-oligotrophic waters of the SPG, the chlorophyll maxima reached as deep as 200 m. Surface primary production seemed limited by nitrogen, as dissolved inorganic carbon uptake was stimulated upon additions of 15N-labeled ammonium and leucine in our incubation experiments. N2-fixation was detectable throughout the upper 200 m at most stations, with rates ranging from 0.001 to 0.19 nM N h−1. N2-fixation in the SPG may account for the production of 8–20% of global oceanic new nitrogen. Interestingly, comparable 15N2-fixation rates were measured under light and dark conditions. Meanwhile, phylogenetic analyses for the functional gene biomarker nifH and its transcripts could not detect any common photoautotrophic diazotrophs, such as, Trichodesmium, but a prevalence of γ-proteobacteria and the unicellular photoheterotrophic Group A cyanobacteria. The dominance of these likely heterotrophic diazotrophs was further verified by quantitative PCR. Hence, our combined results show that the ultra-oligotrophic SPG harbors a hitherto unknown heterotrophic diazotrophic community, clearly distinct from other oceanic gyres previously visited.

Stable isotopic signals and photosymbiosis in Late Paleocene planktic foraminifera

Late Paleocene planktic foraminifera exhibit strong positive correlations between carbon isotopic values and test mass, but negative correlations between oxygen isotopic values and test mass. Based on analogy with modern taxa, these trends are probably ecotypic and may or may not apply to an ontogenetic series. Among Acarinina and Morozovella species, the magnitude and direction of these trends resemble those of modern planktic foraminifera with dinoflagellate photosymbionts. This is consistent with current models of photosymbiosis and calcification in planktic foraminifera and suggests that Acarinina and Morozovella relied heavily on photosymbionts for nutrition. Acarinina and Morozovella species resemble modern, strongly photosymbiotic taxa in their association with low and mid latitude nearsurface water masses. However, their test morphologies differ greatly from those of extant taxa that bear obligate photosymbionts. Earliest Paleocene taxa that exhibit a similar paleohabitat association and similar size-related isotopic trends are characterized by still different test morphologies. These comparisons suggest that (1) throughout geologic time, strong reliance on photosymbiont activity has been closely linked to habitat, but not to test morphology; (2) photosymbiosis has been a common and convergently evolved strategy of planktic foraminifera over geologic time, and (3) modern relationships between planktic foraminiferal test morphology and photosymbiont dependence are largely an artifact of geologically recent phylogenetic relationships and shared ecologic strategies. Intersite comparison suggests that the stable isotopic signals of narrowly constrained size fractions of a late Paleocene Acarinina or Morozovella species can be used to reconstruct the magnitude and direction of relative variation in equilibrium stable isotopic values throughout its geographic and temporal range. This is supported by analogy with extant photosymbiotic taxa. However, since photosynthetic depletion of 12 C leaves 13 C-enriched HCO 3 - for calcification, the carbon isotopic values of Acarinina and Morozovella tests may have been consistently greater than paleoseawater values. Failure to account for this effect could lead to overestimation of late Paleocene marine productivity.

Late Cretaceous Oceans and the Cool Tropic Paradox

Oxygen isotopic proxies of paleo-sea surface temperatures (SSTs) suggest that Maastrichtian (about 66 million years ago) tropical SSTs were lower than those of today. They also demonstrate that Maastrichtian latitudinal SST gradients were much lower than those of the present. The low Maastrichtian SST gradients indicate that meridional heat transport was much greater or latitudinal differences in the balance of radiation to and from the sea surface were much less extreme during the latest Cretaceous than they are today, or that both conditions were true. These findings challenge traditional interpretations of “greenhouse” Late Cretaceous climates.

Cretaceous-Tertiary (Chicxulub) impact angle and its consequences

The Chicxulub impact structure exhibits asymmetries in its geophysical signatures that closely resemble asymmetries produced by oblique impacts in laboratory experiments and recognized on planetary surfaces. These asymmetric signatures suggest a trajectory for the Chicxulub bolide from the southeast to the northwest at a 20°–30° angle from the horizontal. As a result, biotic extinctions may have been most severe and catastrophic in the Northern Hemisphere. Geographic variation in the magnitude of the Cretaceous-Tertiary (K-T) “fern spike” and palynofloral extinctions are consistent with the proposed trajectory.

The impact of the Cretaceous/Tertiary bolide on evaporite terrane and generation of major sulfuric acid aerosol

Abstract Impact glasses from the K/T boundary in Haiti include high-Ca glasses with up to 1 wt.% SO3, formed by the fusion of anhydrite- or gypsum-rich evaporite sediments in the presence of high-silica melts, derived from melting of continental crust. Experimental studies have duplicated these two melts by fusion of gypsum and andesite, and suggest a formation temperature of 1300°C. Geochemical evidence from the glasses is consistent with their derivation from the 180 km diameter Chicxulub impact crater on the Yucatan peninsula (Mexico), situated in a thick late Cretaceous evaporite succession. This large-diameter crater is most likely the result of a cometary impact, on the basis of bolide scaling from the Ir anomaly in K/T boundary sediments. The sulfur degassing associated with formation of the high-Ca glass alone could have formed a2 × 1016 g H2SO4 stratospheric aerosol. However, the total sulfur degassing from the evaporite-rich sediments in the Chicxulub impact site may have led to formation of3.8 × 1018 to1.3 × 1019 g sulfate aerosol, and global atmospheric mass loading from the sulfate aerosol alone is estimated to beof the order of 1–2.6 g cm−2. Combined with the great optical depth estimates of Pollack et al. [1] and Covey et al. [2] resulting from an impact “dust” cloud, the sulfate aerosol may have contributed not only to a rapid decline in global surface temperatures to near-freezing in about one week, but also prolonged the cooling for several years because of the time-dependent conversion of SO2 to H2SO4 aerosol in the atmosphere. The contribution of sulfur to acid rain following the K/T boundary event may have equalled the proposed rainout of nitric acid.