During Leg 202, we recovered a total of 7081 m of sediment at 11 sites, ranging in age from early Oligocene (~31.5 Ma) to Holocene (Tables T1, T2; Fig. F2). Sites 12321242 form an east Pacific latitudinal transect from 41°S8°N as well as an intermediate to deepwater transect from 4904070 m water depth. The pelagic and hemipelagic records provide an excellent base for a broad spectrum of detailed studies that aim to reconstruct the evolution of upwelling, biota, biogeochemistry, sea-surface, and intermediate-water characteristics in the southeast and equatorial Pacific, as well as the closure of the Isthmus of Panama, the history of Andean uplift, and continental climate. Moreover, these records permit paleoceanographic studies on a variety of timescales, including centennial to millennial (102 to 103 yr), orbital (104 to 105 yr), and tectonic (>105 yr) time resolution.
Operational successes during Leg 202 include the ability to make rapid drilling decisions by using a rapid-scanning core logging track, which includes measurements of magnetic susceptibility. Efficient planning, based on the availability of these logging data, made time for extensive overdrilling of APC cores to improve recovery.
One of the most exciting results was the successful recovery of unprecedented ultra high resolution records (Sites 1233, 1334, and 1235) from the Chilean continental margin between 41°S and ~36°S at water depths between 490 and 1115 m. Shipboard stratigraphic data indicate that the base of Sites 12331235 is younger than 260 ka. These records are marked by exceptionally high sedimentation rates of 40200 cm/k.y., apparently driven by extremely high terrigenous sediment supply due to enhanced river discharge in response to heavy continental rainfall. The dominance of siliciclastic sediments here resulted in a high-resolution record of paleomagnetic variations. Among them, the most prominent feature in the late Pleistocene is the Laschamp Event at ~41 ka, which is particularly pronounced at Sites 1233 and 1234 (e.g., covering an interval of 2 m at Site 1233). These extraordinary paleomagnetic records provide opportunities for high-resolution regional and global correlation of marine and terrestrial records using paleomagnetic secular variation and paleointensity variation to test the phasing of climate changes between the Southern and Northern Hemispheres. Together with a rich array of well-preserved biogenic and mineralogic tracers of paleoclimatic utility, these sites provide a unique chance for understanding the role of South Pacific oceanography and Southern Hemisphere climate in a global context on scales from millennia to centuries and perhaps even decades.
In the tropics, climate and oceanographic changes on the scale of centuries and millennia may be well recorded at Site 1240 in the Panama Basin and at Site 1242 on Cocos Ridge. Nannofossils, planktonic foraminifers, and diatoms provided well-constrained biostratigraphic age models and magnetostratigraphic information through the Brunhes and Matuyama Chrons that significantly augmented the age model at Site 1240. Both records comprise a complete stratigraphic sequence of the last ~2.6 m.y., and sedimentation rates vary in the range of 620 cm/k.y., with higher rates in the Pliocene sequence. Persistent decimeter- to meter-scale variability in core logging data (bulk density and magnetic susceptibility) are mainly related to changes in the relative supplies of carbonate and biosiliceous and siliciclastic material and are tentatively interpreted to reflect millennial- to orbital-scale changes in equatorial productivity and/or climate.
These records are ideally located to monitor rapid changes in upwelling and productivity in response to variations in nutrients and wind forcing (Site 1240), as well as changes in sea-surface salinities due to fluctuations in precipitation that are linked to the moisture export from the low-latitude Atlantic and to the latitudinal position of the Intertropical Convergence Zone (Site 1242). Paleomagnetic measurements at Site 1242 indicate a great potential for a good record of magnetic paleointensities that will allow high-resolution chronological comparisons to examine linkages and differences in the hydrological cycle between the mid and low latitudes.
On longer timescales, the response of the southeast and equatorial Pacific to orbital and tectonic forcing is recorded at Sites 1238, 1239, and 1241, covering the interval of the last ~1115 m.y., and at Sites 1236 and 1237, spanning the interval of the last ~2832 m.y. Thus, the geographic distribution of these sites affords a latitudinal comparison for the last 11 m.y. between ~21°S and ~6°N. The water depth distribution of these sites between ~1320 and ~3210 m will allow for the first time detailed insights into the Neogene evolution of equatorial and South Pacific intermediate water chemistry. Most of the pelagic to hemipelagic sequences, especially from the upper Miocene and early Pliocene with sedimentation rates as high as 50110 m/m.y. (Sites 1238, 1239, and 1241), reveal clear lithologic changes on meter and decimeter scales that are most likely related to orbitally induced changes in precession and obliquity. If so, these cycles provide a basis for both (1) developing orbitally tuned age models and (2) testing the phase relationships between major climate and oceanographic components to assess the role of South Pacific oceanography and biogeochemistry within the chain of climate forcing mechanisms.
The predominantly good preservation of calcareous microfossils and the prevalent presence of diatoms resulted in a remarkable biostratigraphic framework that offers the opportunity to generate an improved biostratigraphic scheme for the southeast Pacific, as well as correlations with the low-latitude zonations. In addition, the APC-cored record from Site 1237 yielded an outstanding magnetic polarity reversal stratigraphy for the interval of the last 31 m.y. Although the polarity assignments are still preliminary and allow several possible interpretations in the sequence between 13 and 31 Ma, this site offers an excellent potential to derive an orbitally tuned timescale that can be tied to magnetostratigraphy and biostratigraphy to provide a reference section for the South and equatorial Pacific. Low-resolution Site 1236 revealed a magnetic polarity stratigraphy for the last 13 m.y., significantly augmenting the biostratigraphy and providing a well-constrained chronology. Hiatuses were only registered at Site 1238 from Carnegie Ridge, encompassing the interval from ~7.8 to 13.6 Ma, and at Site 1242 from Cocos Ridge, spanning the interval from ~2.5 to 13 Ma.
On tectonic timescales, significant variability and gradients in the supply of biogenic and siliciclastic sediments, including the abundance of ash layers, are to a large part associated with the history of Andean uplift, closure of the Isthmus of Panama, and the paleodrift of site locations due to tectonic plate motions. Except for Site 1236, no evidence was found for larger changes in paleobathymetry due to the subsidence history of the Cocos, Carnegie, and Nazca Ridges that were formed at hotspots.
The drift of the Cocos plate moved equatorial Sites 1241 and 1242 (Cocos Ridge) from their Miocene paleoposition northeastward away from the equatorial productivity belt closer to the Central American landmass. This resulted in a general decrease in biogenic sediment deposition and may have contributed to the increase in siliciclastics since the Pliocene.
The predominantly eastward migration of the Nazca plate moved South Pacific Sites 12361239 from their Oligocene or Miocene pelagic paleopositions closer to the South American continent and thus closer to hemipelagic environments that are influenced by coastal upwelling processes and enhanced terrigenous eolian sediment supply. However, Sites 1238 and 1239 (Carnegie Ridge) drifted eastward within the equatorial upwelling belt and thus experienced little environmental change during their Miocene to Holocene history, as indicated by permanent and relatively high deposition rates of biogenic silica and calcareous sediments. The Pleistocene increase in siliciclastic sediment, as indicated by an increase in magnetic susceptibility values, may result from changes in continental aridity and/or dust transport rather than from the plate tectonic drift.
The late Oligocene to Holocene sediment deposition at Sites 1236 and 1237 is clearly influenced by environmental changes associated with the eastward drift and the subsidence history of Nazca Ridge. The tectonic backtrack path on Nazca plate moves the sites ~20° westward, toward the center of the subtropical gyre. Site 1236 subsided from a few hundred meters of water depth to its modern depth of 1320 m. The deposition of upper Oligocene fine-grained chalk overlying basaltic basement indicates an early phase of pelagic sedimentation. The presence of authigenic glauconite and the relatively high abundance of buliminids and nododariids suggest shallow-water conditions. After the initial pelagic phase, several ash layers suggest a late stage of volcanic activity near 28 Ma that possibly was related to the well-known tectonic reorganization that created the Nazca plate. This phase was followed by the deposition of neritic, grain-rich, calcareous sediment, typical for a supply from carbonate platforms, which may have surrounded volcanic islands in the vicinity of Site 1236. The sediment supply from the carbonate platform persisted until ~17 Ma and is marked by increased sedimentation rates of ~15 m/m.y. The interval of the last 17 m.y. is characterized by the deposition of nannofossil ooze and very low sedimentation rates of ~5 m/m.y., indicating an oligotrophic pelagic environment and possibly winnowing effects at intermediate water depth.
At deepwater Site 1237, the pelagic upper Oligocene to middle Miocene sequence is dominated by nannofossil ooze, carbonate contents >90wt%, and low sedimentation rates of ~10 m/m.y., typical for oligotrophic conditions in the sphere of the subtropical gyre. The general increase in clay, biogenic opal, sedimentation rates, and the occurrence of diatom species indicative of coastal upwelling reflects the paleodrift toward the Peruvian continental margin since the middle Miocene.
Superimposed on such gradual sedimentological changes are long-term variations that result from changes in climate and oceanography, possibly also associated with tectonic processes like the uplift of the Andes and the closure of the Isthmus of Panama. The Neogene uplift of the Andes is expected to have caused an enhanced steering of the southeast trade winds along the Chilean and Peruvian coasts, resulting in stronger meridional flow, enhanced eolian transport, coastal upwelling, and biogenic productivity. The long-term history of eolian deposition in the southeast Pacific is best recorded at Sites 1236 and 1237, underlying the modern path of eolian transport from the Atacama Desert. There is a stepwise increase in dust flux (Site 1237) and hematite contents (Site 1236) since 8 Ma that may reflect a critical threshold in the uplift history that resulted in a strengthening of trade wind circulation as suggested by enhanced coastal upwelling and productivity. At approximately the same time, discrete ash layers begin to appear at Site 1237, recording the onset of intense volcanism and accompanying the tectonic uplift of the Andes. The sediment records of Leg 202 comprise >200 volcaniclastic horizons that were deposited during the last ~9 m.y. Maxima in ash layer frequency occurred at ~86 Ma and during the last 3 m.y.
The most prominent feature of long-term changes in the Leg 202 sediment records is a pronounced late Miocene to early Pliocene maximum in biogenic accumulation rates suggesting an interval of enhanced oceanic surface productivity between ~8 and ~4 Ma, with a maximum at ~6 Ma. A similar but shorter interval of rapid biogenic accumulation (6.74.5 Ma) was found at equatorial east Pacific sites during Leg 138 and is often referred to as the late Miocene to early Pliocene biogenic bloom. The ultimate cause of the rise and fall of this bloom is unknown but has been linked to a variety of previous hypotheses, including sea level variations, continental weathering, deepwater circulation, trade-wind fluctuations, and the closure of the Isthmus of Panama. Sites 1236 and 1237 trace the productivity maximum for the first time farther south and suggest an early onset of this event in the eastern boundary current of the South Pacific, which was associated with an increase in trade wind circulation since ~8 Ma. Whether the wind-driven northward advection of cool and nutrient-rich Southern Ocean waters via the eastern boundary current and their injection into the South Equatorial Current is a possible source for this bloom will be studied postcruise.
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