Planktic foraminiferal and sea surface temperature record during the last 1 Myr across the Subtropical Front, Southwest Pacific

Planktic foraminiferal faunas and modern analogue technique estimates of sea surface temperature (SST) for the last 1 million years (Myr) are compared between core sites to the north (ODP 1125, 178 faunas) and south (DSDP 594, 374 faunas) of the present location of the Subtropical Front (STF), east of New Zealand. Faunas beneath cool subtropical water (STW) north of the STF are dominated by dextral Neogloboquadrina pachyderma, Globorotalia inflata, and Globigerina bulloides, whereas faunas to the south are strongly dominated by sinistral N. pachyderma (80-95% in glacials), with increased G. bulloides (20-50%) and dextral N. pachyderma (15-50%) in interglacials (beneath Subantarctic Water, or SAW). Canonical correspondence analysis indicates that at both sites, SST and related factors were the most important environmental influences on faunal composition. Greater climate-related faunal fluctuations occur in the south. Significant faunal changes occur through time at both sites, particularly towards the end of the mid-Pleistocene climate transition, MIS18-15 (e.g., decline of Globorotalia crassula in STW, disappearance of Globorotalia puncticulata in SAW), and during MIS8-5. Interglacial SST estimates in the north are similar to the present day throughout the last 1 Myr. To the south, interglacial SSTs are more variable with peaks 4-7 °C cooler than present through much of the early and middle Pleistocene, but in MIS11, MIS5.5, and early MIS1, peaks are estimated to have been 2-4 °C warmer than present. These high temperatures are attributed to southward spread of the STF across the submarine Chatham Rise, along which the STF appears to have been dynamically positioned throughout most of the last 1 Myr. For much of the last 1 Myr, glacial SST estimates in the north were only 1-2 °C cooler than the present interglacial, except in MIS16, MIS8, MIS6, and MIS4-2 when estimates are 4-7 °C cooler. These cooler temperatures are attributed to jetting of SAW through the Mernoo Saddle (across the Chatham Rise) and/or waning of the STW current. To the south, glacial SST estimates were consistently 10-11 °C cooler than present, similar to temperatures and faunas currently found in the vicinity of the Polar Front. One interpretation is that these cold temperatures reflect thermocline changes and increased Circumpolar Surface Water spinning off the Subantarctic Front as an enhanced Bounty Gyre along the south side of the Chatham Rise. For most of the last 1 Myr, the temperature gradient across the STF has been considerably greater than the present 4 °C. During glacial episodes, the STF in this region did not migrate northwards, but instead there was an intensification of the temperature gradient across it (interglacials 4-11 °C; glacials 8-14 °C).

Benthic foraminifera in Southwest Indian Ocean surface sediments

The distribution of deep-sea benthonic foraminifera in core top samples from the southwest Indian Ocean is examined. Principal component analysis reveals two major assemblages. One assemblages between 3600 and 4800-m water depth is dominated by Episominella umbonifera and is associated with cold (Theta = -0.3 to 0.8°C), low salinity (34.66 to 34.72 * 10**-3) Antarctic Bottom Water in the Crozet Basin, in fracture zones, and on the flanks of the Southwest Indian Ridge. A second assemblage, dominated by Planulina wuellerstorfi, Globocassidulina subglobasa, Astrononion echolsi and Pullenia bulloides, is between 1600 and 3800 m on the Crozet Plateau, Madagascar Ridge, Central Indian Ridge, and Southwest Indian Ridge and is associated with relatively warm (Theta = 0.8 to 2.6°C), high salinity (34.72 to 34.76 * 10**-3) North Atlantic Deep Water. The third principal component divides the P. wuellerstorfi assemblage into two subgroups. One is dominated by Epistominella exigua, P. bulloides, P. wuellerstorfi, and A. echolsi and a second is dominated by G. subglobosa. The distribution of the E. umbonifera assemblage and previous hydrographic studies suggest that AABW flows as a western boundary contour current in the Crozet Basin and penetrates fracture zones in the Southwest Indian Ridge between 55 and 57°E and near 66°E as it travels northward into the Madagascar and Mascarene basins. The faunal-water mass associations from the southeast Indian Ocean are compared; the most notable faunal difference is the absence of Uvigerina as a dominant taxon in the southwest Indian Ocean. A comparison of dissolved oxygen and Uvigerina data shows that oxygen is not a major influence upon the distribution of Uvigerina. A correlation analysis of the faunal data and water depth, potential temperature, in situ temperature, salinity, dissolved oxygen, and 1 - Omega, an index of calcium carbonate undersaturation, was carried out to determine the relationships between fauna and hydrography. The second principal component has a significant positive correlation at the 99.9% level with temperature and negative correlations with water depth and 1 - Omega. A general faunal-water mass correlation exists, but it is not possible to determine which variable controls the faunal distributions.

LGM, planktic foraminifera (EPILOG version)

[1] We used planktic foraminiferal assemblages in 70 sediment cores from the tropical and subtropical South Atlantic Ocean (10°N-37°S) to estimate annual mean sea surface temperatures (SSTs) and seasonality for the Last Glacial Maximum with a modified version of the Imbrie-Kipp transfer function method (IKTF) that takes into account the abundance of rare but temperature sensitive species. In contrast to CLIMAP Project Members [1981], the reconstructed SSTs indicate cooler glacial SSTs in the entire tropical/subtropical South Atlantic with strongest cooling in the upwelling region off Namibia (7-10°C) and smallest cooling (1-2°C) in the western subtropical gyre. In the western Atlantic, our data support recent temperature estimates from other proxies. In the upwelling regions in the eastern Atlantic, our data conflict with SST reconstructions from alkenones, which may be due to an environmental preference of the alkenone-producing algae or to an underestimation of foraminiferal SSTs due to anomalous high abundances of N. pachyderma (sinistral).

Distribution of planktonic foraminifera in Quaternary sediments east of New Zealand

A series of cores from east of New Zealand have been examined to determine the paleoceanographic history of the late Quaternary in the SW Pacific using planktonic foraminiferal data. Distinct shifts of species can be seen between glacial and interglacial times especially south of Chatham Rise east of South Island. Foraminiferal fragmentation ratios and benthic/planktonic foraminiferal ratios both show increased dissolution during glacials, especially isotope stage 2 to the south of Chatham Rise. The present-day Subtropical Convergence appears to be tied to the Chatham Rise at 44°S, but during glacial times this rise separated cold water to the south from much warmer water to the north, with an associated strong thermal gradient across the rise. We estimate that this gradient could have presented as much as an 8°C temperature change across 4°C of latitude during the maximum of the last ice age. There is only weak evidence of the Younger Dryas cool event, but there is a clear climatic optimum between 8 and 6.4 ka with temperatures 1°-2°C higher than the present day. The marine changes compare well with vegetational changes on both South and North Island.

Carbonate preservation of sediment cores from the Great Bahama Bank

The oceanic carbon cycle mainly comprises the production and dissolution/ preservation of carbonate particles in the water column or within the sediment. Carbon dioxide is one of the major controlling factors for the production and dissolution of carbonate. There is a steady exchange between the ocean and atmosphere in order to achieve an equilibrium of CO2; an anthropogenic rise of CO2 in the atmosphere would therefore also increase the amount of CO2 in the ocean. The increased amount of CO2 in the ocean, due to increasing CO2-emissions into the atmosphere since the industrial revolution, has been interpreted as "ocean acidification" (Caldeira and Wickett, 2003). Its alarming effects, such as dissolution and reduced CaCO3 formation, on reefs and other carbonate shell producing organisms form the topic of current discussions (Kolbert, 2006). Decreasing temperatures and increasing pressure and CO2 enhance the dissolution of carbonate particles at the sediment-water interface in the deep sea. Moreover, dissolution processes are dependent of the saturation state of the surrounding water with respect to calcite or aragonite. Significantly increased dissolution has been observed below the aragonite or calcite chemical lysocline; below the aragonite compensation depth (ACD), or calcite compensation depth (CCD), all aragonite or calcite particles, respectively, are dissolved. Aragonite, which is more prone to dissolution than calcite, features a shallower lysocline and compensation depth than calcite. In the 1980's it was suggested that significant dissolution also occurs in the water column or at the sediment-water interface above the lysocline. Unknown quantities of carbonate produced at the sea surface, would be dissolved due to this process. This would affect the calculation of the carbonate production and the entire carbonate budget of the world's ocean. Following this assumption, a number of studies have been carried out to monitor supralysoclinal dissolution at various locations: at Ceara Rise in the western equatorial Atlantic (Martin and Sayles, 1996), in the Arabian Sea (Milliman et al., 1999), in the equatorial Indian Ocean (Peterson and Prell, 1985; Schulte and Bard, 2003), and in the equatorial Pacific (Kimoto et al., 2003). Despite the evidence for supralysoclinal dissolution in some areas of the world's ocean, the question still exists whether dissolution occurs above the lysocline in the entire ocean. The first part of this thesis seeks answers to this question, based on the global budget model of Milliman et al. (1999). As study area the Bahamas and Florida Straits are most suitable because of the high production of carbonate, and because there the depth of the lysocline is the deepest worldwide. To monitor the occurrence of supralysoclinal dissolution, the preservation of aragonitic pteropod shells was determined, using the Limacina inflata Dissolution Index (LDX; Gerhardt and Henrich, 2001). Analyses of the grain-size distribution, the mineralogy, and the foraminifera assemblage revealed further aspects concerning the preservation state of the sediment. All samples located at the Bahamian platform are well preserved. In contrast, the samples from the Florida Straits show dissolution in 800 to 1000 m and below 1500 m water depth. Degradation of organic material and the subsequent release of CO2 probably causes supralysoclinal dissolution. A northward extension of the corrosive Antarctic Intermediate Water (AAIW) flows through the Caribbean Sea into the Gulf of Mexico and might enhance dissolution processes at around 1000 m water depth. The second part of this study deals with the preservation of Pliocene to Holocene carbonate sediments from both the windward and leeward basins adjacent to Great Bahama Bank (Ocean Drilling Program Sites 632, 633, and 1006). Detailed census counts of the sand fraction (250-500 µm) show the general composition of the coarse grained sediment. Further methods used to examine the preservation state of carbonates include the amount of organic carbon and various dissolution indices, such as the LDX and the Fragmentation Index. Carbonate concretions (nodules) have been observed in the sand fraction. They are similar to the concretions or aggregates previously mentioned by Mullins et al. (1980a) and Droxler et al. (1988a), respectively. Nonetheless, a detailed study of such grains has not been made to date, although they form an important part of periplatform sediments. Stable isotopemeasurements of the nodules' matrix confirm previous suggestions that the nodules have formed in situ as a result of early diagenetic processes (Mullins et al., 1980a). The two cores, which are located in Exuma Sound (Sites 632 and 633), at the eastern margin of Great Bahama Bank (GBB), show an increasing amount of nodules with increasing core depth. In Pliocene sediments, the amount of nodules might rise up to 100%. In contrast, nodules only occur within glacial stages in the deeper part of the studied core interval (between 30 and 70 mbsf) at Site 1006 on the western margin of GBB. Above this level the sediment is constantly being flushed by bottom water, that might also contain corrosive AAIW, which would hinder cementation. Fine carbonate particles (<63 µm) form the matrix of the nodules and do therefore not contribute to the fine fraction. At the same time, the amount of the coarse fraction (>63 µm) increases due to the nodule formation. The formation of nodules might therefore significantly alter the grain-size distribution of the sediment. A direct comparison of the amount of nodules with the grain-size distribution shows that core intervals with high amounts of nodules are indeed coarser than the intervals with low amounts of nodules. On the other hand, an initially coarser sediment might facilitate the formation of nodules, as a high porosity and permeability enhances early diagenetic processes (Westphal et al., 1999). This suggestion was also confirmed: the glacial intervals at Site 1006 are interpreted to have already been rather coarse prior to the formation of nodules. This assumption is based on the grain-size distribution in the upper part of the core, which is not yet affected by diagenesis, but also shows coarser sediment during the glacial stages. As expected, the coarser, glacial deposits in the lower part of the core show the highest amounts of nodules. The same effect was observed at Site 632, where turbidites cause distinct coarse layers and reveal higher amounts of nodules than non-turbiditic sequences. Site 633 shows a different pattern: both the amount of nodules and the coarseness of the sediment steadily increase with increasing core depth. Based on these sedimentological findings, the following model has been developed: a grain-size pattern characterised by prominent coarse peaks (as observed at Sites 632 and 1006) is barely altered. The greatest coarsening effect due to the nodule formation will occur in those layers, which have initially been coarser than the adjacent sediment intervals. In this case, the overall trend of the grain-size pattern before and after formation of the nodules is similar to each other. Although the sediment is altered due to diagenetic processes, grain size could be used as a proxy for e.g. changes in the bottom-water current. The other case described in the model is based on a consistent initial grain-size distribution, as observed at Site 633. In this case, the nodule reflects the increasing diagenetic alteration with increasing core depth rather than the initial grain-size pattern. In the latter scenario, the overall grain-size trend is significantly changed which makes grain size unreliable as a proxy for any palaeoenvironmental changes. The results of this study contribute to the understanding of general sedimentation processes in the periplatform realm: the preservation state of surface samples shows the influence of supralysoclinal dissolution due to the degradation of organic matter and due to the presence of corrosive water masses; the composition of the sand fraction shows the alteration of the carbonate sediment due to early diagenetic processes. However, open questions are how and when the alteration processes occur and how geochemical parameters, such as the rise in alkalinity or the amount of strontium, are linked to them. These geochemical parameters might reveal more information about the depth in the sediment column, where dissolution and cementation processes occur.

Paleotemperature and productivity reconstruction of sediment core D11957P on the Iberian Margin

Foraminifera counts and climatic assemblages from the Tore Seamount are used to approach the glacial and interglacial changes in temperature and productivity on the Iberian Margin over the last 225 kyr. Chronostratigraphy is based on Globigerinoides ruber and Globigerina bulloides oxygen isotopes and supported by foraminifera and carbonate stadial fluctuations. Foraminifera indicate cooling from late interglacial stage 5 to the beginning of Termination I (TI). Neogloboquadnna pachyderma-s reflects cold conditions during glacial stages 4-2. In contrast, glacial stage 6 is dominated by warmer N. pachyderma-d and dutertrei and a restricted arctic assemblage. Past sea surface temperatures confirm the general cooling, reaching 4.3°C (SIMMAX.28) during stage 2. Multiple productivity proxies such as organic carbon, productivity-related foraminifera, and delta13C constrain the changes observed. A productivity increase occurs after interglacial stage 5, enhanced from late glacial stage 3 to TI Present-day satellite-detected phytoplankton plumes off Portugal would have accounted in the past glacial stages for the general productivity increase over the Tore. On top of this, welldefined peaks of organic carbon and productivity-related foraminifera correspond with Heinrich events 1-4.

Distribution of living benthic foraminifera in sediment cores of cruise M77 Leg 1 and 2, 63-2000 µm fraction

Recent benthic foraminifera and their distribution in surface sediments were studied on a transect through the Peruvian oxygen minimum zone (OMZ) between 10 and 12°S. The OMZ with its steep gradients of oxygen concentrations allows to determine the oxygen-dependent changes of species compositions in a relatively small area. Our results from sediments of thirteen multicorer stations from 79 to 823 m water depth demonstrate that calcareous species, especially bolivinids dominate the assemblages throughout the OMZ. The depth distribution of several species matches distinct ranges of bottom water oxygen levels. The distribution pattern inferred a proxy which allows to estimate dissolved oxygen concentrations for reconstructing oxygen levels in the geological past.

Age determinations on sediment cores from the North Atlantic

Sediments in the North Atlantic ocean contain as eries of layers that are rich in ice-rafted debris and unusally poor in foraminifera. Here we present evidence that the most recent six of the 'Heinrich layers', deposited between 14,000 and 70,000 years ago, record marked decreases in sea surface temperature and salinity, decreases in the flux of planktonic forminifera to the sediments, and short-lived, massive discharges of icebergs originating in eastern Canada. The path of the icebergs, clearly marked by the presence of ice-rafted detrital carbonate, can be traced for more than 3,000 km - a remarkable distance, attesting to extreme cooling of surface waters and enormous amounts of drifiting ice. The cause of these extreme events is puzzling. They may reflect repated rapid advances of the Laurentide ice sheet, perhaps associated with reductions in air temperatures, yet temperature records from Greenland ice cores appear to exhibit only a weak corresponding signal. Moreover, the 5-10,000-yr intervals between the events are inconsistent with Milankovitch orbital periodicities, raising the question of what the ultimate cause of the postulated cooling may have been.

Grain-size, lithic grains, foraminifera-derived and dinocyst-derived data of sediment core MD99-2281

The last glacial period was punctuated by abrupt climatic events with extrema known as Heinrich and Dansgaard-Oeschger events. These millennial events have been the subject of many paleoreconstructions and model experiments in the past decades, but yet the hydrological processes involved remain elusive. In the present work, high-resolution analyses were conducted on the 12-42 ka BP section of core MD99-2281 retrieved southwest of the Faeroe Islands, and combined with analyses conducted in two previous studies (Zumaque et al., 2012; Caulle et al., 2013). Such a multiproxy approach, coupling micropaleontological, geochemical and sedimentological analyses, allows us to track surface, subsurface, and deep hydrological processes occurring during these rapid climatic changes. Records indicate that the coldest episodes of the studied period (Greenland stadials and Heinrich stadials) were characterized by a strong stratification of surface waters. This surface stratification seems to have played a key role in the dynamics of subsurface and deep-water masses. Indeed, periods of high surface stratification are marked by a coupling of subsurface and deep circulations which sharply weaken at the beginning of stadials, while surface conditions progressively deteriorate throughout these cold episodes; conversely, periods of decreasing surface stratification (Greenland interstadials) are characterized by a coupling of surface and deep hydrological processes, with progressively milder surface conditions and gradual intensification of the deep circulation, while the vigor of the subsurface northward Atlantic flow remains constantly high. Our results also reveal different and atypical hydrological signatures during Heinrich stadials (HSs): while HS1 and HS4 exhibit a "usual" scheme with reduced overturning circulation, a relatively active North Atlantic circulation seems to have prevailed during HS2, and HS3 seems to have experienced a re-intensification of this circulation during the middle of the event. Our findings thus bring valuable information to better understand hydrological processes occurring in a key area during the abrupt climatic shifts of the last glacial period.

Dinoflagellate cysts counts of sediment core MD95-2042

Dinoflagellate cysts were analysed from IMAGES core MD952042 (37°48?N; 10°01?W) retrieved from the Tagus Abyssal Plain. Previous results of stable isotope and magnetic susceptibility measurements as well as of planktonic foraminiferal temperature reconstruction from this core, suggest the occurrence of "Heinrich-like events" (i.e. large ice-sheet decay) during Marine Isotopic Stage 5 (MIS 5). Dinoflagellate assemblages of this time period have revealed six dinocyst events that are characterised by peaks in Bitectatodinium tepikiense percentages. These events occur synchronously with "Heinrich-like events" previously identified. They are coeval with major retreats of the forest on land, indicating, therefore, drastic changes in the regional climate. However, results from the Ice-Rafted Detritus (IRD) analysis of the >150 ?m lithic fraction shows that MIS 5 of MD952042 has only recorded one significant input of iceberg discharge, located at the MIS 6/MIS 5 transition. It seems therefore that it is the only event that could be called a "true Heinrich event".