A. C. Ravelo

TROPICAL PACIFIC OCEAN THERMOCLINE DEPTH RECONSTRUCTIONS FOR THE LAST GLACIAL MAXIMUM

We evaluate the relationship between ten surface ocean (0–300 m) hydrographic parameters and the spatial distribution of factor-analyzed core top planktonic foraminiferal abundances in the tropical Pacific Ocean (24°N–24°S) for core tops <3800 m. The spatial distribution of the first three faunal factor loadings (88% of the variance) are most highly correlated to subsurface variability (mixed layer depth, thermocline depth) and resistant species percent (RSP). However, RSP is not related to dissolution but is related to thermocline depth. Factor I (mixed layer species G. glutinata, G. ruber) and factor III (G. ruber) can be distinguished from each other by low abundances of G. glutinata in factor III. Both assemblages spatially comprise the deep mixed layer region of the western tropical and equatorial Pacific Ocean, but are associated with distinct water mass properties. A combination of Factor I and III loadings shows a higher correlation to thermocline depth (R² = 0.70). Factor II loadings (dominated by thermocline dwelling species N. dutertrei) are most significantly correlated with the thermocline depth (R² = 0.73). Most factors show only marginally significant correlation to sea surface temperatures (SSTs), indicating that SST is not the primary forcing factor on the planktonic foraminiferal species distributions in the tropical Pacific. A new transfer function was calculated to predict tropical Pacific thermocline depth from planktonic foraminifera abundances using the Imbrie-Kipp Method (IKM) (standard deviation of residuals ±22 m (1σ)). An additional ±5-m error is attributed to low species counts in the core top database. The modern analog technique (MAT) was also used to predict thermocline depth (standard deviation of residuals ±21 m). While last glacial maximum (LGM) thermocline depth changes by IKM and MAT were generally within error, estimated changes were geographically uniform, suggesting an oceanographic response to climate forcing. We estimate that the thermocline depth of the LGM was shallower than present by ∼20 m south of 8°S, possibly due to a shift in the South Pacific anticyclone to the northeast. Both the IKM and MAT estimate a steeper east-west thermocline slope along the equator, suggesting that zonal wind stress (Walker circulation) was intensified during the LGM. Collectively, the thermocline estimates for the LGM suggest an equatorward compression of the climate zones in both hemispheres.

Pliocene-Pleistocene evolution of eastern tropical Pacific surface water circulation and thermocline depth

We evaluated the response of eastern equatorial Pacific upper water column hydrography to the Panama seaway closure and the initiation of large-scale northern hemisphere glaciation, The δ18O gradient between Globorotalia tumida and Globigerinoides sacculifer indicates a general shoaling of the thermocline between 4.2 and 3.0 Ma which may be related to a recorded decrease in southeast trade wind strength. The δ13C gradients suggest that surface water nutrient concentrations have increased during the last 5 Myr and that upwelling intermediate waters began to change sources at ∼3.2 Ma. Changes in the trans-Pacific east-west temperature and nutrient gradients at 4.0 and 1.5 Ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation. Changes in the sensitivity of surface circulation to Milankovitch forcing, δ18O records linearly related to orbital variations only after 3.2 Ma, are associated with the seaway closure. Pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to Milankovitch forcing is enhanced by air-sea interactions which maintain the relatively steep Pleistocene trans-Pacific gradients. The δ13C records are dominated by the 41 kyr period and are usually coherent with high-latitude climate, suggesting that associated changes in the global carbon reservoir dominate the tropical δ13C signals.

Link between oceanic heat transport, thermohaline circulation, and the Intertropical Convergence Zone in the early Pliocene Atlantic

Planktonic foraminiferal oxygen isotope records from the western and eastern tropical Pacific and Atlantic Oceans suggest a southward shift in the Intertropical Convergence Zone toward its modern location between 4.4 and 4.3 Ma. A concomitant shift in the carbon isotope compositions of Atlantic benthic foraminifera provides strong evidence for an increased thermohaline overturn at this time. We suggest that the southward shift of the Intertropical Convergence Zone and associated change in trade-wind circulation altered equatorial surface hydrography, increased the advection of warmer and more saline surface waters into the subtropical and North Atlantic, and contributed to thermohaline overturn.

Early Pliocene climate: A perspective from the western equatorial Atlantic Warm Pool

High-resolution (∼3–4 kyr) planktonic stable isotope stratigraphies from Site 925 drilled on Ceara Rise (Ocean Drilling Program Leg 154) are used to investigate the role of the western equatorial Atlantic sea surface hydrography in early Pliocene (3.2–4.7 Ma) climate change. Oxygen isotope results from Globigerinoides sacculifer, a mixed layer dweller, suggest that equatorial sea surface temperatures were cooler than today by ∼2°–3°C, consistent with relatively strong northward advection of heat away from the equator. Oxygen isotope results from Neogloboquadrina dutertrei suggest that over the long term this thermocline dweller tracks global ice volume fluctuations. Stable isotope gradients between the two planktonic species throughout the entire interval imply a stable warm pool in the western equatorial Atlantic. We observe a rapid (3.4 kyr) decrease in G. sacculifer and the N. dutertrei δ18O values at 4.36 Ma that may reflect a freshening of the sea surface, a direct response to a southward displacement of the Intertropical Convergence Zone, and perhaps, an indirect response to restricted flow through the Central American Seaway.

The pacemaker always rings twice

We generated new, long, high-resolution, climate proxy records from Deep Sea Drilling Project (DSDP) Sites 607 and 609 in the subpolar North Atlantic over the interval 225–970 ka, which have pronounced variability at periods f > 1/7 kyr) variance in our North Atlantic climate proxy records is shown, using bispectral and cross-bispectral methods, to be explainable as harmonics and/or combination tones of orbital-scale climatic variability of the North Atlantic region itself. Thus the timing and amplitude of high-frequency climate change in the North Atlantic region appears to be a nonlinear function of variations in high-latitude climate at Milankovitch frequencies.

Pliocene carbonate accumulation along the California Margin

Recent modeling studies call on increased ocean heat transport to explain high-latitude warming observed for intervals throughout the middle Pliocene. Possible vehicles for ocean heat transport are the poleward arms of the subtropical gyres. Sites from the California margin (Ocean Drilling Program Leg 167) provide monitors of wind field within the eastern arm of the gyre which may be an indication of basin-wide subtropical gyral strength. At most sites (water depths from 1106 to 4212 m) CaCO3 mass accumulation rate (MAR) was highest in the middle Pliocene (3.5–2.0 Ma). This high CaCO3 MAR “event” is attributed primarily to higher CaCO3 production due to higher offshore upwelling associated with the zone of the greatest wind stress curl. Thus, in the middle Pliocene, there was enhanced wind stress curl along the California margin, and possibly enhanced North Pacific sub-tropical gyral circulation and meridional ocean heat advection.

A Pliocene to recent history of the Bering Sea at Site U1340A, IODP Expedition 323

Fossil diatoms are the principal component of Bering Sea sediments and reflect the paleoceanographic history of the region. Diatom accumulation rates and relative abundances at International Ocean Discovery Program (IODP) Site U1340A are presented. Overall, the total diatom productivity record from 4.9 Ma to the present day reveals a fourfold reduction at circa 4.2 Ma from ~45 × 107 down to 11 × 107 valves/g (wet sediment), signifying a major shift in the upwelling and/or nutrient regime, coinciding with the end of the late Miocene-early Pliocene bloom identified in the eastern equatorial Pacific and California margin. Further abrupt shifts in the diatom assemblage occur at (1) 2.78–2.55 Ma, (2) 2.0–1.8 Ma, and (3) 1.0–0.88 Ma. (1) At 2.78–2.55 Ma, the appearance of sea ice-related species marks the regional cooling associated with the expansion of Northern Hemisphere ice sheets, subsequent development of stratified, nutrient-depleted waters, and increased influence of Western Basin Water masses (most likely due to the suppressed inflow of the Alaskan Stream). (2) Rapid cooling between 2.0 and 1.8 Ma indicates increased sea ice duration and/or frequency. This, coupled with low sea level stands caused prolonged closure of the Aleutian Passes, coupled with further increased Western Basin Water inflow. (3) The shift to 100 ka glacial/interglacial cycles at the middle-Pleistocene transition (1.0–0.88 Ma) marked an increase in upwelling-related species, indicating enhanced surface water mixing. These records confirm that the development and changing dynamics of sea ice in the Bering Sea played a major role in sub-Arctic Ocean circulation and is an integral component of global climate change.

Evaluating drivers of Pleistocene eastern tropical Pacific sea surface temperature

Sea surface temperature (SST) of the eastern equatorial Pacific is a key component of tropical oceanic and atmospheric circulation with global teleconnections. Forcing factors such as local and high-latitude insolation changes, ice sheet size and albedo feedbacks, and greenhouse gas radiation have been proposed as controls of long-term eastern tropical Pacific SST, though the precise role each mechanism plays is not fully known on glacial-interglacial or longer timescales. Here proposed mechanisms are evaluated by comparing orbital-scale records of eastern Pacific SST with forcing variability over the past 1.5 Ma. The primary SST records are a compilation of new and existing data from Ocean Drilling Program Site 1239 at the northeastern margin of the modern eastern Pacific cold tongue and Site 846 SST within the cold tongue. Using time series analysis, we test previously proposed mechanisms for control of long-term tropical SST change and SST gradients in the eastern Pacific. We find that within statistical uncertainties, in the precession band eastern Pacific SST is consistent with direct forcing by equatorial radiation changes in the tropical cold season (summer-fall) rather than inversely correlated as previously suggested. In the obliquity band high-latitude solar forcing leads or is in phase with eastern equatorial Pacific SST, while in the eccentricity band atmospheric greenhouse gas concentrations are closely associated with cold tongue SST. Pleistocene eastern Pacific SST gradients indicate that the gradient on the northern margin of the cold tongue strengthened through the mid-Pleistocene transition, a result compatible with the cold tongue becoming more focused at ~900–650 ka.