Partitioning of the Modern Oceanic Mass and Heat Fluxes
The modern partitioning of mass and heat fluxes between ocean basins and hemispheres is highly structured, and raises interesting questions about how they may have been distributed in the past. Today, oceanic heat fluxes are asymmetric about the equator, but the combined oceanic plus atmospheric heat flux (the real 'global conveyor') is, within error bars, almost indistinguishable from perfect antisymmetry. That is, the total poleward enthalpy flux is nearly the same in both hemispheres despite the very different continental and oceanic distributions. If this antisymmetry is not a coincidence, it implies that shifts in oceanic heat budgets will be compensated by atmospheric ones, and vice-versa. The comparatively minor oceanic contribution to high latitude heat fluxes (in large part a function of the diminishing oceanic surface area with high northern latitude) makes it an implausible trigger of massive climate change. Speculation about past oceanic and atmospheric heat flux pathways has to account for global radiation balance, and for the multiplicity of mechanisms and routes by which the coupled system can move and balance heat and moisture.
Extending the High-Resolution Global Climate Record in Santa Barbara Basin: Preliminary Results and Implications
A major legacy of Michael Sarnthein is his ample demonstration of the necessity for high-resolution paleoceanography and chronology to understand processes of late Quaternary climate change. Consistent with this legacy we have begun to extend the remarkable, high resolution climate record of the Santa Barbara Basin beyond 160 ka found at the base of ODP Site 893. Safety issues prevented drilling of older basinal sediments. However, active folding, uplift and erosion have exposed a sequence of dipping sediments of upper bathyal paleodepths (500-1000 m) now at shallow (160 m) ocean floor depths on the Mid-Channel Trend. Integrative studies of high-resolution seismic and sediment stratigraphy of 33 piston cores have provided an excellent marine climatic sequence that may represent each of the oxygen isotopic stages (OIS) of the last 500 ka (possibly as old as OIS16). Precise core locations on the ocean floor have provided both individual time windows and composite, overlapping sediment records likely to reveal paleoclimatic behavior prior to 150 ka at high stratigraphic resolution not available in ice cores. The lithostratigraphy of the sequence suggests late Quaternary basinal environmental behavior similar to Site 893 and involving major changes in oxygenation state and inferred climatic oscillations over the last 500 kys. Pristinely laminated intervals inferred to represent interglacials and interstadials alternate with massive, bioturbated, sandier intervals representing glacials and stadials. Intermediate oxygenation states are suggested by diffuse or discontinuous laminations. Certain earlier cool episodes record potentially millennial-scale climate oscillations in oxygen minimum zone strength, as during OIS3, in response to changing ventilation and/or surface-ocean productivity. The episodic dysoxic intervals reflect a semi-isolated basin for at least the last 500 ka. Several glacial terminations are well recorded. For example, a composite record of four overlapping cores may have captured the important deglacial episode from OIS12 to 11. This work confirms the presence of an accessible, high-resolution Quaternary climatic sequence for future drilling and piston coring. The success of this expedition suggests that active margin settings should be sought rather than avoided for high-resolution global climate studies.
A possible role for North Pacific salinity in stabilizing North Atlantic climate
A simple ocean/atmosphere feedback may reduce the amplitude of climate variability in around the North Atlantic during interglacial compared to glacial states. When climate is warm in the North Atlantic region, the ITCZ has a relatively northward position and moisture is exported from the tropical Atlantic to the tropical Pacific. At the same time, the east Asian summer monsoon is strong, which helps maintain a positive balance of precipitation over evaporation in the subpolar North Pacific. This is thought to account for lower salinity in the North Pacific relative to the North Atlantic, which in turn drives northward flow through the Bering Strait and freshens the northern North Atlantic. Freshening in the North Atlantic tends to suppress the meridional overturning circulation and reduce the North Atlantic heat flux. The opposite situation exists during cold climate: southward movement of the ITCZ in the Atlantic and a weaker east Asian monsoon cause higher salinity in the North Pacific and reduced flow through Bering Strait. Thus, the combination of atmospheric vapor transport and flow through Bering Strait tend to cool the North Atlantic region when warm and warm the region when cool.
Cooling of the Eastern Tropical North Pacific in Response to a Slow-Down of the Atlantic Meridional Overturning Circulation
Surface ocean conditions in the Pacific Ocean, and in particular in the equatorial Pacific, during millennial-scale climate change in the past are intensely debated because they might hold the clue to whether abrupt global climate change has its origin there, or whether it is controlled by changes in the vigour of the Atlantic meridional overturning circulation (AMOC). A high-resolution alkenone unsaturation-based SST record from the eastern tropical Pacific north of the equator shows a cooling of ca. 1°C during the last glacial-interglacial transition, which is synchronous (within the recognized uncertainties of radiocarbon chronologies) with Heinrich event 1 in the North Atlantic. This cold spell is paralleled by a maximum in surface ocean productivity as inferred from 230-Th normalized organic carbon fluxes, suggesting that it was caused by increased upwelling. The observed cooling of the ETNP during the time interval of H1 is consistent with a southward displacement of the ITCZ caused by a slowdown of the AMOC during this event, but is at odds with model predictions about the occurrence of ENSO shutdowns (La NiÃ±a-like conditions) in the past. The records presented here thus lend strong support to the hypothesis that millennial-scale climate change is driven by a reorganization of the ocean's thermohaline circulation, albeit potentially amplified by tropical ocean-atmosphere interactions.
The Atlantic-Pacific Salinity Contrast and Sea Level During Marine Isotope Stage 11
Several lines of evidence suggest that sea level may have been more than 20 meters higher during Marine Isotope Stage 11 (MIS 11) than during the Holocene. Yet high-resolution oxygen isotope records from the Atlantic for these two time periods display negligible differences. One way to reconcile these apparently conflicting observations would be a greater salinity contrast between the Atlantic and Pacific oceans. This contrast is maintained today by asymmetric atmospheric water vapor transport and by differing salinities in the northern and southern deep waters that combine to fill the deep Pacific. In order to test whether the salinity difference was greater during MIS 11 than the today, we compare isotope records from sites in the Atlantic and Pacific. These sites include ODP Sites 980 (55°N, 15°W, 2.2km) and 983 (60°N, 24°W, 2.0km) in the North Atlantic, ODP Site 925 (4°N, 43°W, 3.0km) in the tropical Atlantic, ODP Site 1238 (2°S, 82°W, 2.4km) in the eastern tropical Pacific, and ODP Site 806 (0°N, 159°W, 2.5km) in the western tropical Pacific. The benthic oxygen isotope records from the Atlantic display no measurable difference between MIS 11 and Holocene, while the Pacific records are more variable, but suggest that MIS 11 values are lower by approximately 0.1 per mil. The results are not easily consistent with a 20-meter higher sea level during MIS 11, but they do allow a smaller rise combined with a greater salinity contrast between oceans.
Plio-Pleistocene Ocean Productivity: A Story of Interbasin Symmetry
The precipitous decline of biological productivity in the high latitude oceans of both the north Pacific and Antarctic synchronous with the intensification of northern hemisphere glaciation (NHG) (2.75 Ma), has previously been attributed to the abrupt onset of ocean stratification in both of these regions. Using the concentration of alkenones, organic compounds exclusively produced by a few species of haptophyte algae, in ocean sediments from the north Atlantic (ODP Site 982), eastern equatorial Pacific (ODP Site 846) and eastern equatorial Atlantic (ODP Site 662) we document the continuous evolution of high latitude and tropical ocean productivity over the past 3.5 Myr. We find that biological productivity in the north Atlantic behaved similarly to that in the high latitude regions of the north Pacific and Antarctic, declining precipitously just after the intensification of NHG. These data suggest that high latitude stratification may have extended into the north Atlantic Ocean, implying that overturning in the north Atlantic after the intensification of NHG may have been significantly weaker than previously proposed. In contrast, productivity in the eastern equatorial Pacific and Atlantic, both low latitude upwelling zones, rose sharply approximately synchronous with the intensification of NHG. We hypothesize that nutrients that were inhibited by the onset of water column stratification from upwelling at high latitudes were instead entrained into the source waters for low latitude upwelling zones feeding the dramatic rise in productivity that occurred in these regions. Our high-resolution (3 kyr) records indicate that on orbital timescales, ocean productivity was anti-correlated to sea surface temperature changes at all sites. In the north Atlantic productivity variations beat at precessional and obliquity periods, while those in the upwelling zones of both the eastern Pacific and Atlantic Oceans beat primarily at obliquity periods. The symmetric behavior of both high latitude and low latitude productivity on both orbital and longer timescales, suggests that nutrient bearing mode waters communicated changes from the high to low latitude ocean in both the Atlantic and Pacific basins.
Climate Models Suggest No Prominent Role of the Panamanian Gateway Closure in Northern Hemisphere Glaciation
The most significant climate transition of the Pliocene was the pronounced intensification of Northern Hemisphere glaciation, which culminated in a synchronous ice-sheet development between Greenland, Eurasia and North America around 2.7 million years ago. Various hypotheses have been proposed to explain the sudden appearance of major ice sheets in the Northern Hemisphere. One hypothesis, which has received much attention, relies on the combined effects of the closure of the Panamanian Gateway and favorable orbital forcing. Utilizing climate models of different complexity, we test the hypothesis that the Pliocene closure of the Panamanian Gateway was a necessary precondition for orbitally triggered Northern Hemisphere glaciation. The first model is an Earth-system model of intermediate complexity (ECBilt-CLIO) while the second is a comprehensive climate model (CCSM 2.01). We conduct a series of sensitivity experiments with both models in order to analyze the isolated and combined effects of Panama closure and orbital forcing on perennial snow cover. In both models, orbital forcing efficiently controls the extension of the snow cover in the Northern Hemisphere. The closure of the Panamanian Gateway leads to an intensification of the thermohaline circulation in the Atlantic Ocean in both models. South of Iceland the associated increase in meridional heat and salt transports raise annual-mean sea-surface temperature by approx. 1-2 K and sea-surface salinity by approx. 0.5 psu. Both changes are in good agreement with recent paleoceanographic reconstructions at ODP Site 984 for the time of the Panama closure [Bartoli et al., 2005; EPSL]. Although the warmer surface layer in the North Atlantic intensifies snowfall on the adjacent continents, the effect on perennial snow cover remains negligible due to enhanced summer melting. Accordingly, the model experiments do not suggest that the closure of the Panamanian Gateway intensifies orbitally forced glaciation in high northern latitudes.
Final closure of the Panamaian Isthmus and the onset of northern hemisphere glaciation
The Greenland ice sheet forms a key factor controlling the Quaternary-style glacial scenario. However, origin and mechanisms of major Arctic glaciation starting at 3.15 Ma and culminating at 2.74 Ma have remained controversial. For this phase of intense cooling Ravelo et al. (2004) proposed a complex gradual forcing mechanism. In contrast, our new submillennial-scale paleoceanographic records from the Pliocene North Atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation (NHG), since it was linked to a 2-3 $/deg$C surface water warming during warm stages from 2.95 to 2.82 Ma (until glacial stage G10). These records support previous models (Haug and Tiedemann, 1998) claiming that the final closure of the Panama Isthmus (3.0 - ~2.5 Ma; Groeneveld, 2005) induced an increased poleward salt and heat transport. Associated strengthening of North Atlantic thermohaline circulation and in turn, an intensified moisture supply to northern high latitudes resulted in the build-Up of NHG, finally culminating in the great, irreversible climate crash at glacial stage G6 (2.74 Ma). In summary, we see a two-step threshold mechanism that marked the onset of NHG with glacial-to-interglacial cycles quasi-persistent until today. G r o e n e v e l d, J., The final closure of the Central American Seaway. PhD Thesis Kiel, 2005. H a u g, G. and T i e d e m a n n , R., 1 9 9 8. Nature 393, 676-678. R a v e l o , A.C., e t a l ., 2004. Nature 429, 263-267.