OVERVIEW OF POSTCRUISE PUBLICATIONS

Fourteen papers appearing in this volume and sixteen papers published outside the Leg 202 Scientific Results volume add significant information about stratigraphy, continent-ocean-atmosphere couplings, and associated biogeochemical interactions on a variety of timescales and with different temporal resolutions. These contributions are briefly summarized below.

Abe et al. (this volume) provide a data report on Miocene to Holocene organic carbon and biomarker variations at Sites 1237 and 1239. Concentrations of organic carbon, carbonate, long-chain n-alkanes, and total alkenones, as well as alkenone-based sea-surface temperature (SST) estimates are presented. The geochemical data can be used to evaluate terrestrial sediment sources. Reconstruction of sea-surface temperatures suggests a significant temperature drop at both sites after ~2.5 Ma, possibly in response to the intensification of Northern Hemisphere glaciation (NHG). The range of temperature variability (~7–8C in this low-resolution data set from Site 1237) is generally consistent with alkenone temperatures reconstructed for the late Pleistocene at a nearby piston core (Calvo et al., 2001; Prahl et al., 2006), with alkenone temperature shifts between the late Pleistocene and mid-Pliocene warm period (3.0–3.3 Ma) at Site 1237 (Haywood et al., 2005), but conflicts with Mg/Ca temperature reconstructions of Wara and Ravelo (this volume), which suggest no significant long-term temperature changes over the past ~6 m.y. For comparison, long-term variations of alkenone temperature estimates in the equatorial cold tongue (Site 846; Lawrence et al., 2006) also document a temperature range of ~7C, although unlike the data from Site 1237, changes at Site 846 include a long-term "ramp" of cooling over the past 5 m.y. superimposed on shorter-term variations.

Abrantes et al. (2007) use core-top samples from throughout the region of Leg 202 to develop transfer functions for quantitatively predicting mean annual SST and primary productivity and qualitatively assessing freshwater input to the marine realm, based on fossil diatom assemblages. This calibration of paleoproxies with careful regional studies of modern sediments provides an important step toward future quantification of oceanographic change in the downcore records.

Benway and Mix (2004) provide oxygen isotope and salinity data on seawater and rainwater sampled during Leg 202 in the eastern tropical Pacific region, providing a basis for reconstructing paleosalinity from estimates of 18O in seawater. Based on an isotope balance, they suggest that about half of the rainfall in the Panama Bight comes from local evaporation and precipitation and the other half comes from long-distance, cross-isthmus transport and intermittent large storm events.

Benway et al. (2006) provide high-resolution 18O and Mg/Ca data on planktonic foraminifers from Site 1242 and other sediment cores and reconstruct substantial millennial-scale changes (but little glacial–interglacial difference) in 18O of seawater (and, by inference, upper ocean salinity, corrected for sea level change) that are significantly larger than those of the Caribbean (Schmidt et al., 2004). Such variations likely reflect the combination of changing location of the intertropical convergence and changes in net freshwater transport across the Panama Isthmus. Surprisingly, millennial-scale changes in freshwater transport across the Panama Isthmus appear to lead millennial-scale changes in thermohaline overturn in the North Atlantic region, suggesting an active role for the tropics in generating such variations, either as an independent tropical trigger or as part of an integrated feedback loop between the tropical and high-latitude climate systems.

Chun and Delaney (this volume) present a detailed examination of particulate phosphorus composition, biogenic barium, manganese, and uranium at Site 1237 in order to characterize the history of nutrient burial and paleoproductivity in the context of major tectonic and climate changes during the past 31 m.y. A stepwise decrease in manganese enrichment observed near 162 meters composite depth (mcd), or ~9 Ma, is consistent with a redox threshold associated with expected increases in paleoproductivity as tectonic drift carried the site under the eastern boundary current system off western South America (Mix, Tiedemann, Blum, et al., 2003).

Flores et al. (this volume) provide a data report that significantly improves the Pleistocene calcareous nannofossil biostratigraphy at Sites 1237, 1238, 1241, and 1242. They confined 11 events of key biostratigraphic marker species (e.g., first occurrence [FO] and last occurrence [LO] datums) using a sampling resolution of 10–30 cm for the last 0.5 m.y. and 75–150 cm for the time interval from 0.5 to 2.0 Ma.

Flower and Chisholm (this volume) present a late Oligocene stable isotope record (Cibicidoides mundulus) from deepwater Site 1237 and tie the onset of the late Oligocene climate optimum to the new geologic timescale (GTS 2004) (Gradstein et al., 2004) by using magnetic polarity stratigraphy and isotope data. Their results suggest an age of 26.35 Ma for the final decrease in 18O, marking the initiation of the late Oligocene climate optimum. This climatic shift is closely associated with the LO of the planktonic foraminifer Paragloborotalia opima.

Grevemeyer et al. (2003) examine the thermal regime of the continental slope and downgoing slab in Chile between 32 and 41S by combining information from seismic reflection data (Empresa Nacional del Petroleo Chilean oil company and Bundesministerium fr Bildung und Wissenschaft, Germany) and downhole temperature measurements obtained during Leg 202 at Sites 1233, 1234, and 1235. Their results suggest that the heat flow through the upper Nazca plate is low with respect to the Eocene age of the incoming oceanic lithosphere. Modeling shows that reduced heat flow can be accounted for by the fact that the downgoing Nazca plate advects heat into the mantle and hence acts as heat sink. The occurrence of bottom simulating reflectors (BSRs) is interpreted to mark the thermally controlled base of gas hydrate layers. BSR heat flow over the accretionary prism is 60%–80% of the regional conductive model and is suggested to reflect the initial thickening of sediment as it is being incorporated into the accretionary wedge. Heat flow over continental basement rocks, however, is generally in accordance with the regional model and may reflect steady-state conditions. The seaward limit of the seismogenic zone is characterized by the 100C isotherm and occurs 60 km landward of the trench axis in the north (33S) and 30 km landward in the south. The seaward shift is related to the subduction of younger lithosphere in the south.

Groeneveld et al. (this volume) present high-resolution Pliocene Mg/Ca and 18O records from tropical east Pacific Site 1241 that were both measured on the planktonic mixed-layer species Globigerinoides sacculifer. Their results suggest a freshening of the mixed layer from 4.8 to 2.4 Ma, which might be coupled with progressive closure of the CAS. Mixed-layer temperatures commenced to decrease after 3.7 Ma, paralleling the Pliocene intensification of NHG that started at ~3.6 Ma, according to the recent work of Mudelsee and Raymo (2005). Groeneveld et al. note, however, that Pliocene SSTs at Site 1241 are similar to modern values and colder than values estimated for the western Pacific warm pool, providing no direct support for the idea of permanent El-Nio conditions during the early Pliocene (Ravelo et al., 2006).

Haywood et al. (2005) present new estimates of mid-Pliocene Pacific SSTs from subtropical Site 1237 using alkenone paleothermometry. These estimates, combined with other SST data from the Pacific and Atlantic, were used to verify model-predicted and fossil-based temperature estimates in the PRISM2 data set that was produced by the Pliocene Research Interpretations and Synoptic Mapping (PRISM) Group. The PRISM Group suggested that mid-Pliocene surface ocean temperatures were the same as, or slightly cooler than, those found today in the tropics and at low latitudes and significantly warmer at higher latitudes. This change in the latitudinal pattern of SSTs has been attributed to enhanced meridional ocean heat transport generated by more vigorous surface ocean gyres and/or thermohaline circulation. In contrast, alkenone and model-based SST estimates of Haywood et al. (2005) suggest warmer SSTs during the mid-Pliocene in the tropics and subtropics. This pattern of SSTs is not characteristic of that produced by enhanced meridional ocean heat transport or thermohaline circulation. Instead, the pattern is similar to that which might be expected as a result of higher concentrations of atmospheric CO2, which would act to warm the oceans at the tropics and other latitudes. Furthermore, model diagnostics indicate that reduced sea ice and terrestrial ice sheet extent as well as a strong ice-albedo feedback played a major role in forcing mid-Pliocene warmth.

Heusser et al. (2006b) compile high-resolution pollen records from Site 1233, providing a continuous, chronostratigraphically controlled 50-k.y. record of regional changes in vegetation from temperate South America. Deposited 38 km west of the transition from northern deciduous lowland forest to southern evergreen rain forest, the pollen record documents the comparatively brief Holocene development of thermophilous vegetation (Lowland Deciduous Beech Forest and Valdivian Evergreen Forest) and the expansion of glacial subantarctic vegetation (North Patagonian Evergreen Forest–Subantarctic Parkland) during marine isotope Stages (MIS) 2 and 3. Systematic variability in these terrestrial climate proxies, which reflect latitudinal movement of the southern westerlies, is mirrored in Antarctic climate records and in coeval ocean conditions inferred from radiolarian census data (Pisias et al., 2006).

Heusser et al. (2006a) present joint pollen and benthic oxygen isotope data from Site 1234 in the southeast Pacific. Their data provide the first continuous, chronostratigraphically controlled record of changes in temperate South American vegetation and climate for the last 140 k.y. Downhole changes in diagnostic pollen assemblages from xeric lowland deciduous forest, mesic Valdivian Evergreen, and Subantarctic Evergreen Rainforest reveal large rapid shifts that likely reflect latitudinal movements in atmospheric circulation and storm tracks associated with the southern westerly winds. During glacial maxima (MIS 2–4 and 6), the prominence of hyperhumid vegetation (north Patagonian and subantarctic forests and parkland) implies sustained northward migration of the southern westerlies. At the MIS 6/5e boundary, coeval with the rapid shift to lower isotopic values, rainforest vegetation was rapidly replaced by xeric plant communities associated with Mediterranean-type climate. An increased prominence of halophytic vegetation suggests that MIS 5e was more arid and possibly warmer than MIS 1. Although rainforest pollen rises again at the end of MIS 5e, lowland deciduous forest pollen persists through MIS 5d and 5c, into MIS 5b. Comparison to Antarctic ice core data suggests that glacial–interglacial advances of the Chilean rainforests occurred in phase with Antarctic cooling. On millennial scales, changes in vegetation lag Antarctic temperature change, suggesting that southern Chilean ecosystems respond to regional climate change with an exponential response time of ~1000 yr.

Holbourn et al. (2005) develop an orbitally tuned age model for the middle Miocene time interval from 12.7 to 14.7 Ma at Site 1237 and examine the impacts of orbital forcing and atmospheric CO2 on Miocene Antarctic ice sheet expansion. Their studies are based on benthic foraminiferal oxygen and carbon isotope (4- to 5-k.y. resolution) and X-ray fluorescence (XRF) scanning records (1-k.y. resolution). The new chronology was generated by initially matching 400- and 100-k.y. amplitude variations in the benthic 18O record to the latest astronomical solution of Laskar et al. (2004) and then fine-tuning obliquity-related 18O cycles to the orbital record. The major increase in mid-Miocene ice sheet expansion at 13.91–13.84 Ma was marked by a striking transition from high-amplitude 41-k.y.-paced 18O variations (prior to 13.9 Ma) to high-amplitude 100-k.y.-paced variations, which are paralleled by a similar shift in the amplitudes of orbital obliquity and eccentricity. The major steps of rapid ice sheet expansion occurred during an extended interval of low seasonal contrast at low eccentricity and coincided with a prominent increase in benthic 13C that was interpreted as reflecting a decrease in atmospheric CO2 contents, promoting global cooling.

Hostetler et al. (2006) assemble available SST data (modern and Last Glacial Maximum [LGM]) from the tropical and subtropical Pacific (including data from Sites 1233 and 1242) and apply a statistical approach to adjust hypothesized biases in the faunal based SST estimates of the Climatic Long-Range Investigation, Mapping, and Prediction Project (CLIMAP Project Members, 1981). The resulting SSTs are generally in better agreement than CLIMAP with recent geochemical estimates of glacial–interglacial temperature changes. In addition, they conduct a series of model experiments using the GENESIS (Global Environmental and Ecological Simulation of Interactive Systems) general atmospheric circulation model to assess the sensitivity of the climate system to their bias-adjusted SSTs. Globally, the new SST field results in a modeled LGM cooling of 6.4C (1.9C cooler than that of CLIMAP). Data model comparisons indicate improvement in agreement relative to CLIMAP, but differences among terrestrial data inferences and simulated moisture and temperature remain. Their SSTs result in positive mass balance over the Northern Hemisphere ice sheets (primarily through reduced summer ablation), supporting the hypothesis that tropical and subtropical ocean temperatures may have played a role in triggering glacial changes at higher latitudes.

Kaiser et al. (2005) extend the alkenone-derived SST record at Site 1233 (Lamy et al., 2004) back to 70 ka. Comparison to other mid-latitude Southern Hemisphere records suggests that the Antarctic timing of SST changes was probably a hemisphere-wide phenomenon. Gradient reconstructions of SSTs over the complete latitudinal range of the Pacific eastern boundary current system suggest a displaced subtropical gyre circulation toward the Equator during glacial MIS 2 and 4.

Kaiser et al. (in press) provide a continuous record of Patagonian ice sheet (PIS) extent (Fe content) and alkenone-based SSTs off Chile during the last 70 k.y. based on Site 1233. In particular, they focus on the millennial- to multicentennial-scale variability of the paleorecords. The close relationship between Fe content and SST pattern in the southeast Pacific (as described by Lamy et al. [2004] for the 50- to 19-k.y. time interval) extends into the older part of the records (70–50 ka). During MIS 4, a delay of ~500 yr of PIS retreats relative to SST increases has been found, similar to that described for the coldest part of MIS 3 and 2. During early MIS 3 (~60–56 ka), synchronous variability in both records resembles the deglacial–Holocene time interval, reflecting either a meltwater pulse or dominant control of Fe input by rainfall changes related to a rather small PIS. Results of spectral analysis on the detrended records show three main periodicity bands at ~4.5–3.1, 2.4–2.2, and 1.2–1 ka. The possible origins of these bands are discussed in terms of stochastic resonance, solar forcing, and Northern Hemisphere high- and/or low-latitude influences on the southeast Pacific, respectively.

Lamy et al. (2004) establish alkenone-derived SST, planktonic 18O, and Fe (XRF scanning) records at the Chilean continental margin Site 1233 and reconstruct millennial-scale changes in southeast Pacific surface water properties and PIS extent for the last 50 k.y. Their results suggest a clear Antarctic timing of SST changes, which appear to be systematically linked to meridional displacements in sea ice, westerly winds, and the circumpolar current system. The changing Fe contents have been related to glaciofluvial sediment flux from the Andes to the continental margin. The patterns of Fe and paleotemperature records are very similar, but Fe changes lag variations in temperature by as long as ~1000 yr. This was interpreted as reflecting the glacier response time to rising SSTs and may explain some of the current discrepancies among terrestrial records in southern South America.

Lund et al. (this volume a) compare detailed rock magnetic and paleomagnetic records of the same age but from different holes of Site 1233 in order to assess whether this late Pleistocene–Holocene archive, deposited at extremely high accumulation rates, contains reproducible evidence for centennial-scale environmental, climatic, and geomagnetic field variability. Their rock magnetic results identify reproducible, pervasive, and distinctive centennial- (~150–300 yr) and millennial-scale variability, representing regional variations in environmental or climate conditions.

Lund et al. (this volume b) summarize the complete paleomagnetic record from Site 1233 and the paleomagnetic record for the uppermost 30 mcd from Site 1234. Their study identifies reproducible high-resolution records of paleomagnetic secular variation, which can be correlated between the two sites. These records provide evidence for three magnetic field excursions. On the basis of high-resolution accelerator mass spectrometry (AMS) radiocarbon stratigraphy, two of these excursions occurred at ~35,000 calendar years before present (cal. yr BP) and the third at ~41,000 cal. yr BP.

Martinez et al. (2006) report a nitrogen isotope record of bulk organic matter from southern Chile margin Site 1233. The site is located slightly south of the Peru-Chile upwelling system and the associated oxygen minimum zone. The 15N values at Site 1233 are relatively high throughout the record, varying between 9 and 13 during the last 50 k.y. The major features are a pronounced 15N increase at the beginning of the deglaciation (~19.5 ka), a large decrease during the early Holocene after 10 ka, and a pattern of millennial-scale variability that resembles the pattern of Antarctic climate change as recorded in the 18O Byrd ice core record. The timing of changes in 15N appears to be synchronous with Antarctic climate changes between 50 and 10 ka. The Holocene drop in 15N is a typical feature of Southern Ocean sites, as suggested by comparisons with other 15N records. The authors propose that the interplay between nutrient demand in the Subantarctic Zone and latitudinal shifts of hydrologic fronts controlled both the concentrations and the isotopic signature of the remaining nitrate delivered to the Chile margin. At that time the glacial surface waters of the southern Chile margin were likely lower in nitrate concentration and higher in 15N than during interglacial periods.

McManus (this volume) provides a data report on major (K, Ca, Mg, Fe, and Ti) and trace (Mn, Cu, Ba, U, Cd, Mo, and V) element concentrations at Sites 1233 and 1234 as background for assessing terrigenous inputs and changes in the reducing nature of these sediments from the Chilean continental margin. In particular, the ratio U/Mo may provide a linear measure of bottom water oxygen concentration, with relatively little local impact from diagenesis of organic matter within the sediment. Preliminary application of this new proxy to Sites 1233 and 1234 is consistent with modern bottom water gradients but suggests substantial variability of the oxygen minimum zone, which overlies Site 1233.

Pena et al. (2005) analyze the geochemical composition of foraminifer shells from Site 1240 (Panama Basin) with several analytical techniques (laser ablation inductively coupled plasma–mass spectroscopy [LA-ICP-MS], ICP-MS, X-ray diffraction [XRD], scanning electron microscopy [SEM], and energy dispersive X-ray [EDX]) in order to identify and evaluate the occurrence of contaminant phases that may bias paleoenvironmental reconstructions. LA-ICP-MS results on uncleaned tests indicate the presence of Mn-Mg–rich contaminant phases at the inner surfaces of the foraminiferal shells. Different cleaning techniques were applied to remove these contaminant phases. Satisfactory results were only produced when a reductive step was included. XRD analysis further revealed that the Mn-Mg–rich phase represents the Ca-Mn-Mg carbonate kutnahorite (Ca[Mn,Mg][CO3]2). Their results demonstrate that the presence of kutnahorite-like minerals can bias Mg/Ca ratios toward higher values (by 7%–36%) and lead to significant overestimation of past seawater temperatures.

Pisias et al. (2006) accomplish multivariate statistical analyses on late Pleistocene–Holocene radiolarian and pollen populations from Chilean continental margin Site 1233. Their sampling intervals yield a temporal resolution of 200–400 yr and provide a detailed record of marine and continental climate change in the southeast Pacific and South American continent for the past 50 k.y. The authors concluded the following:

  1. During the past 50 k.y., the region of the central Chilean coast was not directly influenced by polar water from the Antarctic region;
  2. Changes in ocean conditions off central Chile during this time interval primarily reflect north–south shifts in the position of the South Pacific transition zone;
  3. Changes in Chilean vegetation reflect comparable latitudinal shifts in precipitation and the position of the southern westerlies;
  4. The first canonical variate of radiolarian and pollen records extracted from Site 1233 are remarkably similar to each other as well as to temperature records from the Antarctic, which suggests that marine and continental climate variability in the region is tightly coupled at periods longer than 3000 yr; and
  5. Phase coupling of these climate records, which leads to variations of continental erosion based on iron abundance at the same site, is consistent with the hypothesis that erosion is linked to relatively long response times (i.e., a few thousand years) of the PIS and thus is not a direct indicator of regional climate.

Prokopenko et al. (this volume) compare ammonium and total nitrogen concentrations as well as nitrogen isotope composition of solid phase and dissolved pore water ammonium from Pleistocene sediments that accumulated rapidly at continental margin Sites 1234, 1235, and 1238 and discuss the impact of diagenesis on the 15N preserved in organic matter. Their results suggest that the 15N composition of organic matter at Sites 1234 and 1235 is not significantly affected by diagenesis, even though ~20% of organic nitrogen is lost to diagenesis. This is indicated by the strong similarity between 15N of ammonium and 15N of organic matter, which implies that the process of organic matter decomposition is not associated with an intrinsic fractionation factor.

Robinson et al. (2007) generate high-resolution oxygen isotope and bulk sediment 15N/14N records (Site 1234) in order to assess denitrification changes within the Peru-Chile upwelling system over the last ~70 k.y. Denitrification changes at Chilean margin Site 1234 are coherent with Antarctic climate changes recorded by the Byrd ice core 18O record rather than with Northern Hemisphere climate change. The Southern Hemisphere character of the Chile margin 15N record suggests that episodes of reduced denitrification in the eastern Pacific represent times when more oxygen was supplied as the result of changes in the chemical composition of Subantarctic Mode Water (SAMW), which forms in the Subantarctic Zone of the Southern Ocean and ventilates the low-latitude thermocline.

Steph et al. (this volume) provide Pliocene Mg/Ca temperature and stable isotope records from shallow- and deep-dwelling planktonic foraminifers (Site 1241) that span the time interval from 5.5 to 2.5 Ma. The combination of paired Mg/Ca and 18O measurements allowed them to differentiate between temperature and salinity changes and therefore assess changes in upper ocean stratification. Their study indicates an early Pliocene shoaling of the tropical east Pacific thermocline, marked by a 6C temperature decrease at the bottom of the photic zone between 5.4 and 4.0 Ma. The deviation of 18O records and Mg/Ca temperature estimates from thermocline-dwelling planktonic foraminifers suggests that local changes in salinity exerted a much stronger control on Pliocene tropical east Pacific upper ocean water mass signatures than previously assumed. Whether these variations are triggered by changes in the configuration of low-latitude ocean gateways is being discussed.

Steph et al. (2006) compare Pliocene 18O records of shallow- and deep-dwelling planktonic foraminifers from the tropical east Pacific (Sites 1241 and 851), Caribbean (ODP Sites 999 and 1000), and Atlantic (Site 925, Ceara Rise, and Site 1006, western Great Bahama Bank) to assess Atlantic-Caribbean-Pacific atmospheric and oceanic linkages associated with progressive closure of the CAS. Comparisons suggest development of an inner-Caribbean salinity gradient in the mixed layer and salinity changes on precessional periodicities after 4.4 Ma (Site 1000) when Pacific-Caribbean throughflow became significantly restricted. Precession-induced variations in the volume transport of Pacific surface water masses through the Panamanian Seaway are considered a main factor to explain the Caribbean salinity minima. Results from a coupled climate model point to changes in the El Nio Southern Oscillation (ENSO) state as a potential trigger for changes in the amount of Pacific inflow into the Caribbean.

Tiedemann et al. (this volume) present consistent high-resolution, orbitally tuned age models for Sites 1237 and 1241 as well as benthic 18O and 13C stratigraphies for Sites 1236, 1237, 1239, and 1241, which cover the time interval from 2.5 to 6 Ma. The age models for Sites 1237 and 1241 were generated by correlating the high-frequency variations in gamma ray attenuation (GRA) density, sand percentages of the carbonate fraction, and benthic 13C to the orbital solution of Laskar et al. (1993). The tuned ages of the Pliocene polarity reversals at Site 1237 agree well with those of GTS 2004 (Gradstein et al., 2004).

Wara and Ravelo (this volume) present a data report on geochemical analyses performed on planktonic and benthic foraminifers from Site 1237. Records of planktonic and benthic 18O and 13C as well as planktonic Mg/Ca, Sr/Ca, and Mn/Ca concentrations were generated for the time interval of the last 6 m.y. to provide critical information regarding the late Miocene to present history of climate and oceanographic variability in the southeast Pacific. These records can be used to infer changes in SST and salinity.

Weber and Pisias (this volume) provide an initial radiolarian biostratigraphic framework for Site 1237 that is in agreement with the biostratigraphic zonation of Sanfilippo and Nigrini (1998) and Moore (1995) and the shipboard magnetostratigraphy using the age assignments of Cande and Kent (1995). According to the Geomagnetic Polarity Timescale, the ages of 21 radiolarian datums are defined, covering the time interval of the last 11.5 m.y.

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