Delia W. Oppo

Evolution of Atlantic―Pacific δ13C 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 δ13C 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, δ13C values at DSDP Site 607 in the North Atlantic were indistinguishable from eastern equatorial Pacific δ13C values from approximately the same depth (ODP Site 677), indicating significant incursions of low δ13C water into the deep North Atlantic. Atlantic/Pacific δ13C 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 δ18O and δ13C 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.

The Mid‐Pleistocene climate transition: A deep sea carbon isotopic perspective

Five δ13C records from the deep ocean, extending back to 1.3 Ma, were examined in order to constrain changes in mean ocean carbon isotope composition and thermohaline circulation over the 41- to 100-ka climate transition. These data show that significant perturbations in mean ocean carbon chemistry were associated with the mid-Pleistocene climate transition. Notable features of the last 1.3 Myr are (1) a pronounced ∼0.3‰ decrease in mean ocean δ13C between 0.9 and 1.0 Myr, followed by a return to pre-1.0 Ma values by 400 ka B.P., which we propose was due to the onetime addition of isotopically depleted terrestrial carbon to the ocean, possibly associated with an increase in global aridity (and decrease in the size of the biosphere) across the 41- to 100-ka transition; (2) no change in the Atlantic-Pacific (A-P) δ13C gradient over the last 1.3 Myr, suggesting no change in mean ocean nutrient content accompanied the addition of light carbon; and (3) stronger vertical nutrient fractionation in the North Atlantic in the middle Pleistocene between sites 607 and 552, suggesting weaker North Atlantic Deep Water formation at this time relative to the early and late Pleistocene. We also find evidence for a more pronounced deep recirculation gyre in the western North Atlantic basin in the early Brunhes, as evidenced by “aging” of deep northern basin water (site 607) relative to deep water in the equatorial Atlantic (site 664).

Mid-Depth Circulation of the Subpolar North Atlantic During the Last Glacial Maximum

Holocene and glacial carbon isotope data of benthic foraminifera from shallow to mid-depth cores from the northeastern subpolar Atlantic show that this region was strongly stratified, with carbon-13—enriched glacial North Atlantic intermediate water (GNAIW) overlying carbon-13—depleted Southern Ocean water (SOW). The data suggest that GNAIW originated north of the polar front and define GNAIW end-member carbon isotope values for studies of water-mass mixing in the open Atlantic. Identical carbon isotope values in the core of GNAIW and below the subtropical thermocline are consistent with rapid cycling of GNAIW through the northern Atlantic. The high carbon isotope values below the thermocline indicate that enhanced nutrient leakage in response to increased ventilation may have extended into intermediate waters. Geochemical box models show that the atmospheric carbon dioxide response to nutrient leakage that results from an increase in ventilation rate may be greater than the response to nutrient redistribution by conversion of North Atlantic deep water into GNAIW. These results underscore the potential rule of Atlantic Ocean circulation changes in influencing past atmospheric carbon dioxide values.

Suborbital timescale variability of North Atlantic Deep Water during the past 200,000 years

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Palaeo-oceanography: Deepwater variability in the Holocene epoch.

The conversion of surface water to deep water in the North Atlantic results in the release of heat from the ocean to the atmosphere, which may have amplified millennial-scale climate variability during glacial times and could even have contributed to the past 11,700 years of relatively mild climate (known as the Holocene epoch). Here we investigate changes in the carbon-isotope composition of benthic foraminifera throughout the Holocene and find that deep-water production varied on a centennial–millennial timescale. These variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period.

Millennial-scale changes in North Atlantic circulation since the last glaciation

Ocean circulation is closely linked to climate change on glacial–interglacial and shorter timescales. Extensive reorganizations in the circulation of deep and intermediate-depth waters in the Atlantic Ocean have been hypothesized for both the last glaciation and the subsequent Younger Dryas cold interval,, but there has been little palaeoceanographic study of the subtropical gyres. These gyres are the dominant oceanic features of wind-driven circulation, and as such they reflect changes in climate and are a significant control on nutrient cycling and, possibly, atmospheric CO2 concentrations. Here we present Cd/Ca ratios in the shells of benthic foraminifera from the Bahama banks that confirm previous suggestions, that nutrient concentrations in the North Atlantic subtropical gyre were much lower during the Last Glacial Maximum than they are today (up to 50% lower according to our data). These contrasting nutrient burdens imply much shorter residence times for waters within the thermocline of the Last Glacial Maximum. Below the glacial thermocline, nutrient concentrations were reduced owing to the presence of Glacial North Atlantic Intermediate Water. A high-resolution Cd/Ca record from an intermediate depth indicates decreased nutrient concentrations during the Younger Dryas interval as well, mirroring opposite changes at a nearby deep site,. Together, these observations suggest that the formation of deep and intermediate waters — North Atlantic Deep Water and Glacial North Atlantic Intermediate Water, respectively — wax and wane alternately on both orbital and millennial timescales.

Millennial-scale climate instability during the early Pleistocene epoch

Climate-proxy records of the past 100,000 years show that the Earths climate has varied significantly and continuously on timescales as short as a few thousand years (refs 1–7). Similar variability has also recently been observed for the interval 340–500 thousand years ago. These dramatic climate shifts, expressed most strongly in the North Atlantic region, may be linked to — and possibly amplified by — alterations in the mode of ocean thermohaline circulation. Here we use sediment records of past iceberg discharge and deep-water chemistry to show that such millennial-scale oscillations in climate occurred over one million years ago. This was a time of significantly different climate boundary conditions; not only was the early Pleistocene epoch generally warmer, but global climate variations were governed largely by changes in Earths orbital obliguity. Our results suggest that such millennial-scale climate instability may be a pervasive and long-term characteristic of Earths climate, rather than just a feature of the strong glacial–interglacial cycles of the past 800,000 years.

Synchronous, high-frequency oscillations in tropical sea surface temperatures and North Atlantic Deep Water production during the Last Glacial Cycle

Stable isotopic measurements of G. sacculifer and C. wuellerstorfi in a core from the western equatorial Atlantic imply that there are parallel, suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3. Low values of G. sacculifer δ18O accompany high values of C. wuellerstorfi δ13C, linking warmer sea surface temperatures (SSTs) in the tropics with increased production of lower North Atlantic Deep Water (NADW). The amplitude of the δ18O oscillations is 0.6‰ (or 2°–3°C), which is superimposed on a glacial/interglacial amplitude of about 2.1‰. Using the G. sacculifer δ18O data, we calculate that surface waters were colder during stage 2 than calculated by CLIMAP [1976, 1981]. The longer-period (>2 kyr) oscillations in air temperature recorded in the Greenland and Antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial Atlantic. The magnitude of these oscillations in tropical SST is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone. The apparent synchroneity of equatorial SST and polar air temperature changes, as well as the amplitude of the SST changes at the equator, are consistent with the climate effects expected from changes in the atmospheres greenhouse gas content (H2Ovapor, CO2, and CH4).

Persistent suborbital climate variability in marine isotope stage 5 and termination II

New surface water records from two high sedimentation rate sites, located in the western subtropical North Atlantic near the axis of the Gulf Stream, provide clear evidence of suborbital climate variations through marine isotope stage (MIS) 5 persisting even into the warm peak of the interglaciation (substage 5e). We found that the amplitude of suborbital climate oscillations did not vary significantly for the whole of MIS 5, implying that ice volume has little or no influence on the amplitude of suborbital climate variability in this region. Although some records suggest that longer suborbital variations (4–10 kyr) during MIS 5 are linked to deepwater changes, none of the existing records is of sufficient resolution to assess if a linkage occurred for oscillations shorter than 4 kyr. However, when examined in conjunction with published data from the Norwegian Sea, new evidence from the subpolar North Atlantic suggests that coupled surface-deepwater oscillations occurred during the penultimate deglaciation. This supports the hypothesis that during glacial and deglacial times, ocean-ice interactions and deepwater variability amplify suborbital climate change at higher latitudes. We suggest that during the penultimate deglaciation the North Atlantic deepwater source varied between Nordic Sea and open North Atlantic locations, in parallel with surface temperature oscillations.

Glacial enrichments of authigenic Cd And U in subantarctic sediments: A climatic control on the elements' oceanic budget?

We examine the possibility that glacial increase in the areal extent of reducing sediments might have changed the oceanic Cd inventory, thereby decoupling Cd from PO4. We suggest that the precipitation of Cd-sulfide in suboxic sediments is the single largest sink in the oceanic Cd budget and that the accumulation of authigenic Cd and U is tightly coupled to the organic carbon flux into the seafloor. Sediments from the Subantarctic Ocean and the Cape Basin (South Atlantic), where oxic conditions currently prevail, show high accumulation rates of authigenic Cd and U during glacial intervals associated with increased accumulation of organic carbon. These elemental enrichments attest to more reducing conditions in glacial sediments in response to an increased flux of organic carbon. A third core, overlain by Circumpolar Deep Water (CPDW) as are the other two cores but located south of the Antarctic Polar Front, shows an approximately inverse pattern to the Subantarctic record. The contrasting patterns to the north and south of the Antarctic Polar Front suggest that higher accumulation rates of Cd and U in Subantarctic sediments were driven primarily by increased productivity. This proposal is consistent with the hypothesis of glacial stage northward migration of the Antarctic Polar Front and its associated belt of high siliceous productivity. However, the increase in authigenic Cd and U glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts, suggesting greater partitioning of organic carbon into the sediments during glacial intervals. Lower oxygen content of CPDW and higher organic carbon to biogenic silica rain rate ratio during glacial stages are possible causes. Higher glacial productivity in the Cape Basin record very likely reflects enhanced coastal up welling in response to increased wind speeds. We suggest that higher productivity might have doubled the areal extent of suboxic sediments during the last glacial maximum. However, our calculations suggest low sensitivity of seawater Cd concentrations to glacial doubling of the extent of reducing sediments. The model suggests that during the last 250 kyr seawater Cd concentrations fluctuated only slightly, between high levels (about 0.66 nmol kg−1) on glacial initiations and reaching lowest values (about 0.57 nmol kg−1) during glacial maxima. The estimated 5% lower Cd content at the last glacial maximum relative to modern levels (0.60 nmol kg−1) cannot explain the discordance between Cd and δ13C, such as observed in the Southern Ocean. This low sensitivity is consistent with foraminiferal data, suggesting minimal change in the glacial Cd mean oceanic content.