Paleoceanography of the eastern equatorial Pacific during the Neogene: Synthesis of Leg 138 drilling results

The primary objective of Leg 138 was to provide detailed information about the ocean\u27s response to global climate change during the Neogene. Two north-south transects were drilled (95° and 110°W) within the region of equatorial divergence-driven upwelling (and thus high accumulation rates and resolution) and spanning the major equatorial ocean current boundaries (and thus recording a high-amplitude signal of the response of the sediment to climatically and/or tectonically driven changes in ocean circulation). The Neogene is marked by a number of well-known climatic and tectonic events (the closing of the Isthmus of Panama, the onset of North Atlantic Deep Water (NADW), the rapid uplift of the Himalayas, the major intensification of Northern Hemisphere glaciation), and the response of the ocean before and after these events was a key focus of Leg 138 drilling. To address these objectives at the highest resolution possible, the Leg 138 scientific staff developed a number of new shipboard strategies and analytical procedures. These included the real-time analysis of the near-continuous gamma ray attenuation porosity evaluator (GRAPE) and susceptibility profiles produced by the multisensor track (MST) on unsplit cores to monitor core recovery and, if necessary, to modify the drilling strategy to ensure proper offset of coring gaps; the collection of near-continuous color reflectance data on split cores; the logging of the first hole drilled at each site to optimize drilling and sampling strategies for subsequent holes; and the use of multiple continuous records to unambiguously construct complete composite sections for each site. The complete, continuous records provided by the GRAPE (with a temporal resolution of often yr), in conjunction with an excellent microfossil stratigraphy and often excellent magnetostratigraphy, allowed for astronomical tuning of the stratigraphic record and resulted in a set of internally consistent, high-resolution age models that provide a secure, absolute time scale for the past 6 m.y. For the period before 6 m.y., the absolute time calibration is less secure, but it is still better than any previously offered. The high-resolution stratigraphic framework of Leg 138 provided new insight into the previously ambiguous tectonic history of the region. By assuming that maximum sedimentation rates along the north-south transect would be expected at the equator, the Leg 138 stratigraphy supports the 1985 work of Cox and Engerbretson, which calls for two different poles of rotation of the Pacific Plate during the interval 0-20 Ma. The Leg 138 plate reconstructions also support several previously hypothesized ridge crest jumps and a slowing of the absolute motion of the Nazca Plate at about 5 Ma. Although Leg 138 data that predates about 13 Ma is limited, the impression that one can gain from these data is that the eastern equatorial Pacific was characterized by relatively high carbonate concentrations and accumulation rates before about 11 Ma. This pattern was interrupted occasionally by rapid massive outpourings of near-monospecific laminated diatom oozes that probably represent the formation of massive mats along strong surface-water fronts. The laminated diatom oozes (LDO) continue to be present in the Leg 138 record (many of them being expressed as seismic reflections) until about 4.4 Ma. Carbonate accumulation rates begin to decline slowly between 11 and 9.8 Ma, when, at about 9.5 Ma, a near-complete loss of carbonate (the carbonate crash ) takes place everywhere in the Leg 138 region (and beyond), except at the westernmost sites close to the equator. The carbonate crash was a time of fundamental change for the eastern equatorial Pacific, and perhaps for most of the ocean basins. Unlike many of the carbonate variations that precede and postdate it, this crash represents a major dissolution event whose effects can be traced seismically in the central and western Pacific. The changes in bottom-water chemistry associated with this event (or series of events) appear to be related to the early phases of the closing of the Panama Gateway. The role of NADW initiation and intensification for controlling carbonate accumulation in the eastern equatorial Pacific is still not resolved; however, ocean modeling demonstrates that the closing of the Panama Gateway may also have a direct influence on NADW production. Therefore, the effects of changes in the Panama Gateway sill depth and the production of NADW may be manifested in the history of eastern equatorial Pacific sedimentation. The carbonate crash was followed by a recovery of the carbonate system (except in the Guatemala and Peru basins, which never recovered) that led up to the late Miocene/early Pliocene sedimentation rate maxima, during which equatorial sedimentation rates are as much as five times greater than those of the late Pliocene or Pleistocene. Examination of modern productivity/preser vation relationships implies that the sedimentation rate maximum was the result of enhanced productivity. The distribution of eolian sediments and isotopic gradients, along with an analysis of the modes of variance in carbonate deposition over the last 6 m.y., suggest a more northerly position of the Intertropical Convergence Zone (ITCZ), a stronger north-south gradient across the equator, and a more zonal circulation focused along the equator during the time of maximum sedimentation. The mechanisms suggested for these changes in circulation patterns include the response of the eastern equatorial Pacific to the closing of the Isthmus of Panama, as well as a global increase in the flux of Ca and Si into the oceans, a possible response to evolution of the Himalayas and the Tibetan Plateau. In an effort to understand the response of the climate system to external (orbital) forcing, 6-m.y.-long, continuous records of carbonate (derived from GRAPE), δ 1 8 and insolation were analyzed and compared. Evolutionary spectral calculations of the variance and coherence among these records indicate that the insolation record is dominated by precessional frequencies, but that the relative importance of the two precessional frequencies has changed significantly over the last 6 m.y. In general, precessional forcing is not found in the carbonate or isotopic records. In the tilt band, however, a linear response is present between solar forcing and the carbonate and isotope records over some intervals. The carbonate record appears to be tightly coupled to the tilt component of insolation before about 1.9 Ma; however, the isotope record does not begin to show sensitivity to orbital tilt until about 4.5 Ma, the time of significant changes in sedimentation patterns in the eastern equatorial Pacific. Only during the last 500,000 yr do all frequencies respond in a similar manner; we also see a marked increase in the response of the isotopic record to orbital forcing (including 100,000- and 400,000-yr periods)

1995, Spatial and temporal variability of late Neogene equatorial Pacific carbonate

High-resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138 provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y. The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to 10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series. Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific car bonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence in carbonate sedimentation is strongest in the precession band (19-23 k.y.) and weakest though present at lower frequencies. The second mode of variability has a consistent spatial pattern of east-west asymmetry over the past 4 m.y. only; prior to 4 Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO3 dissolution. The second mode of variability is not highly coherent with insolation. Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ 1 8 indicates that although overall correlation between carbonate and δ 1 8 is low, both modes of variability in carbonate sedimentation are coherent with δ 1 8 changes at some frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ 1 8 at the dominant insolation periods, and the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ 1 8 in both EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene

Leg 202 synthesis: Southeast Pacific paleoceangraphy

Ocean Drilling Program (ODP) Leg 202 has opened a new window into understanding late Paleogene and Neogene global environmental change by providing high-quality sediment sequences from a previously unsampled region, the eastern South Pacific. Eleven sites (1232– 1242) that record variations on timescales ranging from decades to tens of millions of years were drilled and investigated on transects of both depth (489–4072 m) and latitude (41°S–8°N). Building on the shipboard results presented in the Leg 202 Initial Reports volume, postcruise research has significantly improved the stratigraphic framework and provided new insights into climate-related processes, which operate on different timescales and are relevant to hypotheses concerning the bipolar “see-saw” climate mechanism, orbitally driven changes in the continent-ocean-ice-atmosphere system and tectonic processes associated with the uplift of the Andes, closure of the Central American Seaway, and major expansions of polar ice sheets. Stable isotope records and refinements in bio- and magnetostratigraphy in combination with orbitally tuned cyclostratigraphy significantly improved the Pleistocene stratigraphy at Sites 1233 and 1234, the Miocene– Pliocene stratigraphy at Sites 1236, 1237, 1239, and 1241, and the Oligocene stratigraphy around the late Oligocene climate optimum at Site 1237. Site 1233 filled a crucial gap in the stratigraphic and paleoceanographic archive of the South Pacific sector of the Southern Ocean by providing an outstanding reproducible accelerator mass spectrometry (AMS) ¹⁴C-dated paleomagnetic record of centennial- and millennial- scale variability, representing regional variations in environmental/climatic conditions for the past 70,000 yr. With regard to the Cenozoic timetable, the continuous and complete sedimentary sequence of Site 1237, spanning the last ~31 m.y., has the potential of becoming a stratigraphic reference section for the South Pacific. Postcruise work demonstrated the excellence of this record for producing a stratigraphic framework that combines the biostratigraphy and terrific magnetostratigraphy with orbitally tuned stable isotope records. With respect to the bipolar see-saw hypotheses, Leg 202 studies on high-resolution records changed the view of the global distribution of millennial-scale climate change. These studies clearly demonstrate that millennial-scale climate and biogeochemical systems of the southeast Pacific and Chile closely align with those recorded in Antarctica and the southern oceans and that these climate patterns extend to the equatorial Pacific, either transmitted directly by the eastern boundary current or indirectly by “the oceanic tunnel” (subsurface transport via Antarctic Mode or Intermediate Water, Equatorial Undercurrent) injecting Southern Hemisphere extratropical water masses into the equatorial upwelling system. On orbital timescales, most spectacular was the finding that Earth’s final transition into an “icehouse” climate ~13.9 m.y. ago, the middle Miocene intensification of Antarctic glaciation, coincided with a striking transition from obliquity to eccentricity as the drivers of climate change. Thus, the late Pleistocene 100-k.y. climate cycles are not unique in Earth’s history, and although the examples from the Miocene and Pleistocene are both associated with climate cooling, they occur under significantly different global boundary conditions. This important contribution from Leg 202 issues a challenge to climatologists to understand multiple origins of 100-k.y. climate cycles that are now well documented in the geologic record. On timescales of millions of years, late Neogene upper ocean temperature reconstructions in combination with salinity assessments at selected sites from Leg 202 provide further insights into the reorganization of ocean-atmosphere couplings that are linked to the shoaling of the Central American Seaway (CAS), uplift of the Andes, and Pliocene amplification of polar ice sheet expansion. Regional shoaling of the thermocline in the low-latitude eastern Pacific from 5.3 to 4.0 Ma most likely resulted from shoaling of the CAS, as suggested by model experiments. Mixed-layer cooling and freshening in the tropical northeast Pacific warm pool as well as declining sea-surface temperature (SST) and increasing biological productivity off Chile parallel intensification of Northern Hemisphere glaciation from 3.6 to 2.4 Ma. The similarity of temporal changes in SST, upwelling, and dust flux between the Benguela and Chile upwelling systems suggests that uplift of the Andes was probably of secondary importance for generating the observed changes in the southeast Pacific within the last 6 m.y., as the Benguela upwelling system was not affected by mountain uplift. The expected atmosphere- oceanic response of the southeast Pacific to uplift of the Andes probably played a larger role during the late Miocene, prior to 6 Ma. A particular scientific challenge of future Leg 202 paleoclimate research is to better understand the transitions between these different timescales with respect to couplings between global, regional, and local processes

Millennial-scale deep water oscillations: Reflections of the North Atlantic in the deep Pacific from 10 to 60 ka

Northeast Pacific benthic foraminiferal δ¹⁸O and δ¹³C reveal repeated millennial-scale events of strong deep-sea ventilation (associated with nutrient depletion and/or high gas exchange) during stadial (cool, high ice volume) episodes from 10 to 60 ka, opposite the pattern in the deep North Atlantic. Two climate mechanisms may explain this pattern. North Pacific surface waters, chilled by atmospheric transmission from a cold North Atlantic and made saltier by reduced freshwater vapor transports, could have ventilated the deep Pacific from above. Alternatively, faster turnover of Pacific bottom and mid-depth waters, driven by Southern Ocean winds, may have compensated for suppressed North Atlantic Deep Water production during stadial intervals. During the Younger Dryas event (~11.6-13.0 cal ka), ventilation of the deep NE Pacific (~2700 m) lagged that in the Santa Barbara Basin (~450 m) by >500 years, suggesting that the NE Pacific was first ventilated at intermediate depth from above and then at greater depth from below. This apparent lag may reflect the adjustment time of global thermohaline circulation.Copyright 1998 by the American Geophysical Union

Planktonic foraminifera, sea surface temperatures, and mechanisms of oceanic change in the Peru and south equatorial currents, 0–150 ka BP

Planktonic foraminiferal faunas of the southeast Pacific indicate that sea surface temperatures (SST) have varied by as much as 8–10°C in the Peru Current, and by ~5–7°C along the equator, over the past 150,000 years. Changes in SST at times such as the Last Glacial Maximum reflect incursion of high-latitude species Globorotalia inflata and Neogloboquadrina pachyderma into the eastern boundary current and as far north as the equator. A simple heat budget model of the equatorial Pacific shows that observed changes in Peru Current advection can account for about half of the total variability in equatorial SSTs. The remaining changes in equatorial SST, which are likely related to local changes in upwelling or pycnocline depth, precede changes in polar climates as recorded by δ¹⁸O. This partitioning of processes in eastern equatorial Pacific SST reveals that net ice-age cooling here reflects first a rapid response of equatorial upwelling to insolation, followed by a later response to changes in the eastern boundary current associated with high-latitude climate (which closely resembles variations in atmospheric CO₂ as recorded in the Vostok ice core). Although precise mechanisms responsible for the equatorial upwelling component of climate change remain uncertain, one likely candidate that may operate independently of the ice sheets is insolation-driven changes in El Nin˜o/Southern Oscillation (ENSO) frequency. Early responses of equatorial SST detected both here and elsewhere highlight the sensitivity of tropical systems to small changes in seasonal insolation. The scale of tropical changes we have observed are substantially greater than model predictions, suggesting a need for further quantitative assessment of processes associated with long-term climate change.Keywords: paleoceanography, eastern boundary current, sea surface temperature, Pacific Ocean, foraminiferaKeywords: paleoceanography, eastern boundary current, sea surface temperature, Pacific Ocean, foraminifer

Testing the effects of tropical temperature, productivity, and mixed-layer depth on foraminiferal transfer functions

Statistical transfer functions relate living planktonic foraminiferal species of the central equatorial Pacific to measured sea surfce temperature, integrated primary productivity, and mixed-layer depth. The faunal estimates successfully reconstruct latitudinal patterns observed in both warm (El Niño, February-March 1992) and cool (La Niña, August-September 1992) seasonal settings. Predictions of mixed-layer depth appear to be unbiased by temperature or productivity in our data set but tend to underestimate deep mixed layers. Interactions between productivity and temperature, perhaps through their common influence on respiration and growth rates, bias foraminiferal transfer functions for both properties. Paleoceanographic estimates may be improved by accounting for such biological processes that translate the environment into a faunal response preserved in the geologic record

Eolian Evidence for Spatial Variability of Late Quaternary Climates in Tropical Africa

Study of the eolian fraction of late Quaternary sediments from the tropical Atlantic reveals that two modes of long-term climate variability have existed in tropical Africa during the last 150,000 yr. Tropical northwest Africa (i.e., the southwestern Sahara and Sahel) was driest during glacia­tions and stades, but wetter than at present during interglaciations and interstades. This may be a response to ice sheets at higher latitudes, via equatorward displacement of the westerlies and the subtropical high. In contrast, central equatorial Africa (southeast of the Sahara) was most arid during interstades and times of ice growth, and most humid during deglaciation. Wet periods in this area correspond to insolation maxima in northern hemisphere summer. A 23,000-yr preces­sional rhythm is suggested, supporting a direct link between African Monsoon intensity and or­bitally modulated insolation. The late Holocene is the only time observed when both areas are arid during an interglacial episode. This may reflect, in part, anthropogenic disturbance of late Holocene climates

Vegetation and climate of the northwest coast of North America during the last 500 K.Y. : high–resolution pollen evidence from the northern California margin

Pollen analyses of sediments from Holes 1019C, 1019E, 1020C, and 1020D as well as piston Core EW9504-17 provide continuous, chronostratigraphically controlled proxy vegetation and climate data for coastal northwest North America for the last ~500 k.y. Systematic changes in the representation of the diagnostic components of northern California plant assemblages clearly show orbital-scale variations. Interglacials are all marked by an abrupt increase in alder followed by expansion of lowland oak woodland and redwood forests. Glacials are dominated by montane forest and woodland assemblages. This sequence reflects large-scale climatic controls (e.g., orbital-scale variation in insolation and Northern Hemisphere ice sheets) in western North America over the last five glacial cycles. Regional climatic control (variations in sea-surface conditions) is implied by the differential development of xeric oak and mesic redwood communities

Leg 202 summary

More than 7 km of long and relatively continuous sediment sequences from 11 sites in the southeast and equatorial Pacific were recovered during Leg 202 for the study of the Earth’s climate and biogeochemical systems on scales that range from tectonic (millions of years) to orbital (tens to hundreds of thousands of years) and centennial to millennial (hundreds to thousands of years). These materials will be used to test a broad set of hypotheses on (1) the evolution of the South Pacific Ocean as it responds to and modulates the effects of major tectonic and climatic events, such as the opening of the Drake Passage, uplift of the Andes Mountains, closure of the Isthmus of Panama, and major expansion of polar ice sheets; (2) linkage between climate and biogeochemical changes in the high latitudes and the equatorial Pacific, related to rhythmic changes in Earth’s orbit, and the relationship of such changes to well-known glacial events of the Northern Hemisphere; and (3) global and regional changes in climate, biota, and ocean chemistry on timescales of centuries to millennia to millions of years. Three sites (1236, 1237, and 1241) targeted sequences with relatively low sedimentation rates of <30 m/m.y. to obtain long records of climate and oceanographic change representing the Neogene and, in some cases, the late Paleogene that are not subject to severe burial diagenesis. Two sites (1238 and 1239) targeted moderate sedimentation rates of 30– 80 m/m.y. to assess orbital-scale climate and biogeochemistry oscillations at a resolution suitable for the tuning of timescales and examination of changing responses to orbital forcing during the late Neogene. Six sites (1232 through 1235, 1240, and 1242) recovered sediments that accumulated rapidly, at rates of 80–2000 m/m.y., near the equator and in the higher southern latitudes to assess equator-to-pole climate and biogeochemical linkages at centennial, millennial, and orbital timescales. Drilling strategy and near real-time stratigraphic correlation played significant roles in the successful recovery of these sequences. Drilling multiple holes at each site and extensive use of overdrilling with the advanced piston corer (APC) provided long records with continuous recovery. Innovative use of rapid core logging allowed for real-time optimization of drilling strategies that maximized recovery and minimized redundant coring. Analysis of core expansion, as well as core-log integration and double extended core barrel (XCB) coring at some sites, facilitated the assembly of cores into a depth framework that will improve the quantitative analyses of sediment accumulation rates. On Nazca Ridge, Site 1237 provides a continuous sediment sequence, recovered in overlapping APC cores, that spans >30 m.y. (modern to middle Oligocene). Exceptional preservation of the flora and fauna in this long, continuous record indicates that this site will provide a much-needed stratigraphic reference in the southeast Pacific. Abrupt changes in the presence of volcanic ash layers here document an increase in tectonic activity during the late Miocene, while nearly at the same time an increase in dust flux and biogenic components associated with productive upwelling systems, such as diatoms, are associated with late Cenozoic cooling that may be associated with uplift of the Andes. Site 1236 provides an equally good record from shallower water depths for the last 28 m.y. that, when paired with Site 1237, will document variations of deep, intermediate, and surface water masses in the subtropical South Pacific. Near the equator, Sites 1238–1241 provide evidence for rhythmic oscillations of pelagic and hemipelagic sediments on the scale of Earth’s orbital cycles, which will help to test hypotheses on tropical vs. polar origins of the well-known 100-k.y. climate cycle that characterizes the late Pleistocene, as well as the response of the equatorial Pacific to closure of the Isthmus of Panama over millions of years. Again, complete recovery of long and well-preserved sediment sequences will provide unprecedented resolution of biotic and environmental changes. Century- to millennial-scale climate changes can be addressed with the records from rapidly accumulating (40–200 cm/k.y.) sediments recovered at Sites 1233–1235 from the central Chile margin. These sites will provide important data related to the southern westerlies and Antarctic Intermediate Water variability. A detailed record of paleomagnetic intensity and secular variability will link these records into a global chronological framework. At Site 1232, in the Chile Basin, a rapidly accumulating Pleistocene sequence documents terrigenous sediments eroded from the southern Andes and transported to the deep via turbidity currents. Near the equator, Sites 1240 and 1242 have moderately high sedimentation rates (~8–13 cm/k.y.), which will help to test linkages of millennial-scale climate changes between low and high latitudes. Together, the array of sites recovered during Leg 202 provides a new view of Southern Hemisphere and tropical climate variability and biogeochemical systems across a broad range of spatial and temporal scales in a region of the ocean that has received relatively little study in the past.Shipboard scientific party : Chapter 1, Leg 202 Summar