Tom Jilbert

Iron-mediated anaerobic oxidation of methane in brackish coastal sediments

Methane is a powerful greenhouse gas and its biological conversion in marine sediments, largely controlled by anaerobic oxidation of methane (AOM), is a crucial part of the global carbon cycle. However, little is known about the role of iron oxides as an oxidant for AOM. Here we provide the first field evidence for iron-dependent AOM in brackish coastal surface sediments and show that methane produced in Bothnian Sea sediments is oxidized in distinct zones of iron- and sulfate-dependent AOM. At our study site, anthropogenic eutrophication over recent decades has led to an upward migration of the sulfate/methane transition zone in the sediment. Abundant iron oxides and high dissolved ferrous iron indicate iron reduction in the methanogenic sediments below the newly established sulfate/methane transition. Laboratory incubation studies of these sediments strongly suggest that the in situ microbial community is capable of linking methane oxidation to iron oxide reduction. Eutrophication of coastal environments may therefore create geochemical conditions favorable for iron-mediated AOM and thus increase the relevance of iron-dependent methane oxidation in the future. Besides its role in mitigating methane emissions, iron-dependent AOM strongly impacts sedimentary iron cycling and related biogeochemical processes through the reduction of large quantities of iron oxides.

Coupled dynamics of iron and phosphorus in sediments of an oligotrophic coastal basin and the impact of anaerobic oxidation of methane.

Studies of phosphorus (P) dynamics in surface sediments of lakes and coastal seas typically emphasize the role of coupled iron (Fe), sulfur (S) and P cycling for sediment P burial and release. Here, we show that anaerobic oxidation of methane (AOM) also may impact sediment P cycling in such systems. Using porewater and sediment profiles for sites in an oligotrophic coastal basin (Bothnian Sea), we provide evidence for the formation of Fe-bound P (possibly vivianite; Fe3(PO4)2 .8H2O) below the zone of AOM with sulfate. Here, dissolved Fe2+ released from oxides is no longer scavenged by sulfide and high concentrations of both dissolved Fe2+ (>1 mM) and PO4 in the porewater allow supersaturation with respect to vivianite to be reached. Besides formation of Fe(II)-P, preservation of Fe-oxide bound P likely also contributes to permanent burial of P in Bothnian Sea sediments. Preliminary budget calculations suggest that the burial of Fe-bound P allows these sediments to act as a major sink for P from the adjacent eutrophic Baltic Proper.

Hypoxia Sustains Cyanobacteria Blooms in the Baltic Sea

Nutrient over-enrichment is one of the classic triggering mechanisms for the occurrence of cyanobacteria blooms in aquatic ecosystems. In the Baltic Sea, cyanobacteria regularly occur in the late summer months and form nuisance accumulations in surface waters and their abundance has intensified significantly in the past 50 years attributed to human-induced eutrophication. However, the natural occurrence of cyanobacteria during the Holocene is debated. In this study, we present records of cyanobacteria pigments, water column redox proxies, and nitrogen isotopic signatures for the past ca. 8000 years from Baltic Sea sediment cores. Our results demonstrate that cyanobacteria abundance and nitrogen fixation are correlated with hypoxia occurring during three main intervals: (1) ca. 7000–4000 B.P. during the Littorina transgression, (2) ca. 1400–700 B.P. during the Medieval Climate Anomaly, and (3) from ca. 1950 A.D. to the present. Issues of preservation were investigated, and we show that organic matter and pigment profiles are not simply an artifact of preservation. These results suggest that cyanobacteria abundance is sustained during periods of hypoxia, most likely because of enhanced recycling of phosphorus in low oxygen conditions.

Sedimentation Pulse in the NE Gulf of Mexico following the 2010 DWH Blowout

The objective of this study was to investigate the impacts of the Deepwater Horizon (DWH) oil discharge at the seafloor as recorded in bottom sediments of the DeSoto Canyon region in the northeastern Gulf of Mexico. Through a close coupling of sedimentological, geochemical, and biological approaches, multiple independent lines of evidence from 11 sites sampled in November/December 2010 revealed that the upper ~1 cm depth interval is distinct from underlying sediments and results indicate that particles originated at the sea surface. Consistent dissimilarities in grain size over the surficial ~1 cm of sediments correspond to excess 234Th depths, which indicates a lack of vertical mixing (bioturbation), suggesting the entire layer was deposited within a 4–5 month period. Further, a time series from four deep-sea sites sampled up to three additional times over the following two years revealed that excess 234Th depths, accumulation rates, and 234Th inventories decreased rapidly, within a few to several months after initial coring. The interpretation of a rapid sedimentation pulse is corroborated by stratification in solid phase Mn, which is linked to diagenesis and redox change, and the dramatic decrease in benthic formanifera density that was recorded in surficial sediments. Results are consistent with a brief depositional pulse that was also reported in previous studies of sediments, and marine snow formation in surface waters closer to the wellhead during the summer and fall of 2010. Although sediment input from the Mississippi River and advective transport may influence sedimentation on the seafloor in the DeSoto Canyon region, we conclude based on multidisciplinary evidence that the sedimentation pulse in late 2010 is the product of marine snow formation and is likely linked to the DWH discharge.

Solar forcing of Nile discharge and sapropel S1 formation in the early to middle Holocene eastern Mediterranean

We present high-resolution records for oxygen isotopes of the planktic foraminifer Globigerinoides ruber (δ18Oruber) and bulk sediment inorganic geochemistry for Holocene-age sediments from the southeast Mediterranean. Our δ18Oruber record appears to be dominated by Nile discharge rather than basin-scale salinity/temperature changes. Nile discharge was enhanced in the early to middle Holocene relative to today. The timing of the long-term maximum in Nile discharge during the early Holocene corresponds to the timing of maximum intensity of the Indian Ocean-influenced Southwest Indian summer monsoon (SIM). This coincidence suggests a major influence of an Indian Ocean moisture source on Nile discharge in the early to middle Holocene, while, presently, the Atlantic Ocean is the main moisture source. Nile discharge was highly variable on multicentennial time scale during the early to middle Holocene, being strongly influenced by variable solar activity. This solar-driven variability is also recorded in contemporaneous SIM records, however, not observed in an Atlantic Ocean-derived West African summer monsoon record from the Holocene. This supports the hypothesis that the Indian Ocean moisture source predominantly controlled Nile discharge at that time. Solar-driven variability in Nile discharge also influenced paleoenvironmental conditions in the eastern Mediterranean. Bulk sediment Ba/Al and V/Al, used as indicators for (export) productivity and redox conditions, respectively, varied both in response to solar forcing on multicentennial time scales. We suggest that changes in Nile discharge on these time scales have been concordant with nutrient inputs to, and shallow ventilation of, the eastern Mediterranean.

Evidence for active El Niño Southern Oscillation variability in the Late Miocene greenhouse climate

An evaporite varve thickness record from the Late Miocene Mediterranean reveals significant signals of interannual variability, the frequency and persistence of which are compared with climatic oscillations affecting the region today. Sustained variability in the 2–7 yr band resembles the modern spectrum of the El Nino Southern Oscillation (ENSO) and contrasts with that of the North Atlantic Oscillation (NAO), the cyclicity of which is less stationary in frequency and less sustained in duration. Fully coupled climate model simulations demonstrate not only that ENSO variability persisted during the Late Miocene, but also that its teleconnections may have extended further than today, as high-latitude climate modes weakened due to a reduced meridional temperature gradient. ENSO appears to have exerted a stronger influence on the evaporative balance of the Mediterranean in the Late Miocene than it does today. This evidence suggests that the Pacific prior to the Northern Hemisphere glaciation was characterized by ongoing interannual variability.

Climate-controlled multidecadal variability in North African dust transport to the Mediterranean

High-resolution laser ablation–inductively coupled plasma–mass spectroscopy scanning of resin-embedded laminated sediments is used to detail variability in the composition and magnitude of recent eolian dust deposition in the Eastern Mediterranean. The composition of dust accumulating in the anoxic Atalante basin varies in response to the strength of the summer blocking mode of Mediterranean climate. Dust sources located upwind on the westerly airflow are favored during phases of weaker blocking (hence stronger summer westerlies). This mode is in turn correlated to the pronounced multidecadal oscillation in Mediterranean sea-surface temperature (related to the Atlantic Multidecadal Oscillation), suggesting that coupled ocean-atmosphere dynamics control the large-scale transport of dust in the region. Variable precipitation in dust source regions may also exert an influence on the relative flux of dust from each source, and hence the net composition of dust deposited in the basin. Persistent oscillations in the composition of deeper sediments indicate that the basin offers a high-potential archive for reconstruction of climate-controlled variability in dust transport prior to the instrumental era.

Glacio-isostatic control on hypoxia in a high-latitude shelf basin

In high-latitude continental shelf environments, late Pleistocene glacial overdeepening and early Holocene eustatic sea-level rise combined to create restricted marine basins with a high vulnerability to oxygen depletion. Here we show that ongoing glacio-isostatic rebound during the Holocene may have played an important role in determining the distribution of past hypoxia in these environments by controlling the physical exchange of water masses and the distribution of large-scale phosphorus (P) sinks. We focus on the Baltic Sea, where sediment records from a large, presently oxic sub-basin show evidence for intense hypoxia and cyanobacteria blooms during the Holocene Thermal Maximum. Using paleobathymetric modeling, we show that this period was characterized by enhanced deep-water exchange, allowing widespread phosphorus regeneration. Intra-basin sills then shoaled over a period of several thousand years, enhancing P burial in one of the sub-basins. Together with climate forcing, this may have caused the termination of hypoxia throughout the Baltic Sea. Similar rearrangements of physical and chemical processes likely occurred in response to glacio-isostatic rebound in other high-latitude shelf basins during the Holocene.

Hypoxia‐driven variations in iron and manganese shuttling in the Baltic Sea over the past 8 kyr

The Baltic Sea has experienced three major intervals of bottom water hypoxia following the intrusion of seawater circa 8 kyr ago. These intervals occurred during the Holocene Thermal Maximum (HTM), Medieval Climate Anomaly (MCA), and during recent decades. Here we show that sequestration of both Fe and Mn in Baltic Sea sediments generally increases with water depth, and we attribute this to shelf-to-basin transfer (“shuttling”) of Fe and Mn. Burial of Mn in slope and basin sediments was enhanced following the lake-brackish/marine transition at the beginning of the hypoxic interval during the HTM. During hypoxic intervals, shelf-to-basin transfer of Fe was generally enhanced but that of Mn was reduced. However, intensification of hypoxia within hypoxic intervals led to decreased burial of both Mn and Fe in deep basin sediments. This implies a nonlinearity in shelf Fe release upon expanding hypoxia with initial enhanced Fe release relative to oxic conditions followed by increased retention in shelf sediments, likely in the form of iron sulfide minerals. For Mn, extended hypoxia leads to more limited sequestration as Mn carbonate in deep basin sediments, presumably because of more rapid reduction of Mn oxides formed after inflows and subsequent escape of dissolved Mn to the overlying water. Our Fe records suggest that modern Baltic Sea hypoxia is more widespread than in the past. Furthermore, hypoxia-driven variations in shelf-to-basin transfer of Fe may have impacted the dynamics of P and sulfide in the Baltic Sea thus providing potential feedbacks on the further development of hypoxia.

Pore water and solid-phase measurements on sediment cores from the Black Sea

The surface sediments in the Black Sea are underlain by extensive deposits of iron (Fe) oxide-rich lake sediments that were deposited prior to the inflow of marine Mediterranean Sea waters ca. 9000 years ago. The subsequent downward diffusion of marine sulfate into the methane-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ), including anaerobic oxidation of methane (AOM) with sulfate. While the sedimentary cycles of sulfur (S), methane and Fe in the SMTZ have been extensively studied, relatively little is known about the diagenetic alterations of the sediment record occurring below the SMTZ. Here we combine detailed geochemical analyses of the sediment and pore water with multicomponent diagenetic modeling to study the diagenetic alterations below the SMTZ at two sites in the western Black Sea. We focus on the dynamics of Fe, S and phosphorus (P) and demonstrate that diagenesis has strongly overprinted the sedimentary burial records of these elements. Our results show that sulfate-mediated AOM substantially enhances the downward diffusive flux of sulfide into the deep limnic deposits. During this downward sulfidization, Fe oxides, Fe carbonates and Fe phosphates (e.g. vivianite) are converted to sulfide phases, leading to an enrichment in solid phase S and the release of phosphate to the pore water. Below the sulfidization front, high concentrations of dissolved ferrous Fe (Fe2+) lead to sequestration of downward diffusing phosphate as authigenic vivianite, resulting in a transient accumulation of total P directly below the sulfidization front. Our model results further demonstrate that downward migrating sulfide becomes partly re-oxidized to sulfate due to reactions with oxidized Fe minerals, fueling a cryptic S cycle and thus stimulating slow rates of sulfate-driven AOM (~ 1-100 pmol/cm**3/d) in the sulfate-depleted limnic deposits. However, this process is unlikely to explain the observed release of dissolved Fe2+ below the SMTZ. Instead, we suggest that besides organoclastic Fe oxide reduction, AOM coupled to the reduction of Fe oxides may also provide a possible mechanism for the high concentrations of Fe2+ in the pore water at depth. Our results reveal that methane plays a key role in the diagenetic alterations of Fe, S and P records in Black Sea sediments. The downward sulfidization into the limnic deposits is enhanced through sulfate-driven AOM with sulfate and AOM with Fe oxides may provide a deep source of dissolved Fe2+ that drives the sequestration of P in vivianite below the sulfidization front.