Calcite accumulation in the eastern equatorial Pacific

The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores collected beneath the equatorial upwelling and the South Equatorial Current (SEC) by measuring ex230Th-normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for "fraction of calcite preserved". This method produced very similar results at the four sites and revealed that the export flux of calcite was 30-50% lower during the LGM compared to the Holocene. The internal consistency of these results supports our interpretation, which is also in agreement with emerging data indicating lower glacial productivity in the EEP, possibly as a result of lower nutrient supply from the southern ocean via the Equatorial Undercurrent. However, these findings contradict previous interpretations based on mass accumulation rates (MAR) of biogenic material in the sediment of the EEP, which have been taken as reflecting higher glacial productivity due to stronger wind-driven upwelling.

Ages and geochemical and geophysical characteristics of sediment core MD99-2294, Lofoten Contourite Drift

A sediment core from the Lofoten Contourite Drift on the continental slope off Northern Norway, proximal to the former Vestfjorden-Trsnadjupet Ice Stream, details the development, variability and decline of marine margins of the northwestern Fennoscandian Ice Sheet during the time interval 25.3-14 cal ka BP, including the Last Glacial Maximum and onset of the deglaciation based on high-resolution IRD records. From the core interval between 25.3 and 17.7 cal ka BP we report data points with a mean time step of 10 years, between 17.7 cal ka BP and the Holocene time steps are typically 50 years. The core is divided into 7 informal ice-rafted debris (IRD) zones based on the variations in IRD including 7 major IRD maxima (A-G), inferred to represent periods of high iceberg production. Petrological identification reveals dominance of crystalline IRD (monocrystalline, plutonic and metamorphic rock fragments) accounting for 75-80% of total IRD assemblages, while sedimentary fragments generally account for 15-20%. The crystalline fragments (including eclogite and mangerite from a nearby terrestrial source) increase across the IRD peaks while the sedimentary fragments remain constant. This points to the importance of erosional products from icebergs originating from fast-flowing paleo-ice streams including the Vestfjorden-Trsnadjupet Ice Stream draining from the Fennoscandian mainland during the IRD maxima periods. Increased temperature of the adjacent surface water masses was probably an important external forcing factor on the Fennoscandian Ice Sheet behavior because some IRD maxima and plumite deposition from meltwater plumes post-date periods of increased sea surface temperatures. The peak IRD depositions occur in centennial and millennial time cycles (~200, 1030 and 3900 year) indicating some external forcing by solar variation. Both mechanisms could explain the observed synchronous instability of the northwestern Fennoscandian Ice Sheet to other European Ice Sheets.

Age determination of hornblende in ODP Site 188-1165 and DSDP Hole 28-268

Sediments from Ocean Drilling Program Site 1165 in the Indian Ocean sector of the Southern Ocean (off Prydz Bay) contain a series of layers that are rich in ice-rafted debris (IRD). Here we present evidence that IRD-rich layers at Site 1165 at 7, 4.8, and 3.5 Ma record short-lived, massive discharges of icebergs from Wilkes Land and Adélie Land, more than 1500 kilometers to the east of the depositional site. This distant source of icebergs is clearly defined by the presence of IRD hornblende grains with 40Ar/39Ar ages of 1200-1100 Ma and 1550-1500 Ma, ages that are not found on the East Antarctic continent in locations closer to Site 1165. This observation requires enormous amounts of detritus-carrying drifting icebergs, most likely in the form of large icebergs. These events probably reflect destabilization, surge, and break-up of ice streams on the Wilkes Land and Adélie Land margins of the East Antarctic Ice Sheet, in the vicinity of the low-lying Aurora and Wilkes Basins. They occurred under warming conditions, but each coast seems to have produced ice-rafting events independently, at different times. The data presented here constitute the first evidence of far-traveled icebergs from specific source areas around the East Antarctic perimeter. Launch of these icebergs may have happened during quite dramatic events, perhaps analogous to "Heinrich Events" in the North Atlantic.

Sea surface reconstruction of sediment cores off West Spitsbergen

Two sediment cores from the West Spitsbergen area, Euro-Arctic margin, MD99-2304 and MD99-2305, have been investigated for paleoceanographic proxies, including benthic and planktonic foraminifera, benthic foraminiferal stable isotopes and ice rafted debris. Core MD99-2304 is located on the upper continental margin, reflecting variations in the influx of Atlantic Water in the West Spitsbergen Current. Core MD99-2305 is located in Van Mijenfjord, picturing variations in tidewater glacier activity as well as fjord-ocean circulation changes. Surface water warmer than today, was present on the margin as soon as the Van Mijenfjord was deglaciated by 11,200 cal. years BP. Relatively warm water invaded the fjord bottom almost immediately after the deglaciation. A relatively warm early Holocene was followed by an abrupt cooling at 8800 cal. years BP on the continental margin. Another cooling in the fjord record, 8000-4000 cal. years BP, is documented by an increase in ice rafted debris and an increase in benthic foraminiferal delta18O. The IRD-record indicates that central Spitsbergen never was completely deglaciated during the Holocene. Relatively cool and stable conditions similar to the present were established about 4000 cal. years BP.

Benthic foraminifera extinctions in the Cenozoic record

In the late Pliocene-middle Pleistocene a group of 95 species of elongate, cylindrical, deep-sea (lower bathyal-abyssal) benthic foraminifera became extinct. This Extinction Group (Ext. Gp), belonging to three families (all the Stilostomellidae and Pleurostomellidae, some of the Nodosariidae), was a major component (20-70%) of deep-sea foraminiferal assemblages in the middle Cenozoic and subsequently declined in abundance and species richness before finally disappearing almost completely during the mid-Pleistocene Climatic Transition (MPT). So what caused these declines and extinction? In this study 127 Ext. Gp species are identified from eight Cenozoic bathyal and abyssal sequences in the North Atlantic and equatorial Pacific Oceans. Most species are long-ranging with 80% originating in the Eocene or earlier. The greatest abundance and diversity of the Ext. Gp was in the warm oceanic conditions of the middle Eocene-early Oligocene. The group was subjected to significant changes in the composition of the faunal dominants and slightly enhanced species turnover during and soon after the rapid Eocene-Oligocene cooling event. Declines in the relative abundance and flux of the Ext. Gp, together with enhanced species loss, occurred during middle-late Miocene cooling, particularly at abyssal sites. The overall number of Ext. Gp species present began declining earlier at mid abyssal depths (in middle Miocene) than at upper abyssal (in late Pliocene-early Pleistocene) and then lower bathyal depths (in MPT). By far the most significant Ext. Gp declines in abundance and species loss occurred during the more severe glacial stages of the late Pliocene-middle Pleistocene. Clearly, the decline and extinction of this group of deep-sea foraminifera was related to the function of their specialized apertures and the stepwise cooling of global climate and deep water. We infer that the apertural modifications may be related to the method of food collection or processing, and that the extinctions may have resulted from the decline or loss of their specific phytoplankton or prokaryote food source, that was more directly impacted than the foraminifera by the cooling temperatures.

Sea surface temperature estimation for the last 70 ka from IODP Site 306-U1313

During the six Heinrich Events of the last 70 ka episodic calving from the circum-Atlantic ice sheets released large numbers of icebergs into the North Atlantic. These icebergs and associated melt-water flux are hypothesized to have led to a shutdown of Atlantic Meridional Overturning Circulation (AMOC) and severe cooling in large parts of the Northern Hemisphere. However, due to the limited availability of high-resolution records the magnitude sea surface temperature (SST) changes related to the impact of Heinrich Events on the mid-latitude North Atlantic is poorly constrained. Here we present a record of UK37'-based SSTs derived from sediments of Integrated Ocean Drilling Project (IODP) Site U1313, located at the southern end of the ice-rafted debris (IRD)-belt in the mid-latitude North Atlantic (41°N). We demonstrate that all six Heinrich Events are associated with a rapid warming of surface waters by 2 to 4°C in a few thousand years. The presence of IRD leaves no doubt about the simultaneous timing and correlation between rapid surface water warming and Heinrich Events. We argue that this warming in the mid-latitude North Atlantic is related to a northward expansion of the subtropical gyre during Heinrich Events. As a wide-range of studies demonstrated that in the central IRD-belt Heinrich Events are associated with low SSTs, these results thus identify an anti-phased (seesaw) pattern in SSTs during Heinrich Events between the mid-latitude (warm) and northern North Atlantic (cold). This highlights the complex response of surface water characteristics in the North Atlantic to Heinrich Events that is poorly reproduced by fresh water hosing experiments and challenges the widely accepted view that within the IRD-belt of the North Atlantic Heinrich Events coincide with periods of low SSTs.

Palynology of the Middle to Late Miocene from ODP Hole 175-1085A off the west coast of South Africa

Aim To test whether the radiation of the extremely rich Cape flora is correlated with marine-driven climate change. Location Middle to Late Miocene in the south-east Atlantic and the Benguela Upwelling System (BUS) off the west coast of South Africa. Methods We studied the palynology of the thoroughly dated Middle to Late Miocene sediments of Ocean Drilling Program (ODP) Site 1085 retrieved from the Atlantic off the mouth of the Orange River. Both marine upwelling and terrestrial input are recorded at this site, which allows a direct correlation between changes in the terrestrial flora and the marine BUS in the south-east Atlantic. Results Pollen types from plants of tropical affinity disappeared, and those from the Cape flora gradually increased, between 10 and 6 Ma. Our data corroborate the inferred dating of the diversification in Aizoaceae c. 8 Ma. Main conclusions Inferred vegetation changes for the Late Miocene south-western African coast are the disappearance of Podocarpus-dominated Afromontane forests, and a change in the vegetation of the coastal plain from tropical grassland and thicket to semi-arid succulent vegetation. These changes are indicative of an increased summer drought, and are in step with the development of the southern BUS. They pre-date the Pliocene uplift of the East African escarpment, suggesting that this did not play a role in stimulating vegetation change. Some Fynbos elements were present throughout the recorded period (from 11 Ma), suggesting that at least some elements of this vegetation were already in place during the onset of the BUS. This is consistent with a marine-driven climate change in south-western Africa triggering substantial radiation in the terrestrial flora, especially in the Aizoaceae.

Organic sedimentation of ODP Sites 124-767 and 124-768

This study relates the organic sedimentation characteristics to the lithostratigraphic successionsthat were observed at Site 767 (Celebes Sea) and Site 768 (Sulu Sea) during ODP Leg 124. It is based on the total organic carbon content (TOC) of the sediments, on the petrographictype and maturity of the organic matter, and on the TOC accumulation rates calculated for the lithostratigraphic units. In the Celebes and Sulu Seas sediments, the organic matter is mainly of terrestrial origin with the highest concentrations and TOC accumulation rates occurring in the middle Miocene turbiditic sequences that correspond to a major compressive event between the Philippine Mobile Belt and the Palawan, Cagayan, and Sulu Ridges. Petrographic analysis of the Eocene and lower Miocene organic matter in the Celebes Sea shows that it consists only of highly degraded terrestrial particles. This observation and the very low TOC accumulation rates indicate poor conditions for organic carbon preservation during this open-ocean phase of the Celebes Basin formation. The organic matter, either of marine or terrestrial origin, is much better preserved in the younger sediments, suggesting physico-chemical changes in the depositional environment. Because of the dilution phenomena by turbidites, it is difficult to observe the progressive improvement of the organic matter preservation throughout the turbiditic series. The same change in preservation is broadly observed in the Sulu Sea from the early Miocene (rapid opening phase of the basin with massive pyroclastic deposits) to the present.

Radiocarbon dates, accumulation rates, d13C and July temperature of profiles obtained from Selwyn and Misaw Lake, subarctic Canada

The rapid warming of arctic regions during recent decades has been recorded by instrumental monitoring, but the natural climate variability in the past is still sparsely reconstructed across many areas. We have reconstructed past climate changes in subarctic west-central Canada. Stable carbon and oxygen isotope ratios (d13C, d18O) were derived from a single Sphagnum fuscum plant component; alpha-cellulose isolated from stems. Periods of warmer and cooler conditions identified in this region, described in terms of a "Mediaeval Climatic Anomaly" and "Little Ice Age" were registered in the temperature reconstruction based on the d13C record. Some conclusions could be drawn about wet/dry shifts during the same time interval from the d18O record, humification indices and the macrofossil analysis. The results were compared with other proxy data from the vicinity of the study area. The amplitude of the temperature change was similar to that in chironomid based reconstructions, showing c. 6.5 ±2.3 °C variability in July temperatures during the past 6.2 ka.

Late Miocene to Pleistocene paleoceanographic records from the Feni and Gardar Drifts: Pliocene reduction in abyssal flow

Magnetic fabric analyses from two North Atlantic drift deposits provide proxies for determining relative variations in the strength of abyssal flow over the last 10 my. The data show a cessation of current-controlled sedimentation at the shallower Feni Drift (2417 m) at the time of onset of Northern Hemisphere glaciation (2.6 Ma). Drift formation ended nearly 2 my earlier (4.2 Ma) at the deeper Gardar Drift (3220 m), implying stepwise reduction in deep-water flow. Relatively light delta18O values at the deeper Gardar Drift indicate a warmer, thus also more salty, water mass site prior to 6 Ma. We interpret this as representing Mediterranean Sea water, which flowed north at depths greater than that of the Feni Drift Site. The supply of Mediterranean Water to the North Atlantic was shut off as the Gibraltar Straits closed, causing the Messinian salinity crisis, and never returned to that position in the water column after the Mediterranean opened again.

Geochemistry of Cretaceous sediments from Argo and Gascoyne Abyssal Plains

The accumulation of organic matter, ferrous and pyrite iron, and the ratios of organic carbon/total sulfur and organic carbon/total phosphorus in the Lower Cretaceous sediments from the Argo and Gascoyne abyssal plains have been used as indicators of both the source and reactivity of organic matter in the sediments and the depositional environment. Total sulfur, used as an indicator of pyrite sulfur, is more abundant in sediments from the Gascoyne Abyssal Plain than in those from the Argo Abyssal Plain. Sulfur positively correlates with TOC at both sites (although poorly at the Argo Abyssal Plain site, R = 0.48), with an extension of the line of best-fit through the origin, indicating that pyrite (TOC <2 wt%) is diagenetic and deposited from normal marine conditions. The average ratio of C/S for samples of TOC <2 wt% is 5.4 at Argo Abyssal Plain (compared to the modern normal marine value of 2.8) indicating deposition of organic matter probably of mixed terrestrial and oxidized marine sources that is unreactive to the sulfate-reducing bacteria. One sample from the Aptian sediments is rich in TOC (5.1 wt%) and has a C/S ratio of 0.5. The average C/S ratio in Gascoyne Abyssal Plain sediments is 0.8 (R = 0.97), which indicates the formation of abundant pyrite in addition to burial and preservation of relatively fresh organic matter that is reactive to the sulfate-reducing bacteria. Organic carbon to phosphorus ratios (C/P) in the sediments indicate preferential remobilization of organic carbon over phosphorus with increasing water depth. Estimates of the degree of pyritization (DOP) increase with increasing TOC at both sites, indicating iron is not limiting and pyrite is formed diagenetically. The one sample with a TOC content of 5.1 wt%, from the Argo Abyssal Plain near the Barremian-Aptian boundary, is composed mostly of framboidal pyrite, finely laminated and not bioturbated, and hence may have been deposited during a brief period of anoxia in the overlying waters.

Paleomagnetic properties of ODP Hole 178-1101A

Sediments recovered from a drift deposit located on the Pacific side of the Antarctic Peninsula (ODP Leg 178, Site 1101) give a physical record of a bottom current, sourced from the Weddell Sea Deep Water, for the past 3 Ma. Sediment grain size and magnetic fabric analyses indicate a contourite depositional environment and little change in the average intensity of this current. Terrigenous fluxes decreased around the time of the onset of Northern Hemisphere Glaciation, which we interpret as a freezing of the base of the Antarctic Peninsula Ice Cap. Terrigenous fluxes have increased since 1.7 Ma implying a possible return of the Antarctic Peninsula Ice Cap to a more wet-based ice sheet.

Stable isotope and geochemical record of ODP Hole 124-769A

Deep marine late Pleistocene sediments from Ocean Drilling Program Sulu Sea Site 769 contain a high-resolution record of paleoceanographic change in this strongly monsoonal climatic setting in the tropical western Pacific. Detailed time series of planktonic foraminifer (G.ruber; white variety) d18O, d13C, and bulk CaCO3 mass accumulation rate (MAR) were generated, spanning the last 750 k.y. Sedimentation rates in this portion of the record average 8.5 cm/k.y., and vary from 4 to 16 cm/k.y. Cross spectral analysis of the d18O and d13C time-series demonstrate that each contains increased variance at the primary orbital periodicities. The d18O record shows strong variability in the precessional-band and closely correlates with the SPECMAP d18O record and other high-resolution records. The dominance of a 23-k.y cycle in the d18O record agrees with other studies of the monsoon system in the Indian Ocean that have documented the importance of precessional insolation as a monsoon-forcing mechanism. In addition, d13C is strongly coherent, with d18O at a period of 41 k.y (obliquity), suggesting a connection between surface water CO2 chemistry in the Sulu Sea and high- latitude climatic change. The d18O and d13C time-series both contain increased spectral variance at a period of 30 k.y. Although the source of 30-k.y. variability is unknown, other studies have documented late Pleistocene Pacific Oceanographic variability with a period of 30 k.y. Major- and trace-metal analyses were performed on a second, less-detailed sample series to independently assess paleoproductivity changes and bottom-water conditions through time. Glacial periods are generally times of increased calcium carbonate and copper accumulation. The positive association between these independent indicators of paleoproductivity suggests an increase in productivity in the basin during most glacial episodes. Changing bottom-water redox conditions were also assessed using the geochemical data. Low concentrations of molybdenum throughout the record demonstrate that bottom waters at this site were never anoxic during the last 750 k.y. The bioturbated character of the sediments agrees with this interpretation.