Glacial Isotope Stage Research Articles

Challenging the hypothesis of an Arctic Ocean lake during recent glacial episodes

ABSTRACTThe Arctic Ocean is one of the last frontiers on Earth with many unknowns about its geological and climate history and considerable speculation on its role in the Earth's climate and ocean system. It has been proposed recently that it was occupied by a freshwater body of more than 9.5 × 106 km3 underneath a thick ice mass during part of glacial isotopic stages 6 and 4. We argue that such a dramatic scenario, implying replacement of marine waters by freshwater throughout the entire Arctic Ocean and Nordic Seas, is physically implausible. The very low 230Th excesses (230Thxs) observed in sediments from these intervals were used as evidence for the presence of a U‐depleted overlying freshwater column. We show here that they may simply result from short, sporadic sedimentary pulses, below multiyear sea ice or ice shelves, linked to deglacial ice streaming and surging events interrupting long‐duration sedimentary gaps. Due to this sporadic sedimentation regime, interpolating time from 230Thxs data or between benchmark ages in sedimentary sequences would simply be erroneous.

Increased aridity in southwestern Africa during the warmest periods of the last interglacial

Abstract. Terrestrial and marine climatic tracers from marine core MD96-2098 were used to reconstruct glacial–interglacial climate variability in southwestern Africa between 194 and 24 thousand years before present. The pollen record documented three pronounced expansions of Nama-karoo and fine-leaved savanna during the last interglacial (Marine Isotopic Stage 5 – MIS 5). These Nama-karoo and fine-leaved savanna expansions were linked to increased aridity during the three warmest substadials of MIS 5. Enhanced aridity potentially resulted from a combination of reduced Benguela Upwelling, expanded subtropical high-pressure cells, and reduced austral-summer precipitation due to a northward shift of the Intertropical Convergence Zone. Decreased austral-winter precipitation was likely linked to a southern displacement of the westerlies. In contrast, during glacial isotopic stages MIS 6, 4 and 3, fynbos expanded at the expense of Nama-karoo and fine-leaved savanna indicating a relative increase in precipitation probably concentrated during the austral winter months. Our record also suggested that warm–cold or cold–warm transitions between isotopic stages and substages were punctuated by short increases in humidity. Increased aridity during MIS 5e, 5c and 5a warm substages coincided with minima in both precessional index and global ice volume. On the other hand, austral-winter precipitation increases were associated with precession maxima at the time of well-developed Northern Hemisphere ice caps.

Biogenic magnetite, detrital hematite, and relative paleointensity in Quaternary sediments from the Southwest Iberian Margin

Magnetic properties of late Quaternary sediments on the SW Iberian Margin are dominated by bacterial magnetite, observed by transmission electron microscopy (TEM), with contributions from detrital titanomagnetite and hematite. Reactive hematite, together with low organic matter concentrations and the lack of sulfate reduction, lead to dissimilatory iron reduction and availability of Fe(II) for abundant magnetotactic bacteria. Magnetite grain-size proxies (κARM/κ and ARM/IRM) and S-ratios (sensitive to hematite) vary on stadial/interstadial timescales, contain orbital power, and mimic planktic δ18O. The detrital/biogenic magnetite ratio and hematite concentration are greater during stadials and glacial isotopic stages, reflecting increased detrital (magnetite) input during times of lowered sea level, coinciding with atmospheric conditions favoring hematitic dust supply. Magnetic susceptibility, on the other hand, has a very different response being sensitive to coarse detrital multidomain (MD) magnetite associated with ice-rafted debris (IRD). High susceptibility and/or magnetic grain-size coarsening, mark Heinrich stadials (HS), particularly HS2, HS3, HS4, HS5, HS6 and HS7, as well as older Heinrich-like detrital layers, indicating the sensitivity of this region to fluctuations in the position of the polar front. Relative paleointensity (RPI) records have well-constrained age models based on planktic δ18O correlation to ice-core chronologies, however, they differ from reference records (e.g. PISO) particularly in the vicinity of glacial maxima, mainly due to inefficient normalization of RPI records in intervals of enhanced hematite input.

Stable-isotope stratigraphy of the Pliocene–Pleistocene climate transition in the northwestern subtropical Pacific

From Ocean Drilling Program (ODP) Site 1208 on Shatsky Rise below the Kuroshio Current Extension, we present the North Pacific's first orbital-scale benthic-foraminiferal δ18O and δ13C time series to span the Pliocene–Pleistocene climate transition. Excellent agreement between the Site 1208 δ18O record and the global δ18O stack of Lisiecki and Raymo (2005) provides orbital-scale age control and confirms continuous stratigraphy from 3.7 to 1.8Ma at the single-hole site. Cross-spectral analysis of the δ18O and δ13C time series reveals that these are coherent to 80% confidence at the 41-kyr obliquity band prior to 3.3Ma (glacial isotope stage M2) and increase to the 95% level thereafter. Throughout, δ18O cycles consistently lead δ13C cycles by ~3kyr. This suggests that global-ocean δ13C variations, as produced by terrestrial–marine 12C transfers, were responsive to obliquity-induced climate changes before the Northern Hemisphere glaciations (NHG) reached mid latitudes at 2.7Ma. In contrast, 41-kyr carbonate sedimentation (as derived from sediment reflectance) cycles, maxima tightly coupled to (>95% confidence) and in phase with minima in the δ18O record, do not emerge until 2.7Ma. Foraminiferal fragmentation counts indicate that carbonate preservation is not the primary process behind enhanced carbonate deposition during interglacials. Thus, we surmise that hydrography-related changes in biogenic opal and carbonate production in surface water best explain glacial–interglacial carbonate cycles beginning with significant NHG. Firm establishment of orbital-scale age control on the stratigraphically complete Site 1208 section now provides a platform for high-resolution paleoceanographic reconstruction of the relatively understudied North Pacific.

Occurrence of an exceptional carbonate dissolution episode during early glacial isotope stage 6 in the Southeastern Atlantic

Concentration and mass accumulation rate profiles from Southeastern Atlantic sediment cores located off Namibia show that an exceptional episode in benthic carbonate dissolution occurred during early glacial isotope stage 6 (substages 6.6 and 6.5) between about 186 000 and 170 000 yr BP. Although this episode is restricted to or is more pronounced in this region than in other areas of the Atlantic Ocean, its exceptional character with respect to older and younger climatic episodes at the same site cannot be fully explained by local factors alone, but requires a combination of local and global influences. The onset of the carbonate dissolution episode is related to a more efficient transfer of organic matter from surface eutrophic areas to the lower and is due to low sea level, while its termination relates to a change in either global ocean alkalinity or bottom water circulation. An evaluation of the magnitude of this local carbonate dissolution episode suggests that its contribution to a global alkalinity change may have been significant. Carbonate dissolution was probably amplified by stronger upwelling activity of the Benguela System linked to an exceptional northern excursion of the boreal summer ITCZ during early glacial isotope stage 6. This low latitude global linkage may explain how this carbonate dissolution event as well as other ‘anomalies’ observed for early stage 6, like an important Dole effect minimum or a ‘cold’ Mediterranean sapropel, are related.

Pelagic productivity changes in the equatorial area of the northwest Indian Ocean during the last 400,000 years

Major and selected trace element (Al, Fe, Mg, K, Ba, P, Mn, V, Cu, Ni, and Ti) records were analyzed in two cores recovered from the northwestern Indian Ocean (core MD 90940 on the Madingley Rise and core MD 85668 in the Somali Basin) in order to monitor the sedimentary response to pelagic productivity in this area during the last 400,000 years. Geochemical proxies such as bulk mass accumulation rate (MAR), Al-normalized major and trace element contents, major and trace element MARs were compared to biologic markers of palaeoproductivity: a new radiolarian index (rad ratio), and the variations of foraminiferal and radiolarian assemblages. Special attention being paid to the barium record, a sequential leaching procedure determined the main Ba-carrier fraction in the sediment, and, thus, the amount of biogenic barium. The `blue ocean' sedimentation on the Madingley Rise (core MD 90940) is characterized by a low MAR and an insignificant terrigenous input. The bulk MAR variations are directly related to the variations of the rad ratio, reflecting a direct link between surface productivity and pelagic sedimentation at this site. In this sediment, barium is mainly found as barite and can be considered as a palaeoproductivity proxy. In the Somali Basin (core MD 85668), the terrigenous inputs are relatively higher, but the bulk MAR variations are still related to the variations of surface productivity. No barite was found and barium is mainly linked to the terrigenous input. A comparison of the two pelagic sites of the Madingley Rise and the Somali Basin points out the differences between the barium records at both sites. Despite the location of both cores in the pelagic domain, the geochemical record, and especially the barium one, is different from one core to the other. The comparison of the variations of the bulk MAR, which reflects the variations of the surface productivity, presents significant discrepancies between both sites. In the `blue ocean' (core MD 90940), the productivity peak values are mainly recorded during glacial intervals, but their maximum values are observed during interglacial isotope Stage 9. In the Somali Basin (core MD 85668), high bulk MAR periods occurred during glacial isotope stages. The interglacial episode of high productivity recorded on the Madingley Rise, the absence of significant variations of productivity at the 8/7 transition in the Somali basin, as well as the absence of a time lag between both records indicate that the surface productivity is, in this area, controlled more by the local variations of the palaeoceanographic patterns than the global climatic changes, e.g. the glacial/interglacial alternations.

Refractory organic matter in sediments from the North–West African upwelling system: abundance, chemical structure and origin

Refractory (insoluble and nonhydrolyzable) organic matter (ROM) was studied in sediments underlying the north–west African upwelling system. Two core samples: beginning of glacial isotope stage 4 (ca. 70,000 yr BP; depth bsf 360 cm) and interglacial isotope substage 5d (ca. 90,000 yr BP; depth bsf 480 cm) were analyzed, along with a surface sample. ROM, which accounts for a substantial part of the total organic matter (OM) in these sediments (ca. 20% for the two core samples and 10% for the surface sample), was isolated and examined for its morphological and chemical features using a combination of transmission and scanning electron microscopy, spectroscopy (FTIR, solid state 13C NMR) and analytical pyrolysis (Curie point flash pyrolysis–GC–MS). These studies allowed the chemical structure of the three ROMs (chiefly based on melanoidin-type macromolecules) and their mechanism of formation (degradation–recondensation of products mainly derived from proteinaceous material) to be established. Such a formation probably began in the water column and continued within the sediment upper layers. Important differences, concerned both with OM chemical structure and preservation mechanism, were noted upon comparison with recently studied sediments underlying the Peru upwelling system. These differences between the two systems, in spite of similarities in primary production, must reflect the combined influence of factors such as upwelling intensity (primary productivity), water depth and iron supply.

Glacial-interglacial variation in organic carbon burial on the slope of the N.W. European Continental Margin (48°–50°N)

The sources and fate of organic carbon (C org) were examined at two sites on the Goban Spur and a single site within the Porcupine Sea-Bight on the N.W. European slope between 48°N and 50°N. Here we present data on the sedimentary C org concentration, its 13C/ 12C composition (δ 13C org) and C org/N of 224 total and 119 fine fraction (<63 μm) samples as a time series running from the instantaneous marker horizon Heinrich Layer 2 (25.5 ka BP) through the last glacial isotope stage 2 and interglacial isotope stage 1 to the present. The significance of the N.W. European slope as a depositional sink for C org has declined in the past twenty thousand years, with flux values decreasing two to three-fold from the late glacial (60–70 mg C cm −2 ka −1) to low late Holocene values. Significant shifts are seen in the δ 13C org signature of both the total and fine fractions between the present interglacial and last glacial. The open slope of the Goban Spur is characterised by relatively low values over the last glacial, with minimum values for the total sediment of around −24.5‰ occurring at ca. 18 ka BP, and generally increasing values during the Holocene with the most recent samples having values of ca. −19.2 to −20.2‰. In the Porcupine Sea-Bight a different relationship is evident with samples from the Last Glacial Maximum being generally higher, while values from the Holocene are lower with a minimum of −23.3‰ occurring just below the mixed layer. Similar trends are observed in the fine fraction albeit with a different dynamic range. Paired analysis of δ 13C org and C org/N are consistent with the organic matter being a climatically forced mixture from C 3-photosynthetic terrigenous and marine organic matter sources. An increased influence of terrestrial C org is suggested on the glacial Goban Spur and Holocene Porcupine Sea-Bight, and vice versa. We suggest that this may be explained by modern wind-driven flushing of terrigenous carbon across the Armorican and Celtic shelves to the N.W. and thence into Porcupine Sea-Bight. During the last glacial increased strength of westerly winds created a residual current regime driving material to the S.W., where it was transported in vigorous tidal off-shelf flushing to the upper slope, followed by slope current transfer to the Goban Spur. Much of this material was presumably delivered by the `Great Channel River' to the outer shelf, thus leading to enhanced flux of terrestrial C org on the Goban Spur. The influence of a freshwater plume from meltwater discharge of the European ice sheet in this region is supported by foramininiferal δ 18O data from which a palaeo-salinity reconstruction shows a greatest negative anomaly of −2.55 (that is S-35) at the time of maximum terrestrial C org content in the sedimentary material. In addition to showing that surface waters over Goban Spur were relatively fresh throughout the glacial, our results also reveal variability in surface salinity on shorter timescales.

Isolation, extinction and migration within Late Pliocene populations of the planktonic foraminiferal lineage Globorotalia (Globoconella) in the North Atlantic

Quantitative records of Globorotalia puncticulata and Globorotalia inflata, the last two members of the Globorotalia (Globoconella) lineage, obtained from North Atlantic sediments collected at DSDP Site 552, ODP Site 659 and ODP Site 665, are used to examine fluctuations in the biogeographic distribution of these species in the Late Pliocene between 3 and 2 Ma. Abundance data indicate that prior to the expansion of Northern Hemisphere glaciation at about 2.5 Ma, Gr. puncticulata was an important component of the planktonic foraminiferal fauna and had a geographic distribution ranging from 2°N to at least 56°N in the North Atlantic. A previously undescribed 6 chambered variant of Gr. puncticulata is found at both Sites 659 and 665. The stratigraphic distribution of this morphotype is restricted, first occurring at 2.9 Ma and then disappearing when glacial intensity increased at 2.75 Ma (isotope stage 110). Similar declines in Gr. puncticulata abundances occurred during glacial isotope stages 102, 100, and 98 immediately prior to the extinction of Gr. puncticulata during glacial isotope stage 96. It appears that this extinction event was latitudinally diachronous within the North Atlantic, occurring earliest in the north at Site 552 (2.453 Ma), then at Site 659 (2.443 Ma) and later still in the Site 665 equatorial record (2.438 Ma). At Site 665 the first record of Gr. inflata occurs during glacial isotope stage 94 (2.416 Ma), shortly after the extinction of Gr. puncticulata. In the mid latitude North Atlantic there was a 340,000 year period following the disappearance of Gr. puncticulata when the Globoconella lineage was absent (the Gr. inflata gap). The Gr. inflata population found in the equatorial Atlantic must therefore have been introduced from the South Atlantic, probably by the South Equatorial Current. Faunal records from Sites 552 and 659 show that it was not until glacial isotope stage 78 (2.10 Ma) that Gr. inflata became widely established in the North Atlantic. Prior to this large-scale migration event, there were two limited colonisation events during glacial isotope stages 86 and 82 when Gr. inflata populations reached as far as Site 659 in the eastern North Atlantic. These incursions are believed to be reflect the entrainment of Gr. inflata within South Atlantic Central Water and the northward subsurface transport of individuals to the coastal upwelling zone off northwest Africa. It seems likely that the same mechanism was responsible for the re-establishment of the Globoconella lineage in the North Atlantic at 2.10 Ma, but in this instance additional factors, such as enhanced glacial circulation patterns and ecological changes within planktonic foraminiferal faunas, resulted in the successful expansion of Gr. inflata across the North Atlantic and the Mediterranean.

Late Pliocene climatic change and the global extinction of the discoasters: an independent assessment using oxygen isotope records

High-resolution records (2–7 kyr) of Upper Pliocene Discoaster abundances obtained from six ODP/DSDP sites are assessed independently using oxygen isotope stratigraphy. Four Atlantic Ocean sites (DSDP Sites 552 and 607, and ODP Sites 659 and 662) comprise a transect from 56°N to 1°S and provide a record of latitudinal variations in Discoaster biogeography. Low-latitude sites in the Atlantic (ODP Site 662), Pacific (ODP Site 677), and Indian (ODP Site 709) oceans provide additional information about variability in Discoaster abundance patterns within the equatorial region. A common chronology, based on the astronomical time scale developed for ODP Site 677, has been established for all the sites. By integrating oxygen isotope data and Discoaster abundance records at each site we are able to independently evaluate the temporal and spatial distribution of D. brouweri and D. triradiatus in the 500 kyr prior to the extinction of the discoasters near the base of the Olduvai subchron. Major decreases in abundance are evident during some of the more intense late Pliocene glacial events. In particular, glacial isotope stages 82, 96, 98 and 100 are associated with distinct abundance minima. At these times, large-scale changes in surface hydrographic conditions appear to have suppressed Discoaster numbers on a global scale. The increase in abundance of D. triradiatus, which precedes the extinction of the discoasters by around 200 kyr, may also be related to the intensification of environmental pressures that accompanied the build-up of Northern Hemisphere ice sheets during the late Pliocene. In spite of contrasting geographic and oceanographic settings, the various D. brouweri and D. triradiatus records are remarkably similar. This demonstrates that the acme and extinction events are excellent biostratigraphic datums. The simultaneous extinction of D. brouweri and D. triradiatus at 1.95 Ma were synchronous events at both a regional scale within the Atlantic, and on a global scale between the three major oceans. However, the start of the D. triradiatus acme appears to have been diachronous, occurring some 40 kyr earlier in the Atlantic than in the Indo-Pacific, and hence the stratigraphic usefulness of this datum is regional rather than global.

The sources and deposition of organic matter in the Late Quaternary Pigmy Basin, Gulf of Mexico

The concentration and carbon isotopic composition (δ 13C) of sedimentary organic carbon (C org), N/C ratios, and terrigenous and marine δ 13C org endmembers form a basis from which to address problems of Late Quaternary glacial-interglacial climatic variability in a 208.7 m hydraulic piston core (DSDP 619) from the Pigmy Basin in the northern Gulf of Mexico. While interpretations of sedimentary δ 13C org time series records are often not unique, paired analyses of δ 13C org and N/C are consistent with the hypothesis that the C org in the Pigmy Basin is a climatically determined mixture of C 3-photosynthetic terrigenous and marine organic matter, confirming the earlier δ 13C org model of Sackett (1964). A high resolution (∼ 1.4–2.9 Ka/sample) δ 13C org record shows that sedimentary organic carbon in interglacial oxygen isotope (sub)stages 1 and 5a-b are enriched in 13C (average ±1 σ values are −24.2 ± 1.2%o and −22.9 ± 0.7%o relative to PDB, respectively) while glacial isotope stage values 2 are relatively depleted (-25.6 ± 0.5%). Concentrations of terrigenous and marine sedimentary organic carbon are calculated for the first time using δ 13C org and C org measurements, and empirically determined terrigenous and marine δ 13C org endmembers. The net accumulation rate of terrigenous organic carbon is 4.3 ± 2.6 times higher in isotope stages 2–4 than in (sub)stages 1 and 5a-b, recording higher erosion rates of terrigenous organic material in glacial times. The concentration and net accumulation rates of marine and terrigenous C org suggest that the nutrient-bearing plume of the Mississippi River may have advanced and retreated across the Pigmy Basin as sea level fell and rose in response to glacial-interglacial sea level change.

Late Gauss to Early Matuyama invasions of Neogloboquadrina Atlantica in the Mediterranean and associated record of climatic change

A well‐dated, high‐resolution record of climatic change is presented for the late Pliocene Mediterranean. Principal component analysis on abundances of 14 planktonic foraminiferal species reveals a series of late Gauss to early Matuyama surface water cooling events which can be correlated with North Atlantic glacial isotope stages 108 to 94. The abundance record of the benthic foraminifer Trifarina angulosa suggests lowered bottom water temperatures at times of surface water cooling. The record of surface water oxygen and carbon isotopes shows an inverse pattern with δ18O maxima and δ13C minima at times of cool sea surface temperature conditions. Concomitant decrease in P/B value and increased admixtures of clastic material suggest that late Gauss to early Matuyama surface water cooling events in the Mediterranean are accompanied with glacioeustatic sea level low‐stands. Major surface water cooling events are associated with invasions of Neogloboquadrina atlantica. This North Atlantic mid‐ to high‐latitude species is at 2.55 Ma close to the Gibraltar portal whereafter it could invade the Mediterranean repeatedly during periods of climatic cooling until it disappears from all over the North Atlantic and Mediterranean at the end of glacial stage 96.

Late Quaternary carbonate sedimentation at the Sierra Leone Rise (eastern equatorial Atlantic Ocean)

In the eastern equatorial Atlantic Ocean, changes in the concentration of carbonate in Late Quaternary sediments resulted from reduced production of carbonate in surface waters and increased dilution with non-carbonate sediments during glacial maxima. During glacial stages, production of carbonate in surface water (measured as its accumulation rate in shallow, undissolved cores) decreased by one half. The glacial accumulation rate of non-carbonate components increased 1.5 to 4 times over Holocene values; the greatest increases occurred in the deepest cores. Carbonate dissolution during stages 2, 3 and 4 increased the proportion of foraminiferal fragments and decreased the accumulation rate of susceptible species in the deep sites. In shallow sites, slightly increased dissolution can be detected during stage 3 while greatly increased dissolution occurred during stage 4. Bathymetric profiles of foraminiferal fragmentation and accumulation document a shoaling of the foraminiferal lysocline by 1000 m during glacial isotopic stages. We present a mass balance model of sediment accumulation for carbonate and insoluble components and from this model we estimate the rate of downslope transport and dissolution of carbonate at the Sierra Leone Rise. Our results show that during stage 4 the rate of carbonate loss to dissolution was greater than the rate observed today or during other interglacial stages. The calculated rates of dissolution for stages 2 and 3 are not significantly different from those calculated for stage 1.

Near-synchroneity of New Zealand alpine glaciations and Northern Hemisphere continental glaciations during the past 750 kyr

The Brunhes magnetochron (0–730 kyr) at Deep Sea Drilling Project Site 594 off eastern South Island, New Zealand, comprises up to ∼100 m of alternating units of pelagic and hemipelagic ooze formed during interglacial and glacial periods respectively. The youngest hemipelagic interval corresponds to glacial isotope stage 2 and can be correlated with extensive glacial and glacio-fluvial deposits of the late Last Glaciation (∼27–13 kyr BP) on South Island. Inferring the same genetic link for the other hemipelagic sediment units down core, we show here that as many as 12 major episodes of expanded alpine glaciation have occurred on South Island in the past 730 kyr. Each of these glacial episodes coincided with a period of significant 18O enrichment (+2‰) in the core resulting from changes in ocean isotope composition associated with major growth phases of the Northern Hemisphere ice sheets. The inferred history of alpine glaciations on this remote South Pacific island appears to be closely phase-locked (±3 kyr) to world-wide climate changes.

Bottom-current speed in the Vema Channel recorded by particle size of sediment fine-fraction

The particle-size distribution of the carbonate-free silt fraction was determined in forty-two core-top samples from the east flank of the Vema Channel in order to allow comparison of the mean particle size with near-bottom current speed based on nearby CTD and current-meter observations. The silt mean particle size fluctuates insignificantly between 1450–3950 m but coarsens markedly below 4000 m under Antarctic-source bottom currents. The top of the zone of coarse sizes approximates the top of Lower Circumpolar Water (LCPW) at ∼4000 m; the steep gradient to coarsest sizes in the axis of the channel marks the transition to Antarctic Bottom Water (AABW). The vertical profile of mean particle size on the east flank correlates in general respects with the vertical velocity gradient in the channel axis although the core sites on the east flank are as far as ∼200 miles from the current-meter arrays. Nevertheless, this relationship is used to infer that the non-carbonate particle sizes deposited on the east flank of the channel may be an indication of bottom-current speed and, therefore, the mean size is used to reconstruct profiles of mean current speed for three time slices (18, 120 and 140 ka B.P.). During glacial isotopic stages 2 and 6 the particle sizes deposited on the east flank of the channel are characterized by finer sizes at nearly all depths and the zone of coarse sizes corresponding to LCPW and AABW deepens. During interglacial isotopic stage 5e, the particlesize profile is very similar to the modern profile; however, LCPW and AABW appear to be deeper than at present. The finer particle sizes deposited at nearly all depths during glacial periods indicates that the velocity of deep circulation in the Atlantic was reduced in response to the ice ages. Deep circulation during isotopic stage 5e closely resembled modern circulation; however, AABW production rates may have been less that at present.

Holocene and Pleistocene dissolution cycles in deep-sea cores of Baffin Bay and Davis Strait: Palaeoceano-graphic implications

Calcium-carbonate dissolution has been studied in several grab samples and piston cores from Baffin Bay and Davis Strait ranging in water depth from 200 to 2300 m. The intensity of dissolution was determined from examining: (1) the ratio of arenaceous to calcareous benthonic foraminifera; (2) the ratio of benthonic to planktonic foraminifera; (3) the relationships between the dissolution-susceptible and dissolution-resistant foraminifera; (4) the degree of foraminiferal test fragmentation; and (5) the relationships between plankton tows and the fauna in the surface sediments. All core tops and grab samples from Baffin Bay surface sediments below 600–900 m water depth demonstrated intense dissolution of calcium carbonate. Sediments below 900 m were biogenic carbonate free, indicating 100% foraminiferal loss due to dissolution. Possible causes of substantial undersaturation in calcium carbonate of Baffin Bay Bottom Water are very low temperatures, and higher concentrations of carbon dioxide. All cores also demonstrated intense dissolution during interglacial or interstadial isotope stages 1, 3, 5, 7 and 9. Similarities in planktonic foraminiferal assemblages suggested the presence of similar bottom-water masses during these periods. The preservation state of biogenic carbonate debris in glacial isotope stages 2, 4, 6, 8 and 10 is similar to the assemblage found in the water at present.

Carbon isotopic changes in benthic foraminifera from the western South Atlantic: Reconstruction of glacial abyssal circulation patterns

Oxygen- and carbon-isotopic analyses have been performed on the benthic foraminifer Planulina wuellerstorfi in seven Late Quaternary cores from the Vema Channel-Rio Grande Rise region. The cores are distributed over the water-depth interval of 2340 to 3939 m, which includes the present transition from North Atlantic Deep Water (NADW) to Antarctic Bottom Water (AABW). The carbon-isotopic records in the cores vary as a function of water depth. The shallowest and deepest cores show no significant glacial-interglacial difference in δ 13C. Four of the five cores presently located in the NADW have benthic foraminiferal δ 13C that is lower during glacial isotopic stages. Based on bathymetric gradients in δ 13C, we conclude that, like today, there were two water masses present in the Vema Channel during glacial intervals: a water mass enriched in 13C overlying another water mass depleted in 13C. The largest gradient of change of δ 13C with depth, however, occurred at 2.7 km, ∼ 1 km shallower than the present position of this gradient. On the basis of paleontologic and sedimentologic evidence, we consider it unlikely that the NADW:AABW transition shallowed to this level. Reduced carbon-isotopic gradients between the deep basins of the North Atlantic and Pacific Oceans during the last glaciation suggest that production of NADW was reduced. Lower production of NADW may have modified the local abyssal circulation pattern in the Vema Channel region.