Benthic oxygen isotope and dust flux record from ODP Site 108-659 in the North Atlantic

High-resolution benthic oxygen isotope and dust flux records from Ocean Drilling Program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the Atlantic from 2.85 Ma back to 5 Ma. Spectral analysis of the delta18O record indicates that the 41-kyr period of Earth's orbital obliquity dominates the Pliocene record. This is shown to be true regardless of fundamental changes in the Earth's climate during the Pliocene. However, the cycles of Sahelian aridity fluctuations indicate a shift in spectral character near 3 Ma. From the early Pliocene to 3 Ma, the periodicities were dominantly precessional (19 and 23 kyr) and remained strong until 1.5 Ma. Subsequent to 3 Ma, the variance at the obliquity period (41 kyr) increased. The timescale tuned to precession suggests that the Pliocene was longer than previously estimated by more than 0.5 m.y. The tuned ages for the magnetic boundaries Gauss/Gilbert and Top Cochiti are about 6-8% older than the ages of the conventional timescale. A major phase of Pliocene northern hemisphere ice growth occurred between 3.15 Ma and 2.5 Ma. This was marked by a gradual increase in glacial Atlantic delta18O values of 1per mil and an increase in amplitude variations by up to 1.5 per mil, much larger than in the Pacific deepwater record (site 846). The first maxima occured in cold stages G6-96 between 2.7 Ma and 2.45 Ma. Prior to 3 Ma, the isotope record is characterized by predominantly low amplitude fluctuations (< 0.7 per mil). When obliquity forcing was at its minimum between 4.15 and 3.6 Ma and during the Kaena interval, delta18O amplitude fluctuations were minimal. From 4.9 to 4.3 Ma, the delta18O values decreased by about 0.5 per mil, reaching a long-term minimum at 4.15 Ma, suggesting higher deepwater temperatures or a deglaciation. Deepwater cooling and/or an increase in ice volume is indicated by a series of short-term delta18O fluctuations between 3.8 and 3.6 Ma.

Geological Time Scale 2012. Chapter 10. Oxygen isotope stratigraphy, Appendix 1

Variations in the 18O/16O ratios of marine fossils and microfossils record changes in seawater 18O/16O and temperature and form the basis for global correlation. Relying on previous compilations and new data, this chapter presents oxygen isotope curves for Phanerozoic foraminifera, mollusks, brachiopods, and conodonts, and for Precambrian limestones, dolostones, and cherts. Periodic oxygen-isotopic variations in deep-sea foraminifera define marine isotope stages that, when combined with biostratigraphy and astronomical tuning, provide a late Cenozoic chronostratigraphy with a resolution of several thousand years. Oxygen isotope events of early Cenozoic, Mesozoic, and Paleozoic age serve as chemostratigraphic markers for regional and global correlation. Precambrian oxygen isotope stratigraphy, however, is hampered by the lack of unaltered authigenic marine sediments.

Sedimentology and stable carbon and oxygen isotope record of the Central Arctic

Stable oxygen and carbon isotope and sedimentological-paleontological investigations supported by accelerator mass spectrometry 14C datings were carried out on cores from north of 85°N in the eastern central Arctic Ocean. Significant changes in accumulation rates, provenance of ice-rafted debris (IRD), and planktic productivity over the past 80,000 years are documented. During peak glacials, i.e., oxygen isotope stages 4 and 2, the Arctic Ocean was covered by sea ice with decreased seasonal variation, limiting planktic productivity and bulk sedimentation rates. In early stage 3 and during Termination I, major deglaciations of the circum-Arctic regions caused lowered salinities and poor oxygenation of central Arctic surface waters. A meltwater spike and an associated IRD peak dated to ~14-12 14C ka can be traced over the southern Eurasian Basin of the Arctic Ocean. This event was associated with the early and rapid deglaciation of the marine-based Barents Sea Ice Sheet. A separate Termination Ib meltwater event is most conspicuous in the central Arctic and is associated with characteristic dolomitic carbonate IRD. This lithology suggests an origin of glacial ice from northern Canada and northern Greenland where lower Paleozoic platform carbonates crop extensively out.

Stable carbon and oxygen isotope analyses of Neogloboquadrina pachyderma

Detailed records of the carbon and oxygen isotopic ratios of Neogloboquadrina pachyderma are compared between nine high-latitude sediment cores, from the Northern and Southern Hemispheres, covering the last 140000 yrs. The strong analogies between the delta13C records permit to define a delta13C stratigraphic scale, with three clear cut transitions simultaneous with the oxygen isotopic transitions 6/5 (125 kyrs.), 5/4 (65 kyrs.), and 2/1 (13 kyrs.). The delta13C records of N. pachyderma in the high-latitude cores, which follow the changes in delta13C of the surface water TCO2 near areas of deep water formation present trends similar to the benthic foraminifera delta13C records in cores V19-30 and M12-392, although amplitudes of the isotopic shifts are different. This implies that a large part of the observed variations represents global changes in the carbon distribution between biosphere and ocean. The 13C/12C ratios of N. pachyderma in the North Atlantic cores display larger regional variations at 18 kyrs. B.P. than at present. To explain these differences, we have plotted the 18 kyrs. B.P. delta13C values of N. pachyderma from 17 cores distributed N of 40°N. Comparison with published surface water temperature distribution at 18 kyrs. B.P. indicates that a strong divergent cyclonic cell, centered approximatively 55°N and 15°W, was active during most of the last ice-age maximum. This hydrology, analogous to the present Weddell Sea, explains the published evidences of bottom water formation, if located on the northern flank of the gyre, and the strong polar front on the southern flank, probable location of intermediate water formation.

Stable carbon and oxygen isotope ratios of foraminifera from North Atlantic sediments

Oxygen-18 records of benthic foraminifera from the Atlantic Ocean are significantly different from those of the Pacific and Indian Oceans indicating that the Glacial North Atlantic Deep Water was about 1.3°C cooler than today because different deep water sources appeared in the North Atlantic Ocean during glacial times. The present study seeks to interprete carbon-13 records of planktonic and benthic foraminifera as a tracer of the cycle of the CO2 dissolved in surface and deep water of the ocean during the last climatic cycle. Carbon-13 records of planktonic foraminifera indicate that the delta13C of atmospheric CO2 and total CO2 dissolved in surface water did not vary noticeably (-0.2 +/- 0.3 per mil) during glacial times. Carbon-13 records of benthic foraminifera indicate that the eastern North Atlantic Ocean was an area of deep water formation dying isotopic stage 2, but not during most of stage 3. Moreover, large delta13C differences in the NADW between 20°N and 50°N show that the residence time of the glacial NADW was about 4 times that of today.

50 year means of oxygen isotope data from ice core NGRIP

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.

Stable oxygen isotope record of planktonic foraminifera from late Quaternary sediments of the equatorial Pacific

Planktonic foraminiferal test fragmentation in three cores along a depth transect from the western equatorial Pacific (ERDC-93P, 1619 m; RC17-177, 2600 m; V28-238, 3120 m [Thompson, 1976]) were examined for the last 500 kyr at sample intervals from 2.5 to 5 kyr to study the fluctuations of dissolution in the western equatorial Pacific. The age models were constructed by correlating the delta18O records with the SPECMAP stack [Imbrie et al., 1984]. Results showed that intermediate and deep waters experienced the same patterns of dissolution through climatic cycles. Fragmentation varied with a greater amplitude, and the carbonate ion concentration changed less, in the deep than in the intermediate water. Dissolution has significant variance distributions and coherencies with delta18O over the 100, 41, and 23 kyr periods of orbital variations; dissolution maxima lag ice volume minima by 6 to 20 kyr. The dissolution variability was consistent with recent geochemical models which seek to explain the reduction of atmospheric CO2 concentration at the last glacial maximum [Broecker, 1982; Boyle, 1988].

Oxygen isotope and paleomagnetic stratigraphy of sediment core V28-239

Core Vema 28-238 preserves an excellent oxygen isotope and magnetic stratigraphy and is shown to contain undisturbed sediments deposited continuously through the past 870,000 yr. Detailed correlation with sequences described by Emiliani in the Caribbean and Atlantic Ocean is demonstrated. The boundaries of 22 stages representing alternating times of high and low Northern Hemisphere ice volume are recognized and dated. The record is interpreted in terms of Northern Hemisphere ice accumulation, and is used to estimate the range of temperature variation in the Caribbean.

Stable oxygen isotope record of foraminifera from the Ontong Java Plateau

We provide a reconstruction of atmospheric CO2 from deep-sea sediments, for the past 625000 years (Milankovitch chron). Our database consists of a Milankovitch template of sea-level variation in combination with a unique data set for the deep-sea record for Ontong Java plateau in the western equatorial Pacific. We redate the Vostok ice-core data of Barnola et al. (1987, doi:10.1038/329408a0). To make the reconstructions we employ multiple regression between deep-sea data, on one hand, and ice-core CO2 data in Antarctica, on the other. The patterns of correlation suggest that the main factors controlling atmospheric CO2 can be described as a combination of sea-level state and sea-level change. For best results squared values of state and change are used. The square-of-sea-level rule agrees with the concept that shelf processes are important modulators of atmospheric CO2 (e.g., budgets of shelf organic carbon and shelf carbonate, nitrate reduction). The square-of-change rule implies that, on short timescales, any major disturbance of the system results in a temporary rise in atmospheric CO2.

Stable carbon and oxygen isotope composition of Recent, Tertiary and Cretaceous foraminifera (Table III)

Oxygen isotope analyses of Tertiary and Cretaceous planktic foraminifera indicate that species have been stratified with respect to depth in the water column at least since Albian time. There is a relationship between morphology and depth habitat. Species with globigerine morphology have consistently occupied shallower depths than have species with globorotalid morphology. Biserially arranged species occupied both shallow and deep levels in the water column. On the average, it appears that ancient species with shallow habitats have been more susceptible to dissolution and have been preserved less well than species dwelling in deeper habitats. This relationship is similar to that observed for Recent planktic foraminifera. Comparison of carbon isotope ratios of adult and juvenile forms indicates that either the source of the carbon found in the shell or the carbon isotopic fractionations which occur during calcite secretion change during the development of individual foraminifera. The carbon isotopic ratios do not provide a reliable means for reconstructing the depth habitats of ancient species. Temperature-depth profiles for tropical Tertiary oceans have been reconstructed from the isotopic temperatures of planktic and benthic foraminifera. The vertical thermal structure of Oligocene oceans resembled that of modern oceans most closely. Those of Paleocene and Maastrichtian times differed most from that of modern oceans.

Stable carbon and oxygen isotope ratios of benthic foraminifera

Carbon isotopic measurements on the benthic foraminiferal genus Cibicidoides document that mean deep ocean delta13C values were 0.46 per mil lower during the last glacial maximum than during the Late Holocene. The geographic distribution of delta13C was altered by changes in the production rate of nutrient-depleted deep water in the North Atlantic. During the Late Holocene, North Atlantic Deep Water, with high delta13C values and low nutrient values, can be found throughout the Atlantic Ocean, and its effects can be traced into the southern ocean where it mixes with recirculated Pacific deep water. During the glaciation, decreased production of North Atlantic Deep Water allowed southern ocean deep water to penetrate farther into the North Atlantic and across low-latitude fracture zones into the eastern Atlantic. Mean southern ocean delta13C values during the glaciation are lower than both North Atlantic and Pacific delta13C values, suggesting that production of nutrient-depleted water occurred in both oceans during the glaciation. Enriched 13C values in shallow cores within the Atlantic Ocean indicate the existence of a nutrient-depleted water mass above 2000 m in this ocean.

Biogenic opal, carbonat concentrations and stable oxygen isotope ratios of foraminifera from sediment cores of the Southern Ocean

We present records of biogenic opal percentage and burial rate in 12 piston cores from the Atlantic and Indian sectors of the Southern Ocean. These records provide a detailed, quantitative description of changing patterns of opal deposition over the last 450 kyr. The striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal, and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the Southern Ocean under different global climate conditions. Three broad depositional patterns can be distinguished. Northernmost records (39°-42°S latitude) are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity. Records from cores just north of the present Antarctic Polar Front (46°-49°S) show even larger increases in opal burial rate during glacial intervals, but have variance concentrated in the 100 and 23 kyr bands. Southernmost records (51°-55°S) are completely out of phase with those to the north, with greatly reduced opal burial rates during glacial periods. Taken as a whole, the opal records show no evidence for the increased total Antarctic productivity predicted by recent geochemical models of atmospheric CO2 variability. The areal expansion of Southern Ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial-interglacial opal patterns. The excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region. However, local circulation changes may act to modify this basic signal, possibly accounting for the observed differences in the opal variance spectra.