North Atlantic Deep Water Production Research Articles

Influences of Atlantic Ocean thermohaline circulation and Antarctic ice-sheet expansion on Pliocene deep Pacific carbonate chemistry

Quantifying changes in seawater carbonate chemistry is crucial to deciphering of patterns and drivers of the oceanic carbon cycle and climate change. Here, we present a new deep-water carbonate ion saturation state (Δ[CO32−]) record for the Pliocene western tropical Pacific, reconstructed from the size-normalized weight of the planktonic foraminifer Trilobatus sacculifer of IODP Site U1490. A steep decline in deep-water Δ[CO32−] occurred at ∼4.6 Ma synchronous to the enhanced production of North Atlantic Deep Water (NADW) related to the closure of the Panamanian Gateway. Subsequently, at the onset of the Northern Hemisphere glaciation at ∼2.7 Ma the weakening of NADW formation resulted in a deep-water Δ[CO32−] peak. The changes in NADW production rate likely controlled a seesaw-like fluctuation in deep-water Δ[CO32−] between the Pacific and Atlantic oceans. During the late Pliocene (∼3.8–2.8 Ma), Antarctic ice-sheet/sea-ice expansions sequestered CO2 in the deep Pacific through ventilation of the deep watermass, leading to a long-term decrease in deep Pacific Δ[CO32−]. We infer that fluctuating NADW production rates at ∼4.6 Ma and ∼2.7 Ma influenced inter-basinal fractionation of deep-ocean carbon between the Atlantic and Pacific, and that deep Pacific carbon storage linked to expansions of Antarctic ice sheet/sea ice contributed to the lowering of atmospheric pCO2 and global cooling during the late Pliocene.

Age and driving mechanisms of the Eocene–Oligocene transition from astronomical tuning of a lacustrine record (Rennes Basin, France)

Abstract. The Eocene–Oligocene Transition (EOT) marks the onset of the Antarctic glaciation and the switch from greenhouse to icehouse climates. However, the driving mechanisms and the precise timing of the EOT remain controversial mostly due to the lack of well-dated stratigraphic records, especially in continental environments. Here we present a cyclo-magnetostratigraphic and sedimentological study of a ∼ 7.6 Myr long lacustrine record spanning the late Eocene to the earliest Oligocene, from a drill core in the Rennes Basin (France). Cyclostratigraphic analysis of natural gamma radiation (NGR) log data yields duration estimates of Chrons C12r through C16n.1n, providing additional constraints on the Eocene timescale. Correlations between the orbital eccentricity curve and the 405 kyr tuned NGR time series indicate that 33.71 and 34.10 Ma are the most likely proposed ages of the EO boundary. Additionally, the 405 kyr tuning calibrates the most pronounced NGR cyclicity to a period of ∼1 Myr, matching the g1–g5 eccentricity term, supporting its significant expression in continental depositional environments, and hypothesizing that the paleolake level may have behaved as a low-pass filter for orbital forcing. Two prominent changes in the sedimentary facies were detected across the EOT, which are temporally equivalent to the two main climatic steps, EOT-1 and Oi-1. We suggest that these two facies changes reflect the two major Antarctic cooling/glacial phases via the hydrological cycle, as significant shifts to drier and cooler climate conditions. Finally, the interval spanning the EOT precursor glacial event through EOT-1 is remarkably dominated by obliquity. This suggests preconditioning of the major Antarctic glaciation, either from obliquity directly affecting the formation/(in)stability of the incipient Antarctic Ice Sheet (AIS), or through obliquity modulation of the North Atlantic Deep Water production.

Multicentury Instability of the Atlantic Meridional Circulation in Rapid Warming Simulations With GISS ModelE2

AbstractIn multimillennial global warming simulations with the GISS‐E2‐R climate model, we observe multicentennial shutdowns with restoration and fast overshooting in North Atlantic Deep Water production despite the absence of exogenous freshwater input. AMOC (Atlantic Meridional Overturning Circulation) cessation is associated with a sea surface salinity reduction, initiated by increases in precipitation over evaporation as the climate warms. These multicentury shutdowns are the direct result of cooling in the North Atlantic associated with an aerosol indirect effect on cloud cover. The local cooling reduces evaporation within the North Atlantic, while warming elsewhere provides moisture to maintain nearly unperturbed precipitation in this region. As global warming continues, warm temperature (low density) anomalies spread northward at depth in the North Atlantic eventually destabilizing the water column, even though precipitation input at the surface is initially unchanged. Internal ocean freshwater transports do not play an important role in initiating this behavior, as assumed by some standard metrics of AMOC stability. The importance of the aerosol indirect effect in these runs is due to its role in strengthening the sea surface temperature‐evaporation feedback; this suggests a renewed focus on surface flux observations to help assess overturning stability. The length of the AMOC reduction, and its rapid recovery, may be relevant to the onset and end of the Younger Dryas, which occurred within a warming climate during the last deglaciation.

An 8000-year multi-proxy peat-based palaeoclimate record from Newfoundland: Evidence of coherent changes in bog surface wetness and ocean circulation

Energy carried by warm tropical water, transported via the Atlantic Meridional Overturning Circulation (AMOC), plays a vital role in regulating the climate of regions bordering the North Atlantic Ocean. Previous phases of elevated freshwater input to areas of North Atlantic Deep Water (NADW) production in the early to mid-Holocene have been linked with slow-downs in the AMOC and changes in regional climate. Newfoundland’s proximity in the North Atlantic region to the confluence of the Gulf Stream and the Labrador Current and to an area of NADW production in the Labrador Sea makes it an ideal testing ground to investigate the influence of past fluctuations in ocean circulation on terrestrial ecosystems. We use multi-proxy peat-based records from the east coast of Newfoundland to derive a proxy-climate signal for the past 8000 years, which we have compared with changes in ocean circulation. Prominent shifts towards near-surface bog water-table levels, reflecting cooler/wetter climatic conditions, are evident in the early mid-Holocene at c. 7830, 7500, 7220 and 6600 cal. BP with minor changes occurring at c. 6340 and 6110 cal. BP. These events are coherent with evidence of meltwater injections into the N. Atlantic and of reduced NADW production. More recent increases in bog surface wetness in the mid- to late Holocene at c. 4290 and c. 2610 cal. BP are also consistent with reported periods of reduced NADW production. Coherence between the bog-derived palaeoclimate record developed from Newfoundland and evidence of fluctuations in ocean current strength is apparent in the early mid-Holocene.

Ocean-atmosphere interactions as drivers of mid-to-late Holocene rapid climate changes: Evidence from high-resolution stalagmite records at DeSoto Caverns, Southeast USA

Oxygen and carbon isotope time-series derived from an actively growing aragonitic stalagmite in DeSoto Caverns exhibit with unusual clarity rapid hydroclimate changes in the mid-to-late Holocene. Data consist of 1884 δ18O and δ13C determinations whose chronology is anchored on 35 230Th/234U absolute dates in the interval 6.0–1.1 cal ka BP. Exceptional 18O and 13C-enrichments centered at 4.8 ± 0.14 cal ka BP likely represent the imprints of a severe drought. Isotope cycles from 4.7 to 1.3 cal ka BP, exhibit a dominant periodicity of 68 ± 4 yrs. A gradual cooling trend of ∼0.6 °C/103 yrs is attributed to a declining seasonal contrast in insolation. The synchronicity of the mega-drought in the Southeast US with the (1) termination of the African Humid Period; (ii) abrupt reduction of the North Atlantic Deep Water production, and (iii) rapid sea-ice expansion in the polar regions of both Hemispheres testifies to the global extent and rapidity of the “5 ka” event and points to the North Atlantic Deep Water variability as the likely controlling factor. The multidecadal cycles are consistent with alternating dry and wet summers occurring during a long-term switch in the seasonal rainfall amount dominance from winter to summer. The periodic summer droughts in the Southeast US support climate models that predict profound hydroclimate changes in the late Holocene governed by the Atlantic Multidecadal Oscillation. The relatively short and rapid hydroclimate phase transitions documented in this study introduce a complication in the correlation of late Holocene drought events that had significant societal impacts.

Ice–Atmosphere Feedbacks Dominate the Response of the Climate System to Drake Passage Closure

The response of the global climate system to Drake Passage (DP) closure is examined using a fully coupled ocean–atmosphere–ice model. Unlike most previous studies, a full three-dimensional atmospheric general circulation model is included with a complete hydrological cycle and a freely evolving wind field, as well as a coupled dynamic–thermodynamic sea ice module. Upon DP closure the initial response is found to be consistent with previous ocean-only and intermediate-complexity climate model studies, with an expansion and invigoration of the Antarctic meridional overturning, along with a slowdown in North Atlantic Deep Water (NADW) production. This results in a dominance of Southern Ocean poleward geostrophic flow and Antarctic sinking when DP is closed. However, within just a decade of DP closure, the increased southward heat transport has melted back a substantial fraction of Antarctic sea ice. At the same time the polar oceans warm by 4°–6°C on the zonal mean, and the maximum strength of the Southern Hemisphere westerlies weakens by ≃10%. These effects, not captured in models without ice and atmosphere feedbacks, combine to force Antarctic Bottom Water (AABW) to warm and freshen, to the point that this water mass becomes less dense than NADW. This leads to a marked contraction of the Antarctic overturning, allowing NADW to ventilate the abyssal ocean once more. Poleward heat transport settles back to very similar values as seen in the unperturbed DP open case. Yet remarkably, the equilibrium climate in the closed DP configuration retains a strong Southern Hemisphere warming, similar to past studies with no dynamic atmosphere. However, here it is ocean–atmosphere–ice feedbacks, primarily the ice-albedo feedback and partly the weakened midlatitude jet, not a vigorous southern sinking, which maintain the warm polar oceans. This demonstrates that DP closure can drive a hemisphere-scale warming with polar amplification, without the presence of any vigorous Southern Hemisphere overturning circulation. Indeed, DP closure leads to warming that is sufficient over the West Antarctic Ice Sheet region to inhibit ice-sheet growth. This highlights the importance of the DP gap, Antarctic sea ice, and the associated ice-albedo feedback in maintaining the present-day glacial state over Antarctica.

North Atlantic Deep Water Production during the Last Glacial Maximum.

Changes in deep ocean ventilation are commonly invoked as the primary cause of lower glacial atmospheric CO2. The water mass structure of the glacial deep Atlantic Ocean and the mechanism by which it may have sequestered carbon remain elusive. Here we present neodymium isotope measurements from cores throughout the Atlantic that reveal glacial–interglacial changes in water mass distributions. These results demonstrate the sustained production of North Atlantic Deep Water under glacial conditions, indicating that southern-sourced waters were not as spatially extensive during the Last Glacial Maximum as previously believed. We demonstrate that the depleted glacial δ13C values in the deep Atlantic Ocean cannot be explained solely by water mass source changes. A greater amount of respired carbon, therefore, must have been stored in the abyssal Atlantic during the Last Glacial Maximum. We infer that this was achieved by a sluggish deep overturning cell, comprised of well-mixed northern- and southern-sourced waters.

The Plio-Pleistocene development of Atlantic deep-water circulation and its influence on climate trends

Using benthic stable isotope records from 10 sites in the Atlantic Ocean, including two new records from Walvis Ridge in the Southeast Atlantic (Sites 1264 and 1267), we review changes in Atlantic deep-water circulation in the context of Plio-Pleistocene climate. Overall, we find non-linear responses of Atlantic deep-water circulation to a cooling climate, with differently evolving glacial and interglacial states. Our main conclusion is that peak North Atlantic Deep Water (NADW) production was reached between ∼2.0 and 1.5 Ma, most prominently seen by a maximum in ventilated (high δ13C) conditions in the mid-depth Southeast Atlantic (Site 1264). We infer that a major source of NADW at this time was the export of dense overflow water from the Nordic Seas into the abyssal East Atlantic. Sea surface temperature records from the North and South Atlantic support this notion and indicate that the peak NADW production between ∼2.0 and 1.5 Ma was compensated by a stronger warm surface-water return flow (i.e. Atlantic Meridional Overturning Circulation (AMOC) was enhanced), causing long-term (>105 year) heat piracy from the South to the North Atlantic. In the wider picture of Plio-Pleistocene climate evolution, we find that a long-term enhancement in the average state of AMOC (∼2.4–1.3 Ma) coincides with the “41-kyr world”. Hence, we speculate that the transitory negative feedback response of enhanced AMOC to a cooling climate supplied heat to key areas of ice-sheet growth, acting to limit their size and maintain the “41-kyr world”. Once a threshold in global cooling was reached, the strength of AMOC lessened, providing a positive feedback for the Early-Middle Pleistocene Transition and the associated build-up of northern hemisphere ice-sheets.

Northern source for Deglacial and Holocene deepwater composition changes in the Eastern North Atlantic Basin

Over the last decades extensive research has been carried out on changes in the Atlantic Meridional Overturning Circulation and its role during the last deglacial period. At present, only a few high-resolution data sets from the deep eastern North Atlantic exist for this period. It therefore remains uncertain whether deepwater changes in the Eastern North Atlantic Basin were governed by alternating contributions of northern and southern deepwater or whether the Eastern North Atlantic Basin reflects changes in the initial composition and source of North Atlantic deepwater. Furthermore, it is still debated whether such changes are triggered by Northern or Southern Hemisphere climatic changes.In this centennial to decadal scale resolution study we investigate deepwater composition changes in the Eastern North Atlantic Basin over the last 15 ka BP. We used sediment cores GEOFAR KF16 and MD08-3180 (37.984° N; 31.118° W, wd 3050 m/37.999° N; 31.134° W, wd 3064 m), obtained from a small basin at the eastern flank of the Mid Atlantic Ridge south of the Azores Islands. Under modern conditions the coring site is situated at the boundary between southern Lower Deep Water and northern Eastern North Atlantic Deep Water consisting of Iceland Scotland Overflow Water and Labrador Sea Water. Distinct differences between the three water masses in terms of ventilation state, temperature and salinity signatures are ideal for tracking changes in the deglacial North Atlantic deepwater distribution using paired benthic foraminiferal stable isotopes (δ13C, δ18O) and Mg/Ca bottom water temperature reconstructions.The results show a close coupling of low bottom water temperature (1.5 °C) and δ13C(0–0.5‰) values during cold Heinrich event 1, the Younger Dryas and the Preboreal, that were coeval with a major δ18O depletion of deepwater. The strong similarities between subtropical Eastern North Atlantic Deep Water and subpolar North Atlantic surface and deepwater changes indicate that deglacial changes in Eastern North Atlantic Deep Water distribution were triggered in the North Atlantic. Consequently, this would imply that changes in the North Atlantic also contributed to deglacial changes in Antarctic bottom water composition. During the early Holocene stepwise increasing δ13C values suggest increasing Eastern North Atlantic Deep Water production and/or increasing Eastern North Atlantic Deep Water ventilation with minor but distinct interruptions (at 10.8, 10.6, 9.1, 8.4, 8.1 and 7.2 ka BP), which were most probably also triggered in the subpolar North Atlantic.

Dansgaard‐Oeschger oscillations predicted in a comprehensive model of glacial climate: A “kicked” salt oscillator in the Atlantic

AbstractDuring the period from 60,000 to 35,000 years ago, Summit‐Greenland ice core records of the oxygen isotopic ratio 18O/16O exhibit intense millennium time scale oscillations. These Dansgaard‐Oeschger oscillations have been interpreted to represent the variations in North Atlantic air temperature caused by correlative changes in the strength of North Atlantic Deep Water production. We apply a comprehensive model of glacial climate to unambiguously identify the mechanism responsible for this phenomenon. This is shown to involve a salt oscillation of relaxation oscillator form. This nonlinear oscillation does not require the existence of feedback due to freshwater release from grounded ice on the continents during the warm phase of the cycle.

Strengthening of North Atlantic deep water production from the Norwegian-Greenland Sea at onset of the Eocene cooling trend

Strengthening of North Atlantic deep water production from the Norwegian-Greenland Sea at onset of the Eocene cooling trend

Strengthening of North Atlantic deep-water production pre-dates onset of Antarctic glaciation

Strengthening of North Atlantic deep-water production pre-dates onset of Antarctic glaciation

Rapid Reductions in North Atlantic Deep Water During the Peak of the Last Interglacial Period

Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved epibenthic foraminiferal δ(13)C record, we show that the influence of North Atlantic Deep Water (NADW) was strong at the onset of the last interglacial period and was then interrupted by several prominent centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer and fresher than at present.

Onset of North Atlantic Deep Water production coincident with inception of the Cenozoic global cooling trend: COMMENT

Hohbein et al. (2012) propose an early Mid-Eocene onset of North Atlantic Deep Water (NADW) production by interpreting a mounded deposit at the south-west end of the Faroe-Shetland Basin (FSB) as a contourite drift, which they term the ‘Judd Falls Drift (JFD)’. We argue that this deposit is not a contourite drift; we also question how their model of early NADW production fits with current understanding of the development of the Faroe-Shetland Basin and the wider Arctic–NE Atlantic region, neither of which was convincingly discussed by these authors.

Onset of North Atlantic Deep Water production coincident with inception of the Cenozoic global cooling trend: REPLY

Onset of North Atlantic Deep Water production coincident with inception of the Cenozoic global cooling trend: REPLY

Influence of the Amazon River development and constriction of the Central American Seaway on Middle/Late Miocene oceanic conditions at the Ceara Rise

Sediment samples from ODP 154 Site 926A (Ceara Rise, western equatorial Atlantic Ocean) spanning the Neogene from 12.8 to 9.2Ma were investigated on their calcareous dinoflagellate contents to better understand the oceanographic changes in relation to the closure of the Central American Seaway and the development of the Amazon River. Intervals with increased cyst accumulation rates and dissolution sensitive species occur from 12.4Ma onward. They correspond to periods of enhanced North Atlantic Deep Water production indicating the presence of this water mass at the research site during these intervals. This suggests that pulses of North Atlantic Deep waters sporadically flew into the South Atlantic related to uplift phases of the Panama Sill.At about 11.2Ma the first appearance of Leonella granifera indicates river influence at the sample site. This first indication of river influence in the western equatorial Atlantic can be linked to the developing Amazon River. The cyst association changes at about 11.2Ma from almost monospecific to highly diverse and the permanent presence and increased abundance of L. granifera suggest that river waters were able to reach the study site by now, probably as a result of the southward flowing North Brazil Current. After 10.5Ma the cyst association indicates a decrease in Amazon influence at Site 926A. This change can be correlated to a reducing inflow of Pacific waters through the Central American Seaway leading to a reverse of the North Brazil Current to its modern northwards flow pattern.

Late-Holocene NAO and oceanic forcing on high-altitude proglacial sedimentation (Lake Bramant, Western French Alps)

Comparison of glacially derived clastic inputs in high altitude proglacial lake Bramant (Western French Alps) with measured North Atlantic Oscillation winter (NAOw) index reveals an inverse correlation between ad 1884 and 1968 at the pluridecadal timescale (20–25 years). This reflects periodical variations in snow accumulation over Lake Bramant catchment area partly influencing the glacier mass balance in the watershed. Further comparisons with reconstructed NAOw index since ad 1500 highlight spatial and temporal variations of the pluridecadal NAOw influence on this alpine climate, especially at the end of the ‘Little Ice Age’. In addition, wavelet analysis of continuous proxies of clastic sedimentation over the last 4150 years indicates significant pluridecadal variability at frequencies compatible with the NAO (30 years), while periods centered at 60–70 years could also be linked to the North Atlantic Ocean–atmosphere internal variability (Atlantic Multidecadal Oscillation (AMO)). The influence of the North Atlantic deep water production on the regional alpine climate is also suggested by a significant 550 yr cycle of clastic inputs since 2800 cal. BP. Coupling between the North Atlantic Ocean and the atmosphere seems therefore to play a fundamental role on glacier mass balance and climate during the late Holocene in the western Alps.

Patterns of European vegetation changes over the last 15,000 years based on the European Pollen Database with new chronologies

Quantifying changes in seawater carbonate chemistry is crucial to deciphering of patterns and drivers of the oceanic carbon cycle and climate change. Here, we present a new deep-water carbonate ion saturation state (Δ[CO32−]) record for the Pliocene western tropical Pacific, reconstructed from the size-normalized weight of the planktonic foraminifer Trilobatus sacculifer of IODP Site U1490. A steep decline in deep-water Δ[CO32−] occurred at ∼4.6 Ma synchronous to the enhanced production of North Atlantic Deep Water (NADW) related to the closure of the Panamanian Gateway. Subsequently, at the onset of the Northern Hemisphere glaciation at ∼2.7 Ma the weakening of NADW formation resulted in a deep-water Δ[CO32−] peak. The changes in NADW production rate likely controlled a seesaw-like fluctuation in deep-water Δ[CO32−] between the Pacific and Atlantic oceans. During the late Pliocene (∼3.8–2.8 Ma), Antarctic ice-sheet/sea-ice expansions sequestered CO2 in the deep Pacific through ventilation of the deep watermass, leading to a long-term decrease in deep Pacific Δ[CO32−]. We infer that fluctuating NADW production rates at ∼4.6 Ma and ∼2.7 Ma influenced inter-basinal fractionation of deep-ocean carbon between the Atlantic and Pacific, and that deep Pacific carbon storage linked to expansions of Antarctic ice sheet/sea ice contributed to the lowering of atmospheric pCO2 and global cooling during the late Pliocene.

Onset of North Atlantic Deep Water production coincident with inception of the Cenozoic global cooling trend

Here we show that the onset of deep water overflow from the Norwegian-Greenland Sea into the North Atlantic, interpreted to represent the onset of a modern-style North Atlantic Deep Water mass, commenced close to the early to middle Eocene boundary. This finding is based on the identification of a large, elongate contourite sediment drift, the “Judd Falls Drift,” in the Faeroe-Shetland Basin, through detailed mapping of high-resolution two-dimensional and three-dimensional seismic data. This sediment drift covers an area of ∼9000 km2 in one of the critical present-day deep water gateways into the North Atlantic Ocean. Interpretation of the body as a contourite drift is based on standard seismic-stratigraphic diagnostic criteria including tridirectional onlap, lensoid sediment geometry, upslope progradational configurations of stacked onlap units, and an erosional base cut in a deepwater setting. The internal reflection configuration indicates that the drift was deposited under a southwest-flowing current regime, which we propose represents the onset of North Atlantic Deep Water production, with continual deep water flow until at least the latest Eocene (ca. 35 million years ago [Ma]). Onset of drift deposition is dated by several key boreholes as occurring at ca. 50–49 Ma, across the early to middle Eocene boundary, and is coincident with independent geochemical evidence for major changes in the configuration of deep-sea circulation patterns, global biological production, and the onset of Cenozoic climatic cooling. These temporal concurrences suggest a strong link between the initiation of North Atlantic Deep Water production and Cenozoic global climate evolution.

南大西洋ベンゲラ海流系の古海洋学

The Benguela Current is an eastern boundary current in the South Atlantic subtropical gyre, associated with strong coastal upwelling off Namibia, which plays a major role in heat transport from the Indian Ocean to the Atlantic Ocean through the Agulhas Current and in the global carbon cycle through high biologic productivity. Seven sedimentary cores (Sites 1081–1087) recovered during the Ocean Drilling Program Leg 175 cruise from continental slopes consist of upper Cenozoic continuous siliceous and calcareous hemipelagic sequences, which allowed high-resolution paleoceanographic analyses over the upwelling region from 20°S to 30°S. Onboard and post-cruise scientific research has revealed and discussed the history of the oceanic system and the relation to global climate changes as follows. (1) Calcareous sequences (Sites 1085 and 1087) in the southern part of the region record sedimentary inprints that can be correlated with the Miocene global carbonate crash events and latest Miocene to early Pliocene biogenic bloom with the first signal of wind-driven upwelling at 11.2 Ma. (2) Off Namibia diatom concentrations dramatically increased after 3.1 Ma and reached a maximum spanning from 2.6 to 2.0 Ma, which was called the Matuyama Diatom Maximum associated with a moderate increase in organic matter accumulation and lowering of sea-surface temperature. This elevated bioproductivity and cooling occurred in response to changes in water circulation caused by gateway closures and enhanced bipolar glaciation. During the last 2 million years, the decreasing trend of diatom deposition coincided with an overall increase of coastal upwelling intensity. (3) The Walvis Opal Paradox is another prominent feature observed in orbitally controlled climate cycles in the upwelling system during the Quaternary. It is characterized by a decrease of diatom/opal deposition, which coincided with increased upwelling during glacial periods and vice versa during interglacials. Its possible causes include waning of North Atlantic Deep Water production during glacials. Despite these great advances in the reconstruction of the evolution of the Benguela Current upwelling system, causal links to global climate and regional events in other oceans are less well understood. To evaluate the interplay between opal/organic carbon deposition in the upwelling system and a series of climatic, tectonic, oceanographic, and biologic events in the world ocean, a better understanding of sedimentary processes on shelves and slopes in terms of glacio-eustatic sea level changes, improvement of paleoproductivity reconstructions, and reevaluation of dissolution of siliceous microfossil shells is needed.