The El Niño-Southern Oscillation (ENSO) is one of the most important components of the global climate system, but its potential response to an anthropogenic increase in atmospheric CO 2 remains largely unknown. One of the major limitations in ENSO prediction is our poor understanding of the relationship between ENSO variability and long-term changes in Tropical Pacific oceanography. Here we investigate this relationship using palaeorecords derived from the geochemistry of planktonic foraminifera. Our results indicate a strong negative correlation between ENSO variability and zonal gradient of sea-surface temperatures across the Tropical Pacific during the last 22 ky. This strong correlation implies a mechanistic link that tightly couples zonal sea-surface temperature gradient and ENSO variability during large climate changes and provides a unique insight into potential ENSO evolution in the future by suggesting enhanced ENSO variability under a global warming scenario.

The modern Eastern Equatorial Pacific (EEP) Ocean is a large oceanic source of carbon to the atmosphere. Primary productivity over large areas of the EEP is limited by silicic acid and iron availability, and because of this constraint the organic carbon export to the deep ocean is unable to compensate for the outgassing of carbon dioxide that occurs through upwelling of deep waters. It has been suggested that the delivery of dust-borne iron to the glacial ocean could have increased primary productivity and enhanced deep-sea carbon export in this region, lowering atmospheric carbon dioxide concentrations during glacial periods. Such a role for the EEP is supported by higher organic carbon burial rates documented in underlying glacial sediments, but lower opal accumulation rates cast doubts on the importance of the EEP as an oceanic region for significant glacial carbon dioxide drawdown. Here we present a new silicon isotope record that suggests the paradoxical decline in opal accumulation rate in the glacial EEP results from a decrease in the silicon to carbon uptake ratio of diatoms under conditions of increased iron availability from enhanced dust input. Consequently, our study supports the idea of an invigorated biological pump in this region during the last glacial period that could have contributed to glacial carbon dioxide drawdown. Additionally, using evidence from silicon and nitrogen isotope changes, we infer that, in contrast to the modern situation, the biological productivity in this region is not constrained by the availability of iron, silicon and nitrogen during the glacial period. We hypothesize that an invigorated biological carbon dioxide pump constrained perhaps only by phosphorus limitation was a more common occurrence in low-latitude areas of the glacial ocean.

Modern seawater profiles of oxygen, nitrate deficit, and nitrogen isotopes reveal the spatial decoupling of summer monsoon‐related productivity and denitrification maxima in the Arabian Sea (AS) and raise the possibility that winter monsoon and/or ventilation play a crucial role in modulating denitrification in the northeastern AS, both today and through the past. A new high‐resolution 50‐ka record of δ15N from the Pakistan margin is compared to five other denitrification records distributed across the AS. This regional comparison unveils the persistence of east‐west heterogeneities in denitrification intensity across millennial‐scale climate shifts and throughout the Holocene. The oxygen minimum zone (OMZ) experienced east‐west swings across Termination I and throughout the Holocene. Probable causes are (1) changes in ventilation due to millennial‐scale variations in Antarctic Intermediate Water formation and (2) postglacial reorganization of intermediate circulation in the northeastern AS following sea level rise. Whereas denitrification in the world's OMZs, including the western AS, gradually declined following the deglacial maximum (10−9 ka BP), the northeastern AS record clearly witnesses increasing denitrification from about 8 ka BP. This would have impacted the global Holocene climate through sustained N2O production and marine nitrogen loss.

[1] Centennial-scale records of sea-surface temperature and opal composition spanning the Last Glacial Maximum and Termination 1 (circa 25-6 ka) are presented here from Guaymas Basin in the Gulf of California. Through the application of two organic geochemistry proxies, the U 37 K′ index and the TEX 86 H index, we present evidence for rapid, stepped changes in temperatures during deglaciation. These occur in both temperature proxies at 13 ka ($3 C increase in 270 years), 10.0 ka ($2 C decrease over $250 years) and at 8.2 ka (3 C increase in <200 years). An additional rapid warming step is also observed in TEX 86 H at 11.5 ka. In comparing the two temperature proxies and opal content, we consider the potential for upwelling intensity to be recorded and link this millennial-scale variability to shifting Intertropical Convergence Zone position and variations in the strength of the Subtropical High. The onset of the deglacial warming from 17 to 18 ka is comparable to a "southern hemisphere" signal, although the opal record mimics the ice-rafting events of the north Atlantic (Heinrich events). Neither the modern seasonal cycle nor El Niño/Southern Oscillation patterns provide valid analogues for the trends we observe in comparison with other regional records. Fully coupled climate model simulations confirm this result, and in combination we question whether the seasonal or interannual climate variations of the modern climate are valid analogues for the glacial and deglacial tropical Pacific.

Abstraet:The modern sandy Golo turbidite system (500 km 2) is located in a confined basin on the eastern margin of Corsica. The Golo turbidite system is fed by a single river, which supplies coarse sand derived from active weathering of the neighbouring mountains. The late Quaternary deposits have been imaged using a closely spaced grid of 1000 km of sparker seismic-reflection profiles (line spacing close to 1.6 km, vertical resolution of 2 m). The turbidite system is composed of four non-coalescent fans that were at times active simultaneously and of two small deposits onto the slope. The resulting sedimentation pattern is characterized by stacked turbidite deposits. At a regional scale, there is a continuum of fan morphologies and geometries from south to north. The use of both seismic and sedimentary facies, together with mapped seismic geometry of sedimentary bodies, allowed definition of four architectural elements: (1) submarine valley (canyon and gully), (2) sandy channel, (3) muddy levee, and (4) sandy lobe. Some of these architectural elements can be recognized at a scale that is comparable to outcrop examples. Features such as progressive lateral migration and avulsion, or complex longitudinal evolution (progradation and retrogradation), can also be accurately described. Despite the active tectonics along the studied margin, the main variations in sedimentation appear to be controlled by eustatic changes, pre-existing seafloor topography, and sediment source characteristics. The general pattern of sedimentation is controlled by the influence of a confining slope, leading to the predominance of aggradation and to specific morphology and architecture of sedimentary bodies.

[1] Iron is considered to play a large role in the cycling of Si in Fe-limited regions of the ocean, but little is known about its role in Si biogeochemistry outside these areas. Here, we present published sediment trap data, new nutrient profiles and high resolution sedimentary records (Si isotopes, Biogenic silica%, N% and C%) from the Gulf of California, a non-Fe-limited region, to investigate the history of Si cycling in this highly productive basin. Modern nutrient profiles show that silicic acid in subsurface waters is in excess relative to nitrate and is therefore incompletely utilized during moderate winter upwelling events. Modern data, however, suggest that during intense upwelling episodes, silicic acid is preferentially utilized relative to nitrate by the biota, which we suggest reflects transient iron limitation. Our new d 30 Si record from the Guaymas Basin shows dramatic variations at millennial timescales. Low d 30 Si values synchronous with Heinrich events are interpreted as resulting from the decline in Si(OH) 4 utilization at times of decreased upwelling strength, while nearly complete Si(OH) 4 utilization was observed at times of invigorated upwelling and increased opal burial during the Holocene, the Bølling-Allerød and the last glacial period. We attribute the complete utilization of Si(OH) 4 to the occurrence of transient Fe limitation at these times. Our study highlights the importance of Fe limitation on Si and C cycling in coastal upwelling regions and suggests that upwelling dynamics, in combination with Fe availability, have the potential to modulate marine Si distribution and opal burial even at short timescales.

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The Lüderitz upwelling cell is presently the most productive area of the Benguela current system and abundant organic matter (OM) accumulates on the adjacent slope sediments even at great water depth. OM from two cores taken on the slope and covering the last 280 kyear was analysed in terms of "petroleum quality" (Rock-Eval), chemical features (FTIR, EDS) and petrographic composition (light microscopy and TEM). These data indicate that the OM is more oxidized at 3606 m water depth than on the upper slope sediments (1029 m) although the petroleum quality of the OM throughout the deep-water core remains surprisingly high for hemipelagic deepsea sediments (HI=200-400 mg/g). The petroleum quality of OM accumulated on the upper slope is consistently high: HI averages 450 mg/g. Two petrographic types of OM are distinguishable from microscopic observation, each ascribed to distinctive preservation mechanisms: (1) 'Granular' amorphous OM, which dominates in the deep-water core, is formed by organo-mineral aggregates. Aggregation appears to be the primary preservation mode at this depth although is quantitatively limited (maximum TOC value of 4 wt.% of bulk sediment obtained through this process). The ultrastructure of the aggregates highlights an intimate association pattern between sedimentary OM and clays. (2) 'Gel-like' nannoscopically amorphous OM (NAOM) largely dominates at 1000 m water depth and contains sulfur. Thus, early diagenetic sulfurization was probably involved in the preservation of this OM, but a contribution from the classical degradation-recondensation pathway cannot be ruled out. Moreover, selective preservation occurred at both sites but represents an insignificant part of the OM. Organic fluxes mainly control the occurrence and extent of sulfurisation at both water depths by determining the redox conditions at the sea floor. Aggregate formation is limited by both organic and mineral fluxes at the lower slope whereas OM supply is never limiting on the upper slope. Although consistently operating through time at both depths, preservation by organo-mineral association is limited by mineral availability and thus accounts for a relatively minor portion of the OM accumulated on this organic-rich slope. In the case of large organic fluxes, sulfurisation and/or degradation-recondensation is required to obtain TOC contents above 4 wt.% of bulk sediment in the area.