Search

In this study, Arctic Ocean patterns of deep water circulation and sediment provenance during the Last Glacial (LG) were compared with the recent situation by investigating the Nd and Sr isotopic composition ( 143 Nd/ 144 Nd, expressed as e Nd , 87 Sr/ 86 Sr) of the leachable and lithogenic sediment fractions as well as the elemental composition (Mn, Ca, Mg, total inorganic carbon (TIC)) on a spatially extended scale encompassing 11 sediment cores from several Arctic Ocean basins and ridges. The leachable, authigenic signature of the surface sediments implies a North Atlantic deep water source and a late Holocene circulation regime that leads to well-mixed, mostly uniform, e Nd values of − 10.9 to − 10.3. Sediments from the LG have overall more variable authigenic e Nd values (− 13.9 to − 7.4). Lower values specifically occur in the Amerasian and Amundsen Basins, documenting an enhanced flow of deep waters from the Makarov Basin to the Amundsen Basin probably via the Lomonosov Ridge intra-basin. In terms of detrital provenance areas, the lithogenic material deposited at the core locations cannot be unambiguously assigned to distinct source areas based on its radiogenic isotope composition, but represents a diverse mixture. Nevertheless, the Nd and Sr isotope signatures of surface sediments from the Amerasian Basin are dominated by western Arctic sources (e Nd = − 12.8 to − 11.0; 87 Sr/ 86 Sr = 0.7214 to 0.7251), while Eurasian Basin surface sediments are dominated by Eurasian sources (e Nd = − 10.9 to − 9.7, 87 Sr/ 86 Sr = 0.7178 to 0.7299), which is in agreement with previous studies. Bulk element patterns (Ca, Mg, TIC) of the studied sediments support these differences in provenance. The LG samples show the same trends with low e Nd (− 16.1 to − 13.7) and high 87 Sr/ 86 Sr (0.7294) in the Canada Basin, and higher e Nd (− 12.6 to − 8.8) and lower 87 Sr/ 86 Sr (0.7154 to 0.7249) in the Eurasian Basin sediments. However, LG samples have mostly lower and more heterogenous e Nd and 87 Sr/ 86 Sr values than the surface samples. Our geochemical study shows that the overall circulation pattern of the deep water masses during the LG was locally influenced by brine formation but was generally similar to today. The isotopic composition of the lithogenic material from the LG is different from modern times, presumably due to shorter transport pathways of lithogenic material to the Arctic Ocean because of a lower sea level and exposed shelves during the LG. Nevertheless, the lithogenic fraction still represents a mixture from several sources.

Early diagenetic processes play an important role in the environmental cycling of silicic acid (H4SiO4), one of the most important macronutrients in the ocean. Here, we present the first compilation of silicic acid concentrations in sediment pore water sampled in various parts of the Central Arctic Ocean. Based on these data, we calculate benthic fluxes of silicic acid from the sediments to Arctic bottom waters and interpret the pore water profiles in terms of silica dissolution–precipitation reactions deeper within the sediments. Silicic acid concentrations are very low in Arctic pore waters to sediment depths of ~ 8 m, with maxima ranging from 30 μmol/L on the Mendeleev Ridge to 300 μmol/L on the Alpha Ridge. These values are 1–2 orders of magnitude lower than silicic acid pore water concentrations in Atlantic, Indian, and Pacific Ocean sediments, reflecting the low biosilica productivity in the oligotrophic, seasonally ice-covered Arctic Ocean. Low silicic acid pore water concentrations translate into equally low benthic silicic acid fluxes, ranging between 1 and 15 mmol m− 2 a− 1 (1 ∗ 109 to 1.5 ∗ 1010 μmol km− 2 a− 1) at most study sites. With increasing sediment depth, asymptotic patterns typically observed in marine sediments occur at only five of twelve study sites. Deviations from this asymptotic shape at the other sites most likely reflect specific sediment intervals with elevated rates of biosilica dissolution (potentially from underlying Eocene-Cretaceous strata) in association with iron oxide dissolution, or with the consumption of silicic acid by neo-formation of authigenic clay minerals.

[1] Intergrated Ocean Drilling Program Expedition 323 recovered a sediment record covering the last ~4.3 Ma from the Bering Sea (Integrated Ocean Drilling Program Site U1341, Bowers Ridge, 2177 m water depth). To resolve Pliocene-Pleistocene paleoenvironmental changes in this marginal basin, ~190 sediment samples were analyzed for their bulk element composition. Aluminium contents in Bowers Ridge sediments are variable but overall higher toward younger sediments, probably related to the intensification of the Northern Hemispheric Glaciation and increasing sea ice transport in the Bering Sea. The gradual increase of terrigenous input is mirrored by decreasing SiO2 and excess Si (Sixs) contents, but the overall Si enrichment of the deposits reflects continuous opal deposition since the Pliocene at Bowers Ridge. Unlike in the North Pacific, the Sixs record at Site U1341 does not support a dramatic decrease in opal export following the onset of the Northern Hemispheric Glaciation around 2.7 Ma, but SiO2xs have higher accumulation rates (up to ~8 g/cm2/ka) between ~2.6 and ~1.8 Ma BP. During this period, the major oceanic opal deposition centers shifted globally from open marine high latitude regions to upwelling areas. We here discuss how the onset of North Pacific stratification at ~2.7 Ma BP may have caused leakage of nutrient-rich deep/intermediate North Pacific water into the Bering Sea via the deep Kamtchatka Strait, leading to increased opal deposition—and likely reactive organic matter export—at Bowers Ridge. As a result, magnetic and geochemical records were overprinted by intensified diagenesis, significantly affecting their applications as paleoceanographic proxies.

Distinct brown-coloured Mn-rich layers are common features in Quaternary sediments of the central Arctic Ocean, but their paleoenvironmental significance is disputed. Glacial–interglacial changes in bottom water ventilation, variable riverine input, and diagenetic processes have all been suggested to cause the cyclicity in optical and geochemical parameters in these deposits. To better assess the nature and origin of these sediment intervals and their implications for nutrient cycles, we applied bulk geochemical analyses and a sequential Fe extraction scheme to a sediment record covering the last ~ 130 ka on the southern Mendeleev Ridge. All brown layers in this core are enriched in Mn, but also in Fe (oxyhydr)oxides (e.g., ferrihydrite, lepidocrocite, goethite) that contribute to the characteristic sediment colour. Phosphorus and several trace metals (As, Co, Cu, Mo, Ni) are also enriched, indicating that both Mn and Fe (oxyhydr)oxides adsorbed P and trace metals from the water column. The metal enrichments are most probably caused by a combination of enhanced Mn and Fe input from Arctic rivers and/or the erosion of shelf deposits, coupled with early diagenetic processes leading to precipitation of authigenic Mn and Fe (oxyhydr)oxides close to the sediment–water interface. Both primary supply and diagenetic recycling of metals should have been enhanced by a more intense hydrological cycle under warmer climate conditions, combined with seasonal increases in productivity and reactive organic matter export during ice-free conditions in the Central Arctic Ocean. Consequently, the studied metal (oxyhydr)oxide-rich layers were formed under interglacial/interstadial climatic conditions. In contrast, sheet silicate-bound Fe (e.g., chlorite), a second major Fe pool in these sediments (15–35% of total Fe), is enriched between the oxide-rich layers. Thus, the delivery of Fe-rich sheet silicates from the East Siberian hinterland or the Bering Sea, and subsequent deposition on the Mendeleev Ridge, was enhanced by more intense physical weathering during glacial/stadial periods. These contrasts in Fe speciation between the brown layers and the adjacent sediments document the direct response of biogeochemical element cycles to glacial–interglacial variations in river input, weathering regime, and sea level in the Arctic Ocean.

To date, North Sea tidal-flat sediments have been intensively studied down to a depth of 5 m below seafloor (mbsf). However, little is known about the biogeochemistry, microbial abundance, and activity of sulfate reducers as well as methanogens in deeper layers. In this study, two 20 m-long cores were retrieved from the tidal-flat area of Spiekeroog Island, NW Germany. The drill sites were selected with a close distance of 900 m allowing to compare two depositional settings: first, a paleo-channel filled with Holocene sediments and second, a mainly Pleistocene sedimentary succession. Analyzing these cores, we wanted to test to which degree the paleo-environmental imprint is superimposed by present processes. In general, the numbers of bacterial 16S rRNA genes are one to two orders of magnitude higher than those of Archaea. The abundances of key genes for sulfate reduction and methanogenesis (dsrA and mcrA) correspond to the sulfate and methane profiles. A co-variance of these key genes at sulfate-methane interfaces and enhanced ex situ AOM rates suggest that anaerobic oxidation of methane may occur in these layers. Microbial and biogeochemical profiles are vertically stretched relative to 5 m-deep cores from shallower sediments in the same study area, but still appear compressed compared to deep sea sediments. Our interdisciplinary analysis shows that the microbial abundances and metabolic rates are elevated in the Holocene compared to Pleistocene sediments. However, this is mainly due to present environmental conditions such as pore water flow and organic matter availability. The paleo-environmental imprint is still visible but superimposed by these processes.

Salt marsh succession after de-embankment was monitored on the East Frisian barrier island Langeoog by investigating permanent plots. Seventy years after embankment salt marsh plants were once again influenced mainly by the tidal regime. From 2002 to 2004 the former high marsh and glycophytic vegetation died out and was replaced by species of lower salt marsh zones. Nitrophytic halophytes like Suaeda maritima, Atriplex prostrata and Artemisia maritima established first because of the high nutrient content in the soil, a direct result of former vegetation decay. With decreasing nitrogen afterwards other species became more competitive. Until 2007 Atriplex portulacoides became more dominant in the lower marsh and Elymus athericus reached dominance in areas where grazing has been abandoned in the high marsh. The dynamics in the study area is much lower than in natural marshes due to the still existing drainage system. Therefore vegetation units with low species diversity are widespread.

The sulfur isotopic composition of dissolved seawater sulfate is a basic parameter in quantification of fluxes in the global sulfur cycle. It has changed during Earth history but is generally assumed to be constant at a given time due to the long residence time of sulfate in the ocean. Past measurements in the modern ocean, however, have shown an unexpected variability. Since the known marine sedimentary record essentially reflects continental margin processes, we have investigated for the first time the isotopic composition of dissolved sulfate of a marginal sea, the North Sea by means of stable-isotope-ratio-monitoring mass spectrometry. Sampling took place in the water column at 30 stations during two seasons (early spring 1996 and summer 1997). Stable isotope results are compared to measurements of salinity, temperature, and phytoplankton biomass. Except for one profile, the isotopic composition was found to be essentially constant at both the spring ( δ 34 S = + 20.9 ± 0.13‰; n = 131; versus V-CDT) and the summer campaign ( δ 34 S = + 21.1 ± 0.13‰; n = 59). The mean composition of North Sea water sulfate of δ 34 S = + 21‰ is within 0.2‰ of δ 34 S values found for essentially unpolluted coastal waters collected at different stations worldwide (Atlantic and Pacific Ocean) and the water column of the modern open ocean.

A geochemical and stable isotope geochemical study was carried out on sediments and pore waters from the Strait of Sicily at the connection between the western and the eastern Mediterranean Sea (ODP Site 963). Organic-matter-rich layers (ORLs), which are proposed to be time-equivalent to eastern Mediterranean sapropels, only occur in sediments older than 550 kyr, most likely because of a strong dilution of late Pleistocene and Holocene sediments by terrigenous material. The ORLs have total organic carbon (TOC) contents of up to 1.6% and are enriched in pyrite and trace metals. TOC/pyrite ratios indicate the at least temporary presence of an anoxic water column during sediment formation, which is in agreement with the observed sulfur isotope discrimination. These conditions induced the enrichment in sulfide-forming and redox-sensitive trace elements. Trace metal enrichment is lower than at other sites in the Mediterranean Sea, which may be attributed to a less reducing or a less stable anoxic environment that was less favorable for the accumulation of redox-sensitive trace elements and organic matter. However, during sapropel events sedimentary conditions in the Strait of Sicily, based on manganese and barium geochemistry, must have been largely similar to those at other sites in the Mediterranean Sea. According to elemental, Rock-Eval, and stable carbon isotope analyses the organic matter in the ORLs is mainly of marine origin, which appears to have been overprinted significantly during diagenesis. A significant part of the organic matter in sediments from ODP Site 963 may also originate from uplifted land sections eroded during weathering and may primarily also be of marine origin. The results of pore water analyses illustrate the presence of an active deep biosphere in the sediments of Site 963, reflected for example by sulfate, methane and iodine concentrations. A brine, probably originating from underlying Messinian evaporites, was detected, but is only of minor influence.

[1] Integrated Ocean Drilling Program (IODP) Expedition 302 (Arctic Coring Expedition, ACEX) recovered a unique sediment record from the central Arctic Ocean, revealing that this region underwent major environmental fluctuations since the Late Cretaceous. Major and trace element composition of 1,300 samples were determined using X-ray fluorescence (XRF). The results show significant compositional variability of the sediments with depth that can be attributed to changes in (a) provenance and pathways of detrital material, (b) paleoenvironmental conditions and depositional processes, and (c) diagenetic overprint of the primary record. In addition to existing lithological units, we introduce new geochemical units for a more process-related approach interpreting the ACEX record. In detail, via the geochemical signature of Siberian flood basalts we are able to reconstruct the discontinuous rifting and deepening of the central Lomonosov Ridge during the Paleogene, accompanied by changing current regimes and the onset of sea ice. Eocene biosiliceous sedimentation took place in a relatively shallow setting under predominantly anoxic bottom water conditions, causing a positive anoxia-productivity feedback, although water column stratification was repeatedly interrupted by ventilation events. Anoxic to sulfidic conditions were even more extreme after biosilica production ceased, and significant amounts of pyrite were deposited on the Lomonosov Ridge. Especially in organic matter-rich Paleogene deposits, diagenetic processes obscured the paleoenvironmental signals. Fundamental environmental changes occurred in the Middle Eocene, but geochemical and micropaleontological proxies point not to the identical sediment depth. After approximately 26 Ma of non-deposition or erosion, the Middle Miocene record shows the transition to dominantly oxic bottom water conditions, although suboxic diagenesis seemingly affected these deposits.

Drilling results from the Integrated Ocean Drilling Program’s Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown.

The Cenozoic record of the Lomonosov Ridge (central Arctic Ocean) recovered during Integrated Ocean Drilling Program (IODP) Expedition 302 revealed an unexpected 26 Ma hiatus, separating middle Eocene (�44.4 Ma) from lower Miocene sediments (�18.2 Ma). To elucidate the nature of this unconformity, we performed a multiproxy palynological (dinoflagellate cysts, pollen, and spores), micropaleontological (siliceous microfossils), inorganic, and organic (Tetra Ether Index of lipids with 86 carbon atoms (TEX86) and Branched and Isoprenoid Tetraether (BIT)) geochemical analysis of the sediments from �5 m below to �7 m above the hiatus. Four main paleoenvironmental phases (A–D) are recognized in the sediments encompassing the unconformity, two below (A–B) and two above (C–D): (A) Below the hiatus, proxies show relatively warm temperatures, with Sea Surface Temperatures (TEX86-derived SSTs) of about 8�C and high fresh to brackish water influence. (B) Approaching the hiatus, proxies indicate a cooling trend (TEX86-derived SSTs of �5�C), increased freshwater influence, and progressive shoaling of the Lomonosov Ridge drilling site, located close to or at sea level. (C) The interval directly above the unconformity contains sparse reworked Cretaceous to Oligocene dinoflagellate cysts. Sediments were deposited in a relatively shallow, restricted marine environment. Proxies show the simultaneous influence of both fresh and marine waters, with alternating oxic and anoxic conditions. Pollen indicates a relatively cold climate. Intriguingly, TEX86-derived SSTs are unexpectedly high, �15–19�C. Such warm surface waters may be partially explained by the ingression of warmer North Atlantic waters after the opening of the Fram Strait during the early Miocene. (D) Sediments of the uppermost interval indicate a phase of extreme oxic conditions, and a well-ventilated environment, which occurred after the complete opening of the Fram Strait. Importantly, and in contrast with classical postrifting thermal subsidence models for passive margins, our data suggest that sediment erosion and/or nondeposition that generated the hiatus was likely due to a progressive shoaling of the Lomonosov Ridge. A shallow water setting both before and after the hiatus suggests that the Lomonosov Ridge remained at or near sea level for the duration of the gap in the sedimentary record. Interacting sea level changes and/or tectonic activity (possibly uplift) must be invoked as possible causes for such a long hiatus.

[1] The Late Jurassic to Early Cretaceous (Volgian-Ryazanian) was a period of a second-order sea-level low stand, and it provided excellent conditions for the formation of shallow marine black shales in the NorwegianGreenland Seaway (NGS). IKU Petroleum Research drilling cores taken offshore along the Norwegian shelf were investigated with geochemical and microscopic approaches to (1) determine the composition of the organic matter, (2) characterize the depositional environments, and (3) discuss the mechanisms which may have controlled production, accumulation, and preservation of the organic matter. The black shale sequences show a wide range of organic carbon contents (0.5–7.0 wt %) and consist of thermally immature organic matter of type II to II/III kerogen. Rock-Eval pyrolysis revealed fair to very good petroleum source rock potential, suggesting a deposition in restricted shallow marine basins. Well-developed lamination and the formation of autochthonous pyrite framboids further indicate suboxic to anoxic bottom water conditions. In combination with very low sedimentation rates it seems likely that preservation was the principal control on organic matter accumulation. However, a decrease of organic carbon preservation and an increase of refractory organic matter from the Volgian to the Hauterivian are superimposed on short-term variations (probably reflecting Milankovitch cycles). Various parameters indicate that black shale formation in the NGS was gradually terminated by increased oxidative conditions in the course of a sea-level rise. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 4802 Oceanography: Biological and Chemical: Anoxic environments; 9609 Information Related to Geologic Time: Mesozoic; 8105 Tectonophysics: Continental margins and sedimentary basins (1212); KEYWORDS: black shale formation, depositional environment, Norwegian-Greenland Seaway, Mesozoic, organic petrography Citation: Langrock, U., R. Stein, M. Lipinski, and H.-J. Brumsack, Late Jurassic to Early Cretaceous black shale formation and paleoenvironment in high northern latitudes: Examples from the Norwegian-Greenland Seaway, Paleoceanography, 18(3), 1074, doi:10.1029/2002PA000867, 2003.