Carbon Isotope Gradient Research Articles

Water mass characteristics in the Cenomanian US Western Interior seaway as indicated by stable isotopes of calcareous organisms

Stable oxygen isotopes taken from taxon-specific planktic foraminifera are potential sources of water mass density proxy data and paleoclimate information. Material suitable for stable isotope analysis is uncommon in the middle to late Cenomanian strata of the Western Interior of the USA. However, well preserved material from 17 correlative horizons at five localities across SE Montana and NE Wyoming has provided an opportunity to obtain reliable δ 18O data from foraminiferal samples, fine-fraction carbonate (e.g. calcareous nannofossils) and inoceramid prisms. With the exception of inoceramids, which for a benthic organism have stable isotopic compositions resembling surface-dwelling calcareous organisms, these data indicate that the water column was vertically stratified and water masses were anomalously depleted in 18O. Within samples, planktic and benthic foraminifera display strong vertical gradients in δ 18O and δ 13C of several per mil. The vertical gradient of δ 18O ranges from more negative nearer the sea surface to less negative at the sea floor, with a gradient of about 4‰. This implies less saline/warmer waters near the surface with increasingly more saline/colder waters with depth. This isotopic distribution indicates a stably stratified water column at least seasonally. In addition, a persistent vertical carbon isotope gradient of 2–3‰ suggests a strong nutrient gradient that if mixed would result in high surface water fertility. Preservation of high amounts of organic carbon in the sediments corroborates this inference. Although the water column was stratified overall, the nutrients necessary to promote high productivity were clearly renewed at least seasonally. Nutrient renewal could have been achieved by (1) winter cooling of the upper layer allowing deeper seasonal mixing with nutrient-rich deep waters, (2) seasonal shallowing of the pycnocline into the photic zone, (3) entrainment and decomposition of organic matter at the previously proposed frontal convergence, (4) upwelling of nutrient-rich deep waters along the previously proposed oceanic front, or (5) mixing of ‘shelf’ waters with more oceanic waters across an oceanic front.

High-stress paleoenvironment during the late Maastrichtian to early Paleocene in Central Egypt

Biostratigraphic, mineralogical, geochemical and isotopic analyses of the Gebel Qreiya section in the Asyut Basin of central Egypt indicate a depositional environment interrupted by periods of erosion due to local tectonic activity exacerbated by eustatic sea-level fluctuations, and by high-stress environmental conditions akin to those normally experienced during the Cretaceous–Tertiary boundary transition. During the late Maastrichtian (66.8–65.4 Ma) this region experienced a breakdown of the biologically mediated surface to bottom gradient of the 13C/ 12C ratio with planktic δ 13C values 0.2–0.8‰ lighter than benthic values. Planktic foraminiferal species diversity was reduced by more than 50%, with faunal assemblages dominated (75–90%) by the opportunistic disaster species Guembelitria cretacea, which alternate with abundance of small, low oxygen-tolerant heterohelicids ( Heterohelix navarroensis, H. dentata, H. globulosa). This prolonged breakdown in ocean primary productivity occurred during a time of global climate cooling and sea-level regressions (at 66.8 and 65.5 Ma), though clay mineralogy suggests that locally low seasonality warm, wet, tropical and subtropical conditions prevailed. The high detrital influx suggests that the biologically high-stress environment was primarily linked to the existing shallow shelf conditions in southern Egypt, and possibly to local tectonic activity and restricted circulation. A normal carbon isotope gradient was briefly re-established during the short climate warming and rising sea level between 65.4 and 65.2 Ma, a time of increased species diversity, peak abundance of rugoglobigerinids and common heterohelicids. During the last 200 000 years of the Maastrichtian, increased precipitation and terrestrial runoff (increased phyllosilicates and kaolinite) and increasing total organic carbon values are associated with Heterohelix-dominated planktic foraminiferal assemblages. The K/T boundary is marked by a red clay layer and Ir anomaly of 5.4 ppb. During the early Danian, planktic foraminiferal populations and stable isotope data indicate that similarly fluctuating high-stress conditions prevailed in central Egypt as elsewhere in the marginal eastern Tethys.

Late Miocene through early Pliocene deep water circulation and climate change viewed from the sub-Antarctic South Atlantic

Benthic foraminiferal stable isotope records for the past 11 Myr from a recently drilled site in the sub-Antarctic South Atlantic (site 1088, Ocean Drilling Program Leg 177, 41°S, 15°E, 2082 m water depth) provide, for the first time, a continuous long-term perspective on deep water distribution patterns and Southern Ocean climate change from the late Miocene through the early Pliocene. I have compiled published late Miocene through Pliocene stable isotope records to place the new South Atlantic record in a global framework. Carbon isotope gradients between the North Atlantic, South Atlantic, and Pacific indicate that a nutrient-depleted watermass, probably of North Atlantic origin, reached the sub-Antarctic South Atlantic after 6.6 Ma. By 6.0 Ma the relative proportion of the northern-provenance watermass was similar to today and by the early Pliocene it had increased to greater than the modern proportion suggesting that thermohaline overturn in the Atlantic was relatively strong prior to the early Pliocene interval of inferred climatic warmth. Site 1088 oxygen isotope values display a two-step increase between ∼7.4 Ma and 6.9 Ma, a trend that parallels a published δ 18O record of a site on the Atlantic coast of Morocco. This is perhaps best explained by a gradual cooling of watermasses that were sinking in the Southern Ocean. I speculate that relatively strong thermohaline overturn at rates comparable to the present day interglacial interval during the latest Miocene may have provided the initial conditions for early Pliocene climatic warmth. The impact of an emerging Central American Seaway on Atlantic–Pacific Ocean upper water exchange may have been felt in the North Atlantic beginning in the latest Miocene between 6.6 and 6.0 Ma, which would be ∼1.5 Myr earlier than previously thought.

The mid-Pliocene (4.3–2.6 Ma) benthic stable isotope record of the Southern Ocean: ODP Sites 1092 and 704, Meteor Rise

We present mid-Pliocene (4.3–2.6 Ma) benthic stable oxygen and carbon isotope data from Ocean Drilling Program Site 1092 (ODP Leg 177) drilled in the sub-Antarctic sector of the Southern Ocean. The results are compared with the stable isotope results from nearby Site 704 (ODP Leg 114). Oxygen isotope data show that minimum values are about 0.5‰ less than those of the Holocene, which is consistent with the results from Site 704, indicating only minor deglaciation of Antarctica during the studied interval. Oxygen isotope data from both Site 1092 and Site 704 are slightly higher relative to Pacific values during several intervals which could be related to the contribution of warm, saline North Atlantic Deep Water (NADW). Comparisons of benthic carbon isotope gradients between sites located in the North Atlantic, sub-Antarctic sector of the Southern Ocean, and Pacific indicate that at times, the gradient between the Southern Ocean and the Pacific evolved differently than the Atlantic–Pacific gradient. This suggests that variations in NADW strength alone might not be responsible for the observed carbon isotope values in the Southern Ocean.

Late Oligocene to early Miocene geochronology and paleoceanography from the subantarctic South Atlantic

At Ocean Drilling Program (ODP) Site 1090 on the Agulhas Ridge (subantarctic South Atlantic) benthic foraminiferal stable isotope records span the late Oligocene through the early Miocene (25–16 Ma) at a temporal resolution of ∼10 kyr. In the same time interval a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity timescale (GPTS), thereby providing secure correlation of the isotope record to the GPTS. On the basis of the isotope‐magnetostratigraphic correlation we provide refined age calibration of established oxygen isotope events Mi1 through Mi2 as well as several other distinctive isotope events. Our data suggest that the δ18O maximum commonly associated with the Oligocene/Miocene (O/M) boundary falls within C6Cn.2r (23.86 Ma). The δ13C maximum coincides, within the temporal resolution of our record, with C6Cn.2n/r boundary and hence to the O/M boundary. Comparison of the stable isotope record from ODP Site 1090 to the orbitally tuned stable isotope record from ODP Site 929 across the O/M boundary shows that variability in the two records is very similar and can be correlated at and below the O/M boundary. Site 1090 stable isotope records also provide the first deep Southern Ocean end‐member for reconstructions of circulation patterns and late Oligocene to early Miocene climate change. Comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow. Oxygen isotope gradients between sites suggest that the deep Southern Ocean was cold in comparison to the North Atlantic, Indian, and the Pacific Oceans. Dominance of cold Southern Component Deep Water at Site 1090, at least until 17 Ma, suggests that relatively cold circumpolar climatic conditions prevailed during the late Oligocene and early Miocene. We believe that a relatively cold Southern Ocean reflects unrestricted circumpolar flow through the Drake Passage in agreement with bathymetric reconstructions.

A stable isotopic study to determine carbon and nitrogen cycling in a disturbed Southern Californian forest ecosystem

This study utilized isotope analyses to contrast nitrogen and carbon dynamics at four sites located along an air pollution gradient in the San Bernardino National Forest in southern California. Natural 15N and 13C abundances along with nutritional and edaphic properties were determined in soil, litter, and vegetation samples. Mean bulk nitrogen δ15N values of soil and vegetation at Camp Paivika (CP), the most polluted site, were at least 1.7‰ more enriched than the other, less polluted sites. Mean soil δ15NH4+ was also significantly enriched in 15N at CP compared to Barton Flats (BF), the least polluted site, by 3.8‰. Soil δ15NO3− signatures were not statistically different among sites. The litter δ15NH4+ values followed a trend similar to that of the soil. Furthermore, the litter δ15NO3− at CP was significantly depleted in 15N compared to the other sites. The isotopic discrimination for the eventual production of nitrate from organic nitrogen in soil and litter was maximized at CP and minimized at BF. A stable carbon isotopic gradient of decreasing soil, litter, and foliar δ13C was also observed with increasing site pollution level. These results support the hypothesis that chronic atmospheric deposition has enhanced nitrogen cycling processes and has affected carbon metabolism at CP.

Comparison of interglacial stages in the South Atlantic sector of the southern ocean for the past 450 kyr: implifications for Marine Isotope Stage (MIS) 11

Oxygen and carbon isotopic gradients in surface waters were reconstructed for the past 450 kyr by analysis of the planktic foraminifer Neogloboquadrina pachyderma in cores located at approximately 43°, 47°, and 54°S across the Polar Frontal Zone in the South Atlantic sector of the Southern Ocean. Comparison of the oxygen isotopic records for peak interglacial conditions during the past 450 kyr reveals that Marine Isotope Stage (MIS) 11 was not substantially warmer than other interglacials at high southern latitudes, although the period of warmth lasted longer. The carbonate and carbon isotope chemistry of surface and deep water represent the truly distinctive aspects of Stage 11 in the Southern Ocean. Peak carbonate production occurred at high southern latitudes during MIS 11, resulting in light-colored, high-carbonate sediments deposited throughout the Southern Ocean above the lysocline. Carbon isotopic values of benthic foraminifera in cores bathed by Circumpolar Deep Water (CPDW) were highest during MIS11, suggesting strong input of North Atlantic Deep Water (NADW) to the Southern Ocean. Planktic δ 13C values at high southern latitudes were also highest during MIS 11, which may reflect upwelling of CPDW with a greater contribution of NADW, lower whole-ocean nutrient inventories, higher gas exchange rates, and/or lowered alkalinity of Antarctic surface waters (resulting from carbonate precipitation south of the Polar Front).

Palaeoceanographic change in the northeastern South China Sea during the last 15000 years

The sedimentary succession of piston core RC26-16, dated by 14C accelerator mass spectrometry, provides a nearly continuous palaeoceanographic record of the northeastern South China Sea for the last 15000 yr. Planktic foraminiferal assemblages indicate that winter sea-surface temperatures (SSTs) rose from 18°C to about 24°C from the last glacial to the Holocene. A short-lived cooling of 1°C in winter temperature centred at about 11000 14C yr ago may reflect the Younger Dryas cooling event in this area. Summer SSTs have remained between 27°C and 29°C throughout the record. The temperature difference between summer and winter was about ca. 9°C during the last glacial, much higher than the Holocene value of ca. 5°C. During the late Holocene a short-lived cooling event occurred at about 4000 14C yr ago. Oxygen and carbon isotopic gradients between surface (0–50 m) and subsurface (50–100 m) waters were smaller during the last glacial than those in the Holocene. The fluctuation in the isotopic gradients are caused most likely by changes in upwelling intensity. Smaller gradients indicate stronger upwelling during the glacial winter monsoon. The fauna-derived estimates of nutrient content of the surface waters indicate that the upwelling induced higher fertility and biological productivity during the glacial. The winter monsoon became weaker during the Holocene. The carbonate compensation depth and foraminiferal lysocline were shallower during the Holocene, except for a short-lived deepening at about 5000 14C yr ago. A preservation peak of planktic foraminifera and calcium carbonate occurred between 13400 and 12000 14C yr ago, synchronous to the global preservation event of Termination I.

Palaeoceanographic change in the northeastern South China Sea during the last 15000 years

The sedimentary succession of piston core RC26-16, dated by 14 C accelerator mass spectrometry, provides a nearly continuous palaeoceanographic record of the northeastern South China Sea for the last 15 000 yr. Planktic foraminiferal assemblages indicate that winter sea-surface temperatures (SSTs) rose from 18°C to about 24°C from the last glacial to the Holocene. A short-lived cooling of 1°C in winter temperature centred at about 11 000 14 Cy r ago may reflect the Younger Dryas cooling event in this area. Summer SSTs have remained between 27°C and 29°C throughout the record. The temperature difference between summer and winter was about ca. 9°C during the last glacial, much higher than the Holocene value of ca. 5°C. During the late Holocene a short-lived cooling event occurred at about 4000 14 Cy r ago. Oxygen and carbon isotopic gradients between surface (0-50 m) and subsurface (50-100 m) waters were smaller during the last glacial than those in the Holocene. The fluctuation in the isotopic gradients are caused most likely by changes in upwelling intensity. Smaller gradients indicate stronger upwelling during the glacial winter monsoon. The fauna-derived estimates of nutrient content of the surface waters indicate that the upwelling induced higher fertility and biological productivity during the glacial. The winter monsoon became weaker during the Holocene. The carbonate compensation depth and foraminiferal lysocline were shallower during the Holocene, except for a short-lived deepening at about 5000 14 C yr ago. A preservation peak of planktic foraminifera and calcium carbonate occurred between 13400 and 12 000 14 C

Surface ocean and monsoon climate variability in the South China Sea since the last glaciation

Changes in the Southeast Asia monsoon winds and surface circulation patterns since the last glaciation are inferred using multiple paleoceanographic indicators including planktic foraminifer faunal abundances, fauna and alkenones sea-surface temperature (SST) estimates, oxygen and carbon isotopes of planktic and benthic foraminifers, and sedimentary fluxes of carbonates and organic carbon obtained from deep-sea core SCS90-36 from the South China Sea (SCS) (17°59.70'N, 111°29.64'E at water depth 2050 m). All these paleoceanographic evidences indicate marked changes in the SCS ocean system over the last glacial toward the Holocene. Planktic foraminiferal faunal SST estimates show stable warm-season SST of 28.6°C, close to the modern value, throughout the glacial-interglacial cycle. In contrast, cold-season SST increases gradually from 23.6°C in the last glacial to a mean value of 26.4°C in the Holocene with a fluctuation of about 3°C during 13–16 ka. SST estimates by U 37 k′ method reveal less variability and are in average 1–3°C lower than the fauna-derived winter-season SST. These patterns reveal that the seasonality of the SST is not only higher by about 3–4°C in the glacial, but also a function of the winter season SST. Sedimentation rates decrease from the last glacial-deglacial stage to the Holocene due to a reduction in supply of terrigenous components, which led to an increase of carbonate contents. Total organic carbon (TOC) contents of primarily marine sources decrease from the last glacial-deglacial to the Holocene. The last deglaciation is also characterized by high surface productivity as indicated by increased ketones abundances and high mass accumulation rates (MAR) of the TOC and carbonates. The gradient of planktic foraminifer ocygen and carbon isotopes of between surface dwellers and deep dwellers increases significantly toward Termination I and Holocene, and is indiscernibly small in the carbon isotope gradient of between 14 and 24 ka, revealing a deep-mixing condition in surface layers prior to 10 ka. The glacial-interglacial fluctuation of the carbon isotope value of a benthic foraminifer is 0.61%. which is significantly larger than a global mean value. The large carbon isotope fluctuation indicates an amplification of marginal-sea effects which is most likely resulted from an increase in surface productivity in the northern SCS during the last glacial-deglacial stage. The multiple proxies consistently indicate that the last glacial-deglacial stage winter monsoon in the Southeast Asia was probably strengthened in the northern SCS, leading to a development of deep-mixing surface layer conditions and a more efficient nutrient cycling which supports more marine organic carbon production.

Middle Miocene deepwater paleoceanography in the southwest Pacific: Relations with East Antarctic Ice Sheet development

A suite of middle Miocene Deep Sea Drilling Project sites in the southwest Pacific reveals large‐scale changes in deepwater circulation associated with East Antarctic Ice Sheet (EAIS) variations from ∼16.5 to 13.8 Ma. Oxygen and carbon isotopic records based on Cibicidoides benthic foraminifera from a depth transect (sites 590B, 588A, 591B, and 206 from 1200‐ to 3150‐m paleodepth at ∼35°S paleolatitude) and from a meridional transect (sites 588A, 590B, 593, and 594 from 30° to 48°S paleolatitude at intermediate water depth) allow detailed examination of southwest Pacific deepwater circulation from ∼17.5 to 12 Ma. Significantly, intervals of low δ18O from 16.5 to 16.3 Ma and perhaps at 15.7 Ma were marked by similar δ18O values at upper bathyal (∼1200–1500 m; sites 588A and 590B) and midbathyal (∼2100 m; site 591B) water depths. Small vertical δ18O gradients during δ18O minima may indicate warm saline deep water (WSDW) at midbathyal depths in the southwest Pacific during intervals of inferred global warmth and low global ice volume. Increased vertical δ18O gradients after ∼15.6 Ma and especially after 13.8 Ma indicate increased production of Southern Component Water (SCW) in association with EAIS growth. These data are consistent with the hypothesis (Woodruff and Savin, 1989) that major EAIS growth was fostered by diminished meridional heat transport to the high southern latitudes related to the termination of Tethyan Indian Saline Water (TISW) and an increase in SCW production during the early middle Miocene after ∼15.6 Ma. Further, a maximum vertical carbon isotopic gradient of ∼0.8 ‰ at 13.6 Ma suggests that Southern Component Intermediate Water (SCIW) production and Pacific Deep Water (PDW) strength were each at a maximum at this time and were critical to major EAIS growth. The establishment of near‐modern δ13C and δ18O gradients following major EAIS growth from ∼14.0 to 13.8 Ma marks a major step in the development of the Neogene ocean/cryosphere system.

Middle–Late Cretaceous climate of the southern high latitudes: Stable isotopic evidence for minimal equator-to-pole thermal gradients

A detailed δ 18 O and δ 13 C stratigraphy has been generated from analysis of well-preserved Albian–early Maastrichtian foraminifera from Deep Sea Drilling Project (DSDP) Sites 511 and 327 (Falkland Plateau; ≈58° S–62° S paleolatitude) in the southern South Atlantic, and Cenomanian and Coniacian–Santonian foraminifera from DSDP Site 258 (Naturaliste Plateau; ≈58° S paleolatitude) in the southern Indian Ocean. These results, when combined with previously published Maastrichtian stable isotope data from Ocean Drilling Program (ODP) Site 690 (Weddell Sea, ≈65° S paleolatitude), provide new insight into the climatic and oceanographic history of the southern high latitudes during middle–Late Cretaceous time. The planktonic foraminifer δ 18 O curves reveal a gradual warming of surface waters from the Albian through the Cenomanian followed by extremely warm surface waters from the Turonian through the early Campanian. Long-term cooling of surface waters began in the late early Campanian and continued through the end of the Maastrichtian. The benthic foraminifer δ 18 O record generally parallels changes in the oxygen isotopic curves defined by shallow-dwelling planktonic foraminifera. The vertical oxygen and carbon isotopic gradients were relatively low during the Albian–Cenomanian, high from the Turonian–early Campanian, and then low during the late Campanian and Maastrichtian. Foraminiferal oxygen isotopic data from published sources and this study are averaged for each site, corrected for latitudinal changes in salinity based on modern-day surface-water values, and plotted versus paleolatitude for the late Albian, Coniacian–Santonian, and late Maastrichtian. Differences between lowand high-latitude surface-water paleotemperatures are estimated at ≈14 ° C during the late Albian and late Maastrichtian, but the Coniacian–Santonian reconstruction reveals only a 0–4 ° C latitudinal temperature gradient. Uncertainty regarding Cretaceous salinity gradients and possible diagenetic alteration of δ 18 O values introduce error into our estimates of paleolatitudinal thermal gradients; however, apparent low equator-to-pole temperature differences could indicate much higher poleward heat transport than at present.

Late Pleistocene Paleoceanography of the South Atlantic Sector of the Southern Ocean: Ocean Drilling Program Hole 704A

Isotopic and sedimentologic data from Ocean Drilling Program hole 704A suggest that isotopic stages 7, 9, and 11 were marked by unusually strong interglacial conditions in surface waters of the southern ocean. During interglacial stages 9 and 11, warm surface waters penetrated far poleward and may have led to destabilization of the West Antarctic Ice Sheet. In contrast, the strongest glacial conditions in surface waters of the subantarctic South Atlantic occurred during oxygen isotopic stage 12. Comparisons of benthic carbon isotopic gradients between sites located in the North Atlantic, southern ocean, and Pacific indicate that the production of upper North Atlantic Deep Water (uNADW) was strongest during stages 7,9, and 11 and weakest during stage 12, These results suggest a possible link between the flux of uNADW and paleoceanographic change in the southern ocean and support the traditional NADW‐Antarctic connection whereby increased NADW leads to warming of the southern ocean.

Oceanographic and climatic implications of the Palaeocene carbon isotope maximum

ABSTRACTWe have compared detailed planktonic and benthonic foraminiferal carbon and oxygen isotope records from the Palaeocene and early Eocene successions at DSDP Site 577 (Shatsky Rise, North Pacific), a composite section derived from DSDP Leg 74 sites (Walvis Ridge, South Atlantic) and a composite section from ODP Leg 113 sites (Maud Rise, Weddell Sea). The δ13C records of Palaeocene and early Eocene Foraminifera at Site 577 and the Leg 74 sites show that an increase in δ13C values in surface waters at 64 Ma (end of Zone P1) resulted in increased vertical carbon isotope gradients (δ13C) between surface and deeper dwelling planktonic foraminifera, and between surface‐dwelling planktonics and benthonic foraminifera which became progressively steeper until the iniddle Late Palaeocene (Zone P4). This steepening also occurs in the latest Palaeocene of the composite Leg 113 section and can be explained by an increase in surface ocean productivity. This increase in productivity probably resulted in an expansion of the oxygen minimum zone (OMZ).Benthonic δ13C values increased during the late Palaeocene in Site 577 and the composite Leg 74 section, suggesting that the Palaeocene carbon isotope maximum was composed of both within‐ocean reservoir (increased surface water productivity) and between‐reservoir (organic carbon burial) ftactionation effects. The benthonic δ13C increase lags the surface ocean δ13C increase in the early Palaeocene (63–64 Ma) suggesting that surface water productivity increase probably led an increase in the burial rate of organic carbon relative to carbonate sedimentation. Moreover, inter‐site δ13C comparisons suggest that the locus of deep to intermediate water formation for the majority of the Palaeocene and the earliest Eocene was more likely to have been in the high southern latitudes than in the lower latitudes.Oxygen isotope data show a decline in deeper water temperatures in the early and early late Palaeocene, followed by a temperature increase in the late Palaeocene and across the PalaeoceneEocene boundary. We speculate that these changes in deeper water temperatures were related to the flux of CO2 between the oceans and the atmosphere through a mechanism operating at the high southern latitudes.

Stable carbon isotope variation in C3 and C4 plants along the Amazon River

ALL plants assimilate 12C in preference to 13C. As a result of this isotope fractionation, the tissues of subaerial plants have lower 13C/12C ratios than that of atmospheric CO2. By contrast, plant respiration and tissue decomposition are accompanied by little, if any, fractionation and hence release 13C-depleted biogenic CO2 back into the atmosphere. If this biogenic CO2 is reassimilated before it is thoroughly mixed into the atmosphere, a further depletion of 13C in the plant tissue will result. This recycling effect has been found most often in vertical variations of stable isotope composition in tropical forests where plant tissues near the forest floor are more depleted in 13C (refs 1–5). Here we show that the intensity of biogenic CO2 recycling in flood plain forests of the Amazon systematically increases inland, in the western Amazon basin. We also show that a similar recycling mechanism affects the 13C composition of semiaquatic grasses owing to evasion of biogenic CO2 from the Amazon river. But in this case the degree of recycling is more pronounced in the eastern basin because the flux of 13CO2 out of the river is smaller there. Our data indicate that significant spatial carbon isotope gradients can exist across the same general ecosystem, both between different species and also within a single species. Recycling effects therefore need to be taken into account in studies that try to relate plant carbon composition to animal and human diet, and in those attempting to determine the carbon isotope composition of the ancient atmosphere from preserved plant tissues.

Evolution of Atlantic-Pacific δ 13C gradients over the last 2.5 m.y.

The evolution of interocean carbon isotopic gradients over the last 2.5 m.y. is examined using high-resolution δ 13C records from deep sea cores in the Atlantic and Pacific Oceans. Over much of the Northern Hemisphere ice ages, relative reductions in North Atlantic Deep Water production occur during ice maxima. From 2.5 to 1.5 Ma, glacial reductions in NADW are less than those observed in the late Pleistocene. Glacial suppression of NADW intensified after 1.5 Ma, earlier than the transition to larger ice sheets around 0.7 Ma. At a number of times during the Pleistocene, δ 13C values at DSDP Site 607 in the North Atlantic were indistinguishable from eastern equatorial Pacific δ 13C values from approximately the same depth (ODP Site 677), indicating significant incursions of low δ 13C water into the deep North Atlantic. Atlantic/Pacific δ 13C values converge during glaciations between ∼ 1.13-1.05 m.y., 0.83-0.70 m.y., and 0.46-0.43 m.y. This represents a pseudo-periodicity of approximately 300 kyr which cannot easily be ascribed to global ice volume or orbital forcing. This partial decoupling, at low frequencies, of the δ 18O and δ 13C signals at Site 607 indicates that variations in North Atlantic deep water circulation cannot be viewed simply as a linear response to ice sheet forcing.

Geochemical evidence for suppression of pelagic marine productivity at the Cretaceous/Tertiary boundary

The normal, biologically productive ocean is characterized by a gradient of the 13C/12C ratio from surface to deep waters. Here we present stable isotope data from planktonic and benthic micro-fossils across the Cretaceous/Tertiary boundary in the North pacific, which reveal a rapid and complete breakdown in this biologically mediated gradient. The fluxes of barium (a proxy for organic carbon) and CaCO3 also decrease significantly at the time of the major marine plankton extinctions. The implied substantial reduction in oceanic primary productivity persisted for ∼0.5 Myr before the carbon isotope gradient was gradually re-established. In addition, the stable isotope and preservational data indicate that environmental change, including cooling, began at least 200 kyr before the Cretaceous/Tertiary boundary, and a peak warming of ∼3 °C occurred 600 kyr after the boundary event.

Benthic isotope evidence for changes of the Mediterranean outflow during the Late Quaternary

A suit of sediment cores close to and south of the Strait of Gibraltar (12°‐36°N, 500–2800 m water depth) were analyzed for stable isotopes in epibenthic foraminifers Cibicidoides wuellerstorfi and Planulina ariminensis. During peak glacial times, the data exhibit higher δ13C values of up to 1.6‰ at intermediate depths near the Strait of Gibraltar (36°N). The values decrease to the south as evidenced by our data, but also to the north as revealed by data of intermediate depth cores north of 38°N (in Duplessy et al. [1987]). Thus, the distribution pattern of δ13C provides crucial evidence for an increased influence of nutrient depleted Mediterranean Outflow Water (MOW) on the glacial northeast Atlantic hydrography. During oxygen isotope Terminations I and II, the meridional carbon isotope gradient indicates a significantly decreased but still active MOW. As deduced from the δ18O fluctuations, the temperatures of the MOW in the Atlantic were lower during glacial times by as much as 5°C. During glacial times and during Termination I the maximum δ13C values of the MOW correlate with minimum values of the North Atlantic Deep Water (NADW) and vice versa. This inverse response to climatic change of the carbon isotope signals of both water masses indicates, that the supply of saline MOW to the north Atlantic may be less important for the formation of NADW than previously assumed.

Late Pliocene reorganization of deep vertical water-mass structure in the western South Atlantic: Faunal and isotopic evidence

Changes in the vertical water mass structure of the Vema Channel during the Pliocene have been inferred from benthic foraminiferal assemblages and stable isotopic analyses from three sites of DSDP Leg 72 (South Atlantic). Faunal and isotopic results from Sites 516A and 518 suggest that a major change occurred in deep-water circulation patterns in the late Pliocene near 3.2 Ma. Benthic oxygen isotopic records from Sites 516A and 518 show a characteristic increase in δ 18 O values near 3.2 Ma. This has been documented in numerous Pliocene isotopic records. The magnitude of the oxygen isotopic enrichment near 3.2 Ma appears to increase with water depth from an average enrichment of 0.34‰ in Site 516A (1,313 m) to an average enrichment of 0.58‰ in Site 518 (3,944 m). We suggest that this enrichment resulted partly from a change in deep-water circulation patterns which included a decrease in bottom-water temperatures. Planktonic δ 18 O values near 3.2 Ma show no evidence of an enrichment which would be indicative of an increase in global ice volume. On the contrary, δ 18 O values in Sites 517 and 518 become more depleted near 3.2 Ma, indicating a surface-water warming perhaps due to a change in the strength and/or position of the Brazil Current. An increase in the relative abundance of the benthic foraminifer Nuttalides umbonifera , which is associated with Antarctic Bottom Water (AABW) in the modern ocean, coincides with the benthic 18 O enrichment in Site 518. At 3.2 Ma, oxygen and carbon isotopic gradients between Sites 518 (3,944 m) and 516A (1,313 m) show a marked increase such that Site 518 becomes enriched in 18 O and depleted in 13 C relative to Site 516A. This enrichment in δ 18 O is interpreted as partly representing a temperature decrease at Site 518; the depletion in δ 13 C indicates a corrosive water mass which is high in metabolic CO 2 . We suggest that benthic foraminiferal and stable isotopic changes in Site 518 resulted from a pulse-like increase in the formation of AABW near 3.2 Ma. The cause of this circulation event may have been linked to global cooling and/or the final closure of the Central American Seaway.

Oxygen and carbon isotope record of East Pacific core V19-30: implications for the formation of deep water in the late Pleistocene North Atlantic

A detailed oxygen and carbon isotope record has been obtained from benthic Foraminifera in core V19-30 from the Carnegie Ridge on the south side of the Panama Basin. Expressing these records and the oxygen and carbon isotope records previously published for Atlantic core M-12392 on a common timescale, it is apparent that the oxygen isotope records are very similar but that the carbon isotope records are quite different. By obtaining the carbon isotope gradient between the two sites as a function of time we show that the production of North Atlantic Deep Water has varied over a wide range during the late Pleistocene, and that the pattern of variation is not simply related to the well known oxygen isotope record. Although the two oxygen isotope records are very similar, changes in the interoceanic gradient are detectable and support the hypothesis that in the glacial mode the North Atlantic was colder, and less oxygenated, than it is today. Shackleton's [1] 1977 interpretation whereby the carbon isotope record from the Atlantic core reflects changes in the terrestrial biomass, is an over-simplification. However, the record from the Pacific core V19-30 probably can be explained in these terms since it probably approximates the carbon isotope record of global mean oceanic dissolved CO 2.