Relative abundance of dinoflagellate cysts in surface sediments and dinoflagellate counts from three sediment cores from the Atlantic sector of the Southern Ocean

In this study we investigate the potential of organic-walled dinoflagellate cysts (dinocysts) as tools for quantifying past sea-surface temperatures (SST) in the Southern Ocean. For this purpose, a dinocyst reference dataset has been formed, based on 138 surface sediment samples from different circum-Antarctic environments. The dinocyst assemblages of these samples are composed of phototrophic (gonyaulacoid) and heterotrophic (protoperidinioid) species that provide a broad spectrum of palaeoenvironmental information. The relationship between the environmental parameters in the upper water column and the dinocyst distribution patterns of individual species has been established using the statistical method of Canonical Correspondence Analysis (CCA). Among the variables tested, summer SST appeared to correspond to the maximum variance represented in the dataset. To establish quantitative summer SST reconstructions, a Modern Analogue Technique (MAT) has been performed on data from three Late Quaternary dinocyst records recovered from locations adjacent to prominent oceanic fronts in the Atlantic sector of the Southern Ocean. These dinocyst time series exhibit periodic changes in the dinocyst assemblage during the last two glacial/interglacial-cycles. During glacial conditions the relative abundance of protoperidinioid cysts was highest, whereas interglacial conditions are characterised by generally lower cyst concentrations and increased relative abundance of gonyaulacoid cysts. The MAT palaeotemperature estimates show trends in summer SST changes following the global oxygen isotope signal and a strong correlation with past temperatures of the last 140,000 years based on other proxies. However, by comparing the dinocyst results to quantitative estimates of summer SSTs based on diatoms, radiolarians and foraminifer-derived stable isotope records it can be shown that in several core intervals the dinocyst-based summer SSTs appeared to be extremely high. In these intervals the dinocyst record seems to be highly influenced by selective degradation, leading to unusual temperature ranges and to unrealistic palaeotemperatures. We used the selective degradation index (kt-index) to determine those intervals that have been biased by selective degradation in order to correct the palaeotemperature estimates. We show that after correction the dinocyst based SSTs correspond reasonably well with other palaeotemperature estimates for this region, supporting the great potential of dinoflagellate cysts as a basis for quantitative palaeoenvironmental studies.

(Table 1) Helium and Thorium 230 excess concentrations in sediment cores B18 and B25

At four sites in the central equatorial Pacific Ocean the flux of extraterrestrial 3He, determined using the excess 230Th profiling method, is 8 * 10**-13 cm**3 STP/cm**2/ka. This supply rate is constant to within 30%. At these same sites, however, the burial rate of 3He, determined using chronostratigraphic accumulation rates, varies by more than a factor of 3. The lowest burial rates, which occur north of the equator at 1°N, 139°W are lower than the global average rate of supply of extraterrestrial 3He by 20% and indicate that sediment winnowing may have occurred. The highest burial rates, which are recorded at the equator and at 2°S, are higher than the rate of supply of extraterrestrial 3He by 100%, and these provide evidence for sediment focusing. By analyzing several proxies measured in core PC72 sediments spanning the past 450 kyr we demonstrate that periods of maximum burial rates of 230Th, 3He, 10Be, Ti, and barite, with a maximum peak-to-trough amplitude of a factor of 6, take place systematically during glacial time. However, the ratio of any one proxy to another is constant to within 30% over the entire length of the records. Given that each proxy represents a different source (234U decay in seawater, interplanetary dust, upper atmosphere, continental dust, or upper ocean), our preferred interpretation for the covariation is that the climate-related changes in burial rates are driven by changes in sediment focusing.

Gas analysis of sediment cores from the Batumi seep area

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone. Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (>99.85 mol.% of Sum[C1-C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] > 1000) and stable isotopic compositions of methane (d13C = -53.5 per mill V-PDB; D/H around -175 per mill SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by >0.03 mol.% (Sum[C1-C4, CO2]) relative to the other gas types and the virtual lack of 14C-CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely. As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% Sum(C1-C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C-CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.

(Table 2) Ba/Ca ratios of Cibicides wuellerstorfi and Orbulina universa from Bahaman sediment cores

This geochemical investigation utilizes Ba/Ca in the benthic foraminifer Cibicides wuellerstorfi from cores taken from the Bahama Banks and the Caribbean Sea to reconstruct changes in basal thermocline ventilation (800-1000 m) and middepth thermohaline circulation (1000-2000 m) in the western North Atlantic during the last glacial period, focusing on the deglacial transition. Previous studies show that an increase in ventilation of the North Atlantic subtropical gyre during the Last Glacial Maximum (LGM) caused a 30-60% decrease in labile nutrients within the thermocline layer. Using foraminiferal Ba/Ca as a proxy of refractory nutrients, increased ventilation during the LGM produced a depletion of less than 20% compared to Holocene values. Following glaciation, the production of Glacial North Atlantic Intermediate Water (GNAIW) shut down owing to the presence of meltwater in the surface ocean, which resulted in a decrease in ventilation, as seen by an enrichment of barium in the basal thermocline. GNAIW was subsequently replaced by barium-rich southern component water in the middepth western North Atlantic. Foraminiferal Ba/Ca data suggest a 38% contribution from southern component water to a depth as shallow as 1475 m and a 14% contribution at 1123 m during deglaciation.

Gas hydrates of nonpressurized (GC) and pressurized (DAPC) sediment cores from the Hakon Mosby Mud Volcano, SW Barents Sea

The occurrence of gas hydrates at submarine mud volcanoes (MVs) located within the gas hydrate stability zone (GHSZ) is controlled by upward fluid and heat flux associated with MV activity. Determining the spatial distribution of gas hydrates at MVs is crucial to evaluate their sensitivity to known episodic changes in volcanic activity. We determined the hydrocarbon inventory and spatial distribution of hydrates at an individual MV structure. The Håkon Mosby Mud Volcano (HMMV), located at 1,250 m water depth on the Barents Sea slope, was investigated by combined pressure core sampling, heat flow measurements, and pore water chemical analysis. Quantitative pressure core degassing revealed gas-sediment ratios between 3.1 and 25.7, corresponding to hydrate concentrations of up to 21.3% of the pore volume. Hydrocarbon compositions and physicochemical conditions imply that gas hydrates incipiently crystallize as structure I hydrate, with a dissociation temperature of around 13.8°C at this water depth. Based on numerous in situ measurements of the geothermal gradient in the seabed, pore water sulfate profiles and microbathymetric data, we show that the thickness of the GHSZ increases from less than 1 m at the warm center to around 47 m in the outer parts of the HMMV. We estimate the total mass of hydrate-bound methane stored at the HMMV to be about 102.5 kt, of which 2.8 kt are located within the morphological Unit I around the center and thus are likely to be dissociated in the course of a large eruption.

Seismic reflection data, calcium/iron ratios, stable istopes, and magnetic susceptibility of sediment cores from the Pen Duick Escarpment, Gulf of Cadiz

Here we present a case study of three cold-water coral mounds in a juvenile growth stage on top of the Pen Duick Escarpment in the Gulf of Cadiz; Alpha, Beta and Gamma mounds. Although cold-water corals are a common feature on the adjacent cliffs, mud volcanoes and open slope, no actual living cold-water coral has been observed. This multidisciplinary and integrated study comprises geophysical, sedimentological and (bio)geochemical data and aims to present a holistic view on the interaction of both environmental and geological drivers in cold-water coral mound development in the Gulf of Cadiz. Coring data evidences (past or present) methane seepage near the Pen Duick Escarpment. Several sources and pathways are proposed, among which a stratigraphic migration through uplifted Miocene series underneath the escarpment. The dominant morphology of the escarpment has influenced the local hydrodynamics within the course of the Pliocene, as documented by the emplacement of a sediment drift. Predominantly during post-Middle Pleistocene glacial episodes, favourable conditions were present for mound growth. An additional advantage for mound formation near the top of Pen Duick Escarpment is presented by seepage-related carbonate crusts which might have offered a suitable substrate for coral settling. The spatially and temporally variable character and burial stage of the observed open reef frameworks, formed by cold-water coral rubble, provides a possible model for the transition from cold-water coral reef patches towards juvenile mound. These rubble "graveyards" not only act as sediment trap but also as micro-habitat for a wide range of organisms. The presence of a fluctuating Sulphate-Methane Transition Zone has an important effect on early diagenetic processes, affecting both geochemical and physical characteristics, transforming the buried reef into a solid mound. Nevertheless, the responsible seepage fluxes seem to be locally variable. As such, the origin and evolution of the cold-water coral mounds on top of the Pen Duick Escarpment is, probably more than any other NE Atlantic cold-water coral mound province, located on the crossroads of environmental (hydrodynamic) and geological (seepage) pathways.

Pore-water concentration of chloride in sediment cores and equilibrium temperatures and gas bubble analysis at ROV stations from the Vodyanitskii mud volcano

Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to 650 mmol L**-1 at 150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle Quest revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at 60 cm sediment depth that were 0.9 °C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (dD-CH4 = -170.8 per mil (SMOW), d13C-CH4 = -61.0 per mil (V-PDB), d13C-C2H6 = -44.0 per mil (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 ± 0.5 x 10**6 mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column.

Multivariate statistical analysis on the kaolinite/chlorite ratios from 20 South Atlantic sediment cores

Multivariate statistical analysis on the kaolinite/chlorite ratios from 20 South Atlantic sediment cores allowed for the extraction of two processes controlling the fluctuations of the kaolinite/chlorite ratio during the last 130,000 yrs, (1) the relative strength of North Atlantic Deep Water (NADW) inflow into the South Atlantic Ocean and (2) the influx of aeolian sediments from the south African continent. The NADW fluctuation can be traced in the entire deep South Atlantic while the dust signal is restricted to the vicinity of South Africa. Our data indicate that NADW formation underwent significant changes in response to glacial/interglacial climate changes with enhanced export to the Southern Hemisphere during interglacials. The most pronounced phases with Enhanced South African Dust Export (ESADE) occurred during cold Marine Isotope Stage (MIS) 5d and across the Late Glacial/Holocene transition from 16 ka to 4 ka (MIS 2 to 1). This particular pattern is attributed to the interaction of Antarctic Sea Ice extent, the position of the westerlies and the South African monsoon system.

Bulk components and ice rafted debris of five sediment cores of the Sophia Basin, Arctic Ocean

Periods of enhanced terrigenous input to the ocean's basins of the North Atlantic have been reported for the last glacial period. We present a set of new sediment cores recovered from the Sophia Basin north of Svalbard which exhibit wide spread IRD layers reflecting enhanced terrigenous input throughout the last ~200 kyr. BP. Their consistent stratigraphic position, sedimentological character, high sedimentation rate and geochemical characteristic point to synchronously deposited layers which we name terrigenous input events (TIEs). Due to their higher densities, they generate excellent reflectors for sediment penetrating acoustic devices and prominent acoustic layers in the imagery of sedimentary structures. Therefore TIEs can be used for regional acoustic stratigraphy. Each of the events can be linked to major glacial activity on Svalbard. However, the Early Weichselian glaciation is not recorded as a TIE and, in agreement with other work, might not have occurred on Svalbard as a major glacial advance to the shelf break. Non-synchronous timing of western and northern sources on Svalbard points against sea-level induced iceberg discharge events.

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.

Age, 232Th, 230Th, 238U, CaCO3, organic carbon, lithogenic fraction and opal concentrations of 4 sediment cores from the southwest Pacific, with calculated 230Th-normalized mass flux

No clear scenario has yet been able to explain the full carbon drawdown that occurred during the Last Glacial Maximum (LGM); however, increased export production (EP) in the Subantarctic Zone (SAZ) of the Southern Ocean due to iron (Fe) fertilisation has been proposed to have provided a key mechanism affecting the air-sea partitioning of carbon. We chronicle changes in marine EP based on four sediment cores in Subtropical Waters (STW) and SAZ around New Zealand since the LGM. For the first time in this region, we present 230-Thorium normalised fluxes of biogenic opal, carbonate (CaCO3), excess Barium (xsBa), and organic Carbon (Corg). In STW and SAZ, these flux variations show that EP did not change markedly since the LGM. The only exception was a site in the SAZ close to the STF, where we suggest the STF shifted over the core site, driving increased EP. To understand why EP was mostly low and constant we investigated dust deposition changes by measuring lithogenic fluxes at the four sites. These data are coherent with an increased dust deposition in the southwest Pacific during the LGM. Additionally, we infer an increased lithogenic material discharge from erosion and glacier melts during the deglaciation, limited to the Campbell Plateau. Therefore, we propose that even though increased glacial dust deposition may have relieved Fe limitation within the SAZ, the availability of silicic acid (Si(OH)4) limited any resultant increase in carbon export during the LGM. Consequently, we infer low Si(OH)4 concentrations in the SAZ that have not significantly changed since the LGM. This result suggests that both Si(OH)4 and Fe co-limit EP in the SAZ around New Zealand, which would be consistent with modern process studies.

Bulk sediment parameter of three sediment cores from permafrost lowlands, Alaska Arctic Coastal Plain

Arctic lowland landscapes have been modified by thermokarst lake processes throughout the Holocene. Thermokarst lakes form as a result of ice-rich permafrost degradation and they may expand over time through thermal and mechanical shoreline erosion. We studied proximal and distal sedimentary records from a thermokarst lake located on the Arctic Coastal Plain of northern Alaska to reconstruct the impact of catchment dynamics and morphology on the lacustrine depositional environment and to quantify carbon accumulation in thermokarst lake sediments. Short cores were collected for analysis of pollen, sedimentological and geochemical proxies. Radiocarbon and Pb/Cs dating, as well as extrapolation of measured historic lake expansion rates, were applied to estimate a minimum lake age of ~ 1,400 calendar years BP. The pollen record is in agreement with the young lake age as it does not include evidence of the "alder high" that occurred in the region ~ 4.0 cal ka BP. The lake most likely initiated from a remnant pond in a drained thermokarst lake basin (DTLB) and deepened rapidly as evidenced by accumulation of laminated sediments. Increasing oxygenation of the water column as shown by higher Fe/Ti and Fe/S ratios in the sediment indicate shifts in ice regime with increasing water depth. More recently, the sediment source changed as the thermokarst lake expanded through lateral permafrost degradation, alternating from redeposited DTLB sediments, to increased amounts of sediment from eroding, older upland deposits, followed by a more balanced combination of both DTLB and upland sources. The characterizing shifts in sediment sources and depositional regimes in expanding thermokarst lakes were therefore archived in the thermokarst lake sedimentary record. This study also highlights the potential for Arctic lakes to recycle old carbon from thawing permafrost and thermokarst processes.

Clay mineralogy of sediment cores from the South-East Atlantic off Namibia

Independent proxies were assessed in two Late Quaternary sediment cores from the eastern South Atlantic to compare deep-water changes during the last 400 kyr. Two cores were recovered from beneath North Atlantic Deep Water (NADW) at approximately 3 000 m depth. Late Quaternary presence of NADW is indicated by the Cibicidoides wuellerstorfi assemblage on the Walvis Ridge (Core GeoB 1214) and the Bulimina alazanensis assemblage on the Namibian continental slope (Core GeoB 1710). The propagation of NADW is exclusively observed during interglacials, with maximum factor loadings in Stages 1, 5, 7, 9 and 11. These maxima are consistent with peaks in kaolinite/chlorite ratios and maxima of poorly crystalline smectite in the clay-mineral record. Kaolinite and poorly crystalline smectite are products of intense chemical weathering. They are injected into the NADW at low latitudes, north of the study area, and advected south. Chlorite, which is stable under cold weathering regimes, is a characteristic mineral of water masses of southern origin. During glacial stages, it is advected north with Southern Component Water (SCW). Above the NADW/SCW depths, kaolinite/chlorite ratios vary only slightly without a significant glacial-interglacial pattern, as measured in a core (GeoB 1712) from 1 000 m deep on the same profile of the Namibian continental slope off Walvis Bay.