Paleoceanographic Changes Research Articles

Methane release from the southern Brazilian margin during the last glacial

Seafloor methane release can significantly affect the global carbon cycle and climate. Appreciable quantities of methane are stored in continental margin sediments as shallow gas and hydrate deposits, and changes in pressure, temperature and/or bottom-currents can liberate significant amounts of this greenhouse gas. Understanding the spatial and temporal dynamics of marine methane deposits and their relationships to environmental change are critical for assessing past and future carbon cycle and climate change. Here we present foraminiferal stable carbon isotope and sediment mineralogy records suggesting for the first time that seafloor methane release occurred along the southern Brazilian margin during the last glacial period (40–20 cal ka BP). Our results show that shallow gas deposits on the southern Brazilian margin responded to glacial−interglacial paleoceanographic changes releasing methane due to the synergy of sea level lowstand, warmer bottom waters and vigorous bottom currents during the last glacial period. High sea level during the Holocene resulted in an upslope shift of the Brazil Current, cooling the bottom waters and reducing bottom current strength, reducing methane emissions from the southern Brazilian margin.

Astronomically paced changes in deep-water circulation in the western North Atlantic during the middle Eocene

North Atlantic Deep Water (NADW) currently redistributes heat and salt between Earth's ocean basins, and plays a vital role in the ocean-atmosphere CO2 exchange. Despite its crucial role in today's climate system, vigorous debate remains as to when deep-water formation in the North Atlantic started. Here, we present datasets from carbonate-rich middle Eocene sediments from the Newfoundland Ridge, revealing a unique archive of paleoceanographic change from the progressively cooling climate of the middle Eocene. Well-defined lithologic alternations between calcareous ooze and clay-rich intervals occur at the ∼41-kyr beat of axial obliquity. Hence, we identify obliquity as the driver of middle Eocene (43.5–46 Ma) Northern Component Water (NCW, the predecessor of modern NADW) variability. High-resolution benthic foraminiferal δ18O and δ13C suggest that obliquity minima correspond to cold, nutrient-depleted, western North Atlantic deep waters. We thus link stronger NCW formation with obliquity minima. In contrast, during obliquity maxima, Deep Western Boundary Currents were weaker and warmer, while abyssal nutrients were more abundant. These aspects reflect a more sluggish NCW formation. This obliquity-paced paleoceanographic regime is in excellent agreement with results from an Earth system model, in which obliquity minima configurations enhance NCW formation.

Lower Sinemurian - upper Pliensbachian smaller agglutinated foraminiferal events from the eastern part of the Pieniny Klippen Belt (Transcarpathian Ukraine, Western Carpathians)

Diverse and well-preserved foraminifera are documented from the Sinemurian to Pliensbachian deposits from the Priborzhavske-Perechin Zone in the Ukrainian segment of the Pieniny Klippen Belt. The changes in the smaller agglutinated foraminiferal assemblages are compared with the high resolution ammonite biozonation. The foraminifera are observed in four main facies. In biodetritic marls of the Bucklandi - ?Semicostatum zones a response to increasing low-oxic conditions is recorded. Abloom of small Haplophragmoides spp. predates a barren interval of black shales probably in the Semicostatum Zone that in turn precedes an interval of barren black clays in the Turneri Zone. Above this event the FO of Textularia aff. agglutinans is noted. In the lower part of the Raricostatum Zone a bloom of agglutinated foraminifera and subboreal influx is documented. A rapid decrease in agglutinated foraminifera precedes the Sinemurian/Pliensbachian boundary and continues in the early Pliensbachian Jamesoni Zone. Unfavourable conditions for the agglutinated foraminifera culminate in the Ibex – lower Margaritatus zones and represent an interval barren of agglutinated foraminifera. Impoverished assemblages of agglutinated foraminifera reappear higher in the Margaritatus Zone. The paleoecological interpretations based on morphogroup analysis and the relative abundances of agglutinated foraminifera cast light on paleoclimatic and paleoceanographic changes in the Sinemurian-Pliensbachian stratigraphic record of the eastern part of the Pieniny Klippen Belt.

Surface water productivity and sediment transport by Bering Strait throughflow in the Chukchi Shelf (the western Arctic Ocean) during the Holocene

Diverse paleoceanographic proxies from three sediment cores (GC12ex, JPC35, and JPC30) collected from the Chukchi Shelf north of the Bering Strait elucidate the Holocene paleoceanographic changes (surface water productivity and sediment transport) caused by the Bering Strait throughflow from the Bering Sea into the Chukchi Sea. Lithology of three sediment cores identified the same three units. Based on comparison and correlation to adjacent age-dated cores as well as AMS 14C dates of core GC12ex, the boundary between Unit 1 and Unit 2a is dated about 8500 cal. yr BP, and the boundary between Unit 2a and Unit 2b is also dated about 4500 cal. yr BP. Consistent down-core profiles of the geochemical and isotopic properties among the three cores differentiate the paleoceanographic conditions corresponding to lithologic units. Based on the biogenic opal, total organic carbon, and δ13C values, Unit 1 is characterized by low surface water marine productivity under relatively shallow water with weak transport of Bering Strait throughflow. Unit 2a shows a mixture of terrestrial and marine contributions, indicating the onset of increased marine surface water productivity after the main flooding (~11,500 cal. yr BP) of the Bering Strait by the Holocene sea-level rise. Unit 2b exhibits stable and enhanced marine biogenic opal production similar to the present-day oceanographic conditions. Such paleoceanographic changes were confirmed by the clay minerals (smectite, illite, kaolinite, and chlorite) and detrital isotopes (εNd and 87Sr/86Sr). Thus, the Bering Strait throughflow played an important role on surface water productivity and sediment deposition in the Chukchi Shelf in response to Holocene sea-level rise after the opening of the Bering Strait.

Influence of dominant wind patterns in a distal region of the NW Iberian Margin during the last glaciation

This work combines planktonic foraminiferal assemblages, stable isotope, transfer function estimated sea surface paleotemperatures (SST) and export productivity (Pexp) with sedimentological records from the PC7 core to document paleoclimatic and paleoceanographic changes on a distal region of the NW Iberian Margin since the last glaciation (<≈68 ka). Neodymium and strontium isotope data are also used to identify the provenance of ice rafted debris (IRD) during Heinrich event 5 (H5). Our results are compared and combined with previously published records and evidence that melt water and icebergs from the Canada, Greenland, Iceland and Europe ice sheets arrived to the NW Iberian Margin causing a significant decrease of SST during the last six Heinrich Stadials (HS) and some Dansgaard–Oeschger (D-O) stadial events. εNd and 87 Sr/ 86 Sr indicate that the studied locality mostly received sediments from European Ice Sheets (EIS) during H5. During the HS and D–O stadial events, SSTs were in general higher than in other cores collected in inner areas of the NW Iberian Margin. The presence and intensification of Portugal Current probably induced heat transfer to this distal region of the Iberian Margin. Our results also suggest the occurrence of high variability in the intensity and wind patterns during the last glaciation. In some periods, the intensity of northerly winds could have been very strong and contributed to a significant stimulation of ocean productivity in this distal area through the intensification of upwelling. In general, oceanic productivity decreased during the HS and D–O as the presence of cold and less saline water prevented nutrient rich waters from deep levels reaching the surface. The high productivity events were bracketed by periods characterized by the highest influence of the Azores Current eastern branch during which northerly winds and the oceanic productivity should have been weakened. Supplementary material: Geochemical and foraminiferal data analysed in this work as well as Pearson Correlations between the analysed data are available at https://doi.org/10.6084/m9.figshare.c.3931813

Sea-level and surface-water change in the western North Atlantic across the Oligocene–Miocene Transition: A palynological perspective from IODP Site U1406 (Newfoundland margin)

Abstract The Oligocene–Miocene transition (OMT; ~ 23.1 Ma) terminates the late Oligocene warming trend and is marked by a transient, large-amplitude expansion of Antarctic ice sheets. The associated glacial maximum, which is expressed by a ~ 1‰ positive shift in benthic foraminiferal oxygen-isotope values, is commonly referred to as the ‘Mi-1 isotope event’. Whereas the causes for the glacial maximum at the OMT are intrinsically connected to Southern Hemisphere ice-sheet dynamics, the behavior of the surface ocean in the Northern Hemisphere during this time is poorly known. To contribute to a better understanding of the paleoceanographic evolution during the OMT in the higher-latitude North Atlantic, we have analysed both marine and terrestrial palynomorphs from Integrated Ocean Drilling Program (IODP) Site U1406 offshore Newfoundland; this site has yielded a complete OMT section and exhibits a high-quality magnetostratigraphy that provides precise age control and allows reliable correlation to other records beyond Newfoundland. Our palynological data, which span the interval from 23.3 to 22.5 Ma and have a mean temporal resolution of 11.9 kyrs, show strong ~ 110-kyr eccentricity-paced oscillations during the earliest Miocene; these oscillations are in phase with similar cyclicity identified in previously published benthic foraminiferal oxygen-isotope records. More specifically, a pronounced sea-level variability is documented by the abundances of neritic dinoflagellate cysts (dinocysts) and terrigenous palynomorphs, which both reach maxima during peak glacial intervals as inferred from previously published South Atlantic benthic oxygen-isotope data. A decline in the abundance of warmer-water dinocysts suggests a surface-water cooling offshore Newfoundland from the latest Oligocene onwards. Surface-water productivity (as derived from the ratio between heterotrophic and autotrophic dinocysts) remained generally low throughout the studied interval. Notably, this ratio does not exhibit any correlation with changes in surface-water temperature, which is estimated from the ratio of warm-water over cold-water dinocysts. Together with the consistently low surface-water productivity, the lack of a correlation between surface-water productivity and temperature makes it highly unlikely that the observed paleoceanographic change was caused by a southward migration of the Arctic Front. Instead, we argue that our data may document an enhanced influence of the (Proto-) Labrador Current on surface waters offshore Newfoundland during the earliest Miocene that suppressed the influence of the Gulf Stream in this region of the Northwest Atlantic. We speculate that the enhanced influence of the (Proto-) Labrador Current was triggered by cooling of the northern hemisphere and possibly modulated by high-latitude sea-ice expansion.

Central Arctic Ocean paleoceanography from ∼ 50 ka to present, on the basis of ostracode faunal assemblages from the SWERUS 2014 expedition

Abstract. Late Quaternary paleoceanographic changes at the Lomonosov Ridge, central Arctic Ocean, were reconstructed from a multicore and gravity core recovered during the 2014 SWERUS-C3 Expedition. Ostracode assemblages dated by accelerator mass spectrometry (AMS) indicate changing sea-ice conditions and warm Atlantic Water (AW) inflow to the Arctic Ocean from ∼ 50 ka to present. Key taxa used as environmental indicators include Acetabulastoma arcticum (perennial sea ice), Polycope spp. (variable sea-ice margins, high surface productivity), Krithe hunti (Arctic Ocean deep water), and Rabilimis mirabilis (water mass change/AW inflow). Results indicate periodic seasonally sea-ice-free conditions during Marine Isotope Stage (MIS) 3 ( ∼ 57–29 ka), rapid deglacial changes in water mass conditions (15–11 ka), seasonally sea-ice-free conditions during the early Holocene ( ∼ 10–7 ka) and perennial sea ice during the late Holocene. Comparisons with faunal records from other cores from the Mendeleev and Lomonosov ridges suggest generally similar patterns, although sea-ice cover during the Last Glacial Maximum may have been less extensive at the new Lomonosov Ridge core site ( ∼ 85.15° N, 152° E) than farther north and towards Greenland. The new data provide evidence for abrupt, large-scale shifts in ostracode species depth and geographical distributions during rapid climatic transitions.

Correlation between faunal transitions of benthic foraminifera and ballasting of particulate organic carbon by siliceous plankton during the Holocene off San-in district, southwestern Japan

We investigated the biotic response of benthic foraminifera (Rhizaria) to Holocene paleoceanographic changes in a upper bathyal environment by examining fossil benthic foraminifera younger than ~13...

Semi‐quantitative reconstruction of early to late Holocene spring and summer sea ice conditions in the northern Barents Sea

ABSTRACTSemi‐quantitative estimates of early to late Holocene spring sea ice concentration (SpSIC) and occurrence of summer sea ice for the northern Barents Sea have been obtained by analysing the biomarkers IP25, brassicasterol and a tri‐unsaturated highly branched isoprenoid lipid in a Holocene marine sediment core. Sub‐surface water mass variations were derived from planktic foraminiferal assemblages and stable isotopes (δ18O, δ13C). The record indicates paleoceanographic changes over three intervals. During Period I (ca. 9500–5900 cal a BP), the study location experienced the lowest recorded SpSIC (ca. 25%) with short spring seasons and long productive summers, resulting partly from increased Atlantic Water inflow that caused a stronger ocean–atmosphere heat exchange. Throughout Period II (ca. 5900–2700 cal a BP), the winter sea ice margin migrated southwards and an overall cooling trend resulted in higher SpSIC (ca. 60%) and increased delivery of cold Arctic Water. During Period III (ca. 2700 cal a BP to present), SpSIC increased further (ca. 75%) and some sea ice remained during summer months. A sub‐surface warming probably indicates a decoupling of heat exchange between the ocean and the atmosphere. Longer springs and shorter summers were accompanied by the most southerly location of the winter sea ice margin.

Local glaciation in West Greenland linked to North Atlantic Ocean circulation during the Holocene

Abstract Recent observations indicate that ice-ocean interaction drives much of the recent increase in mass loss from the Greenland Ice Sheet; however, the role of ocean forcing in driving past glacier change is poorly understood. To extend the observational record and our understanding of the ocean-cryosphere link, we used a multi-proxy approach that combines new data from proglacial lake sediments, 14C-dated in situ moss that recently emerged from beneath cold-based ice caps, and 10Be ages to reconstruct centennial-scale records of mountain glacier activity for the past ∼10 k.y. in West Greenland. Proglacial lake sediment records and 14C dating of moss indicate the onset of Neoglaciation in West Greenland at ca. 5 ka with substantial snowline lowering and glacier expansion at ca. 3.7 ka followed by additional ice expansion phases at ca. 2.9, ca. 1.7, and ca. 1.4 ka and during the Little Ice Age. We find that widespread glacier growth at ca. 3.7 ka in West Greenland coincides with marked cooling and reduced strength of the West Greenland Current in Disko Bugt. The transition to cooler ocean conditions at ca. 3.7 ka identified in Disko Bugt is registered by marine proxy data farther afield in East Greenland and on the northwestern Icelandic shelf, implying large-scale paleoceanographic changes across the North Atlantic during this interval. The similarity between glacier change on West Greenland and multiple marine and terrestrial records across the North Atlantic suggests that glaciers are strongly influenced by changes in ocean circulation and consequently implies that the ocean-cryosphere teleconnection is a persistent feature of the Arctic system.

How dry was the Mediterranean during the Messinian salinity crisis?

The Messinian salinity crisis (MSC; 5.97–5.33Ma) is an enigmatic episode of paleoceanographic change, when kilometers-thick evaporite units were deposited in the Mediterranean basin. It is generally accepted that during the MSC interval there was a dry climate in the Mediterranean region. It is difficult to assess how dry the climate was during the MSC because a modern analogue, in size and duration, is absent. Here we reconstruct hydrological changes in the Mediterranean basin during the three main MSC stages using excellently preserved biomarkers. We used the hydrogen isotopic composition of the long chain n-alkanes (δDn-alkanes) to reconstruct the hydrological changes on the land adjacent to the Mediterranean Sea. Additionally, the δD of long-chain alkenones (δDalkenones) is used to observe changes in the Mediterranean Sea water source. The δDn-alkanes recorded during the deposition of Primary Lower Gypsum (stage 1) in Monte Tondo indicate a δD of the precipitation comparable to the present-day Mediterranean implying a similar hydrologic regime (indicated by experiments modelling the Miocene-Pliocene transition). Elevated δDalkenones values from halite unit (stage 2) of the Realmonte mine are associated with kainite and giant polygons, consistent with presumably high evaporative conditions during halite deposition. The δDn-alkanes recorded during the deposition of Upper Gypsum (stage 3) in Eraclea Minoa indicate a δDprecipitation typical for much drier settings, similar to the Red Sea region. The relative contribution of the different alkenones from Eraclea Minoa is similar to the one observed in present-day marine settings suggesting that, during stage 3, connections to the open Ocean were likely maintained. However, the δDalkenones records during deposition of the evaporites in Eraclea Minoa are similar to those synchronously registered in the Black Sea implying that a similar hydrologic regime, characterized by extended drought, covered large areas of southeastern Europe. Based on the δDalkenones similarity and the Paratethys type of ´Lago Mare´ fauna in the Mediterranean we speculate that the surface water during stage 3 was, at times, derived from the Black Sea.

The responses of benthic foraminifera to paleoceanographic changes during the last glacial maximum, deglaciation, and the Holocene in the northwestern Pacific

Based on the changes in the species composition, abundance, and species richness of benthic foraminiferal assemblages in sediments of the northwestern Pacific, we reconstructed the paleoenvironment (paleoproductivity and near-bottom water ventilation) during the period of 8000–19000 calendar years ago (ka). A moderate seasonal productivity of the surface water, a relatively high oxygen content of the near-bottom water, and the influence of sea ice occurred during the Last Glacial Maximum in the studied area. An increased primary production and suboxic conditions in the near-bottom water were typical of the cold Heinrich 1 event (17.5–14.7 ka). A further increase in surface-water productivity and a decrease in oxygen content of the near-bottom and pore waters have been reconstructed for the second half of the Bolling/Allerod interstadial (14.7–12.8 ka) and the Pre-Boreal (11.8–9.2 ka). Surface water productivity decreased and oxygen saturation of the near-bottom water increased during the Younger Dryas (12.8–11.8 ka). In the Middle Holocene, the total decrease in productivity led to an increased oxygen content of the near-bottom and pore waters.

Holocene paleoceanography of the Bay of Biscay: Evidence for west-east linkages in the North Atlantic based on dinocyst data

Paleoceanographical changes during the Holocene were reconstructed from the study of core MD95-2002 situated in the northern Bay of Biscay, which is marked by the direct influence of the northeastern return branch of the North Atlantic Drift. Palynological data, sea-surface condition estimates based on dinocyst assemblages and stable isotope measurements in planktic and benthic foraminifera reveal a strong influence of freshwater/meltwaters from both the proximal European sources and the more distal Laurentide Ice Sheet, which experienced delayed deglaciation. The data also indicate the setting of a climate optimum between 7 and 5.5ka followed by a cooling trend, which is consistent with insolation changes and other regional records of climate changes. Superimposed on the long term trends, the reconstructions of sea-surface conditions evidence large amplitude changes at centennial to millennial time-scales, with seven episodes of cooling and low salinity since 11ka that generally match episodes of dense sea-ice cover in the Labrador Sea. The west to east transfer of the sea-ice and/or meltwater signal across the North Atlantic evidenced from core MD95-2002 points to strong linkages between western and eastern North Atlantic, probably in relation to the North Atlantic Oscillation (NAO) mode of variability.

Paleoceanographic changes in the Northeast Indian Ocean during middle Miocene inferred from carbon and oxygen isotopes of foraminiferal fossil shells

The middle Miocene transition witnessed rapid fluctuations in oceanographic and climatic systems. However, very limited work has been attempted in Miocene sediments of the Northern Indian Ocean to identify these changes. Here we present the first detailed δ13C and δ18O records of surface and deep dwelling foraminifera from ODP site 758 in the Northeast Indian Ocean, located atop the Ninetyeast Ridge. Carbon and oxygen isotopic data on Globigerinoides quadrilobatus (planktonic) and Cibicidoides wuellerstorfi (benthic foraminifera) indicated various global and local successions of oceanographic and climatic importance. The climatic events recorded at ODP 758 are; (a) Mid Miocene climatic Optimum (17–15 Ma), (b) Monterey excursion (17–14 Ma), (c) East Antarctica Ice sheet formation (13.8 Ma), (d) Initiation of Indian Summer Monsoon with waning of Antarctica Ice sheet (12.3–10.4 Ma), and (e) evidence of a cooling event (10.2–9.6Ma).Our benthic foraminifera δ18O record showed almost identical deep water evolution to that of the Indo Pacific for 13.9–11.5 Ma interval. A significant decrease in δ13Cben at ~10.2 Ma was indicative of a complete re-adjustment of the carbon cycle after the Monterey excursion and export of the North Atlantic Deep Water (NADW) towards the southern Ocean to give rise to the Circumpolar Deep Water (CDW), with a δ13C value of ~0.4‰. The formation of the NADW had given rise to modern deep water like conditions at ~10 Ma ago. A comparison of our record with previously published isotopic records from other regions suggested a good correlation between the South Pacific, Indo Pacific and Northeast Indian Ocean during the studied time interval. It appears that a strong westerly Indian Ocean current (MIOjet) may have linked these Oceans from 14 to 9 Ma.

Low‐frequency Pliocene climate variability in the eastern Nordic Seas

AbstractThe Pliocene (5.3–2.6 Ma) is often described as a relatively stable climatic period, with warm temperatures characterizing high latitudes. New suborbital resolved stable isotope records from Ocean Drilling Program Hole 642B in the eastern Nordic Seas document that the Pliocene was not a stable period characterized by one climate. Rather, seven distinct climate phases, each lasting between 150,000 and 400,000 years, are identified and characterized in the time interval 5.1–3.1 Ma. Four of the transitions between the defined climate phases occurred close to an eccentricity minimum and a minimum in amplitude of change for Northern Hemisphere summer insolation, while two occurred around an eccentricity maximum and a maximum in amplitude in insolation change. Hence, a low‐frequency response of the Nordic Seas to insolation forcing is indicated. In addition, paleogeographic and related paleoceanographic changes, expansion of the Arctic sea ice cover, and onset of Northern Hemisphere glaciation were important factors behind the evolving Pliocene low‐frequency variability in the eastern Nordic Seas. It is likely that the identified climate phases and transitions are important beyond the Nordic Seas, due to their association with changes to both insolation and paleogeography. However, a strong and variable degree of diagenetic calcite overgrowth is documented for the planktic foraminifera, especially influencing the planktic δ18O results; the absolute values and amplitude of change cannot be taken at face value.

Climatic evolution of the central equatorial Pacific since the Last Glacial Maximum

AbstractThis paper investigates paleoceanographic changes at a central equatorial Pacific site (6°40′N, 177°28′W) since the last glacial maximum using planktic foraminifera assemblages, together with the oxygen isotope (δ18O) and Mg/Ca compositions of three species (Globigerinoides sacculifer, Pulleniatina obliquiloculata, and Globorotalia tumida) that dwell in the mixed layer, upper thermocline, and lower thermocline, respectively. While the Mg/Ca‐derived temperatures of the mixed layer and lower thermocline varied within a narrow range from 18 ka onward, the upper thermocline temperature increased by as much as 3°C during the last deglaciation (18–12 ka) with a simultaneous decrease of δ18O. These changes are best explained by an enhanced mixing of the upper ocean and a reduced habitat depth separation between P. obliquiloculata and G. sacculifer during the 18–12 ka interval. The planktic foraminifera assemblage during the same period resembles modern composition at subtropical central Pacific sites that are strongly influenced by the northeasterly Trades and North Equatorial Current (NEC). We suggest that the study site, presently under the control of the Intertropical Convergence Zone (ITCZ)‐North Equatorial Countercurrent, had been influenced by the northeasterly Trades and NEC during the 18–12 ka interval. This interpretation is consistent with previous documentation of a more southerly location of the ITCZ during two Northern Hemisphere cooling events; the Heinrich Stadial 1 and the Younger Dryas, and implies that the mean annual position of the ITCZ was located south of the study site, by at least 2° of latitude.

The Eocene-Oligocene climate transition in the Central Paratethys

We studied two boreholes (Cserépváralja-1 and Kiscell-1) with continuous sedimentary records across the Eocene-Oligocene climate transition from the Central Paratethyan area. Assemblages of benthic foraminifera display a shift in dominance by epifaunal taxa in the late Eocene to shallow and deep infaunal taxa in the early Oligocene. Using the benthic foraminiferal oxygen index (BFOI), a decreasing trend of bottom-water oxygen levels is established across the Eocene-Oligocene transition (EOT), leading to the development of dysoxic conditions later in the early Oligocene.Trends in δ18O and δ13C values measured on tests of selected benthic and planktic foraminifera roughly parallel those of the global record of stepped EOT δ18O increase and deviate only later in the early Oligocene, related to the isolation of the Paratethys. The overall similarity of the isotope curves and the presence of a planktic-benthic ecological offset suggest that the original isotope trends are preserved, despite the systematically more negative δ18O values. Of different scenarios, a quasi-uniform diagenetic overprint by fluids with low δ18O values, during burial or uplift, appears best supported. We conclude that the globally established isotopic expression of Antarctic ice sheet growth across the EOT may be recognizable in the Paratethys. Deviations from the global trends after the EOT were caused by regional paleoceanographic changes induced by the progressing Alpine orogeny and sea-level change, which led to a restricted connection with the open ocean, freshwater influx from increased precipitation, and gradual development of bottom-water oxygen depletion.

Submarine erosional troughs in the northern South China Sea: Evidence for Early Miocene deepwater circulation and paleoceanographic change

The paleoceanography of the South China Sea (SCS) is poorly understood, although recent cruises and an ODP expedition (Leg 184) have provided a useful general framework. Here we report previously unrecognized multi-km scale sedimentary structures consisting of coupled ridges and troughs within the Early Miocene strata, using high-resolution 3D seismic data from the northern SCS margin. The troughs are interpreted as linear and/or arcuate erosional features which truncate the underlying strata. They range in orientation from E-W in the southern part of the study area to nearly NE-SW in the northwestern part of the study area. The spacing of erosional troughs varies from less than 0.1 km in the southern part to more than 3 km in the northern part, and they are about 50–1500 m in width, with a relief of 10–90 m. The erosional troughs are parallel to the regional strike of the Early Miocene slope of the northern SCS. From their internal geometry, context, regional distribution and most importantly their orientation relative to the paleo-slope gradient at the time of their formation, it is argued that they formed due to the action of bottom currents linked to the broader paleo-circulation regime in the basin. We infer that the paleo-bottom currents intruding from the Pacific flowed along the northern SCS slope, resulting in the occurrence of localized erosional troughs as deep water masses impinged on and interacted with the sea floor topography. This study provides new insights into the Early Miocene paleoceanography of the SCS and demonstrates the utility of 3D seismic data to enhance our understanding of paleo-bottom current circulation regimes.

Paleoceanographic changes through the last 130 ka in the western South Atlantic based on planktonic Foraminifera

This work reconstructs Late Quaternary paleoceanographic changes in the western South Atlantic Ocean based on sedimentary core GL-77, recovered from the lower continental slope in the Campos basin, off shore SE Brazil. The studied interval comprises the last 130 ka. Changes in sea surface temperature (SST) and paleoproductivity were estimated using the total planktonic foraminiferal fauna and oxygen isotope analyses. The age model was based on the oxygen isotope record, biostratigraphic datums and AMS 14C dating. It was observed that the Pleistocene/Holocene transition occurs within Globorotalia menardii Biozone Y, and is not coeval with the base of Biozone Z. The range between summer and winter SST estimates is larger during the glacial period compared to interglacials. Three peaks of low SST around 70, 50-45 and 20 ka coincided with periods of enhanced SE trade winds. Despite faunal diff erences between the last interglacial (MIS 5e) and the Holocene, our SST estimates suggest that SSTs did not diff er signifi cantly between these intervals.

Phosphorite deposits on the Namibian shelf

Phosphorite deposits provide a source of fertilizer to feed an increasingly populated world and they provide a record of paleoceanographic changes in upwelling systems linked to climate. The Benguela Upwelling System (BUS) is among the most productive today and is associated with major phosphorite deposits exposed over an area of 24,700km2 on the Namibian shelf. Analyses of cores associated with recent offshore mineral exploration provide new insights into the age and origin of these phosphorite deposits. The deposit consists of coarsening upward muddy to gravelly pelletal phosphorite sand, up to several meters thick, on the middle to outer shelf (190 and 350m water depth) offshore of Lüderitz and Walvis Bay. Less extensive and less continuous deposits occur offshore of Walvis Bay as far north as the Kunene River mouth on the inner to middle shelf (50 and 250m water depth). Pelletal phosphorite sand (some concentrically banded) and concretionary phosphorite pebbles are the dominant grain types consisting of up to 90wt.% carbonate fluorapatite (francolite) cement and inclusions of organic matter, pyrite and terrigenous mud. Strontium isotope stratigraphy and foraminiferal biostratigraphy indicate that phosphogenesis was initiated in the latest Miocene but that most phosphorite formed in the Plio/Pleistocene during early burial diagenesis of organic-rich mud as it does today in the Holocene diatomaceous mud belt. The highly-condensed, coarsening-upward succession reflects increasingly high-amplitude Pleistocene sea-level fluctuations. Phosphorite formation correlates to terrestrial aridification as well as to marine proxies of intensified coastal upwelling in the 600m thick equivalent successions on the upper slope. Repeated phosphorite formation and reworking over Pleistocene glacial to interglacial cycles resulted in the economic concentration of phosphorite, with an estimated total resource of 7800milliontons of phosphate rock at an average grade of 19wt.% P2O5 on the Namibian shelf.