Lateral Advection Research Articles

The response of excess 230Th and extraterrestrial 3He to sediment redistribution at the Blake Ridge, western North Atlantic

The constant-flux proxies excess 230Th ( 230Th xs) and extraterrestrial 3He ( 3He ET) are commonly used to calculate sedimentary mass accumulation rates and to quantify lateral advection of sediment at core sites. In settings with significant lateral input or removal of sediment, these calculations depend on the assumption that concentrations of 230Th xs and 3He ET are the same in both advected sediment and sediment falling through the water column above the core site. Sediment redistribution is known to fractionate grain sizes, preferentially transporting fine grains; though relatively few studies have examined the grain size distribution of 230Th xs and 3He ET, presently available data indicate that both are concentrated in fine grains, suggesting that fractionation during advection may bias accumulation rate and lateral advection estimates based on these proxies. In this study, we evaluate the behavior of 230Th xs and 3He ET in Holocene and last glacial samples from two cores from the Blake Ridge, a drift deposit in the western North Atlantic. At the end of the last glacial period, both cores received large amounts of laterally transported sediment enriched in fine-grained material. We find that accumulation rates calculated by normalization to 230Th and 3He are internally consistent despite large spatial and temporal differences in sediment advection. Our analyses of grain size fractions indicate that ~ 70% of 3He ET-bearing grains are in the < 20 μm fraction, with roughly equal amounts in the < 4 and 4–20 μm fractions. 230Th xs is concentrated in <4-μm grains relative to 4- to 20-μm grains by approximately a factor of 2 in Holocene samples and by a much larger factor (averaging a factor of 10) in glacial samples. Despite these enrichments of both constant-flux proxies in fine particles, the fidelity of 230Th- and 3He-based accumulation rate estimates appears to be preserved even in settings with extreme sediment redistribution, perhaps due to the cohesive behavior of fine particles in marine settings.

Abrupt warming and salting of the Western Mediterranean Deep Water after 2005: Atmospheric forcings and lateral advection

The recent major production of anomalously warm, salty deep water in the northwestern Mediterranean Sea (winters 2004–2005 and 2005–2006) is linked to extreme winter air‐sea heat and freshwater forcing of the basin. Fields of heat and density fluxes are determined both from the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis and a daily high‐resolution downscaling of the European Centre for Medium‐Range Weather Forecasts reanalysis and analysis data set ARPERA. In the deep water formation region, during winter 2004–2005, the net heat loss exceeds 300 W m−2 compared with typical values of 200 W m−2. The relationship between the deep water formation episodes and large‐scale atmospheric patterns is investigated and found to be more closely related to the East Atlantic Pattern than the North Atlantic Oscillation. The contributions of atmospheric forcing and lateral advection of anomalously warm, salty water to the convection region are discussed in order to determine their relative roles in causing massive renewal of Western Mediterranean Deep Water and its anomalous properties. The main result shows that the net evaporation during winter 2004–2005, even if very high compared to the climatology, could have induced only 49% of the actual observed increase in the salt content of the deep layer. Thus, lateral advection played a major role in setting the new deep water properties.

Three-Dimensional Structure and Temporal Evolution of Submesoscale Thermohaline Intrusions in the North Pacific Subtropical Frontal Zone

AbstractFour instances of persistent intrusive deformation of the North Pacific Subtropical Front were tagged individually by a Lagrangian float and tracked for several days. Each feature was mapped in three dimensions using repeat towed observations referenced to the float. Isohaline surface deformations in the frontal zone included sheetlike folds elongated in the alongfront direction and narrow tongues extending across the front. All deformations appeared as protrusions of relatively cold, and fresh, water across the front. No corresponding features of the opposite sign or isolated lenslike structures were observed. The sheets were O(10 m) thick, protruded about 10 km into the warm saline side of the front, and were coherent for 10–30 km along the front. Having about the same thickness and cross-frontal extent as the sheets, tongues extended less than 5 km along the front. All of the intrusions persisted as long as they were followed, several days to one week. Their structures evolved on both inertial (23 h) and subinertial (∼10 days) time scales in response to differential lateral advection. The water mass surrounding the intrusions participated in gradual anticyclonic rotation as a part of a mesoscale meander of the subtropical front. The intrusions may be interpreted as a manifestation of three-dimensional submesoscale turbulence of the frontal zone, driven by the mesoscale. Absence of large features of the opposite sign may be indicative of the asymmetry of the underlying dynamics.

Intraseasonal response of mixed layer temperature and salinity in the Bay of Bengal to heat and freshwater flux

Buoy and satellite data show pronounced subseasonal oscillations of sea surface temperature (SST) in the summertime Bay of Bengal. The SST oscillations are forced mainly by surface heat flux associated with the active break cycle of the south Asian summer monsoon. The input of freshwater (FW) from summer rain and rivers to the bay is large, but not much is known about subseasonal salinity variability. We use 2002–2007 observations from three Argo floats with 5 day repeat cycle to study the subseasonal response of temperature and salinity to surface heat and freshwater flux in the central Bay of Bengal. About 95% of Argo profiles show a shallow halocline, with substantial variability of mixed layer salinity. Estimates of surface heat and freshwater flux are based on daily satellite data sampled along the float trajectory. We find that intraseasonal variability of mixed layer temperature is mainly a response to net surface heat flux minus penetrative radiation during the summer monsoon season. In winter and spring, however, temperature variability appears to be mainly due to lateral advection rather than local heat flux. Variability of mixed layer freshwater content is generally independent of local surface flux (precipitation minus evaporation) in all seasons. There are occasions when intense monsoon rainfall leads to local freshening, but these are rare. Large fluctuations in FW appear to be due to advection, suggesting that freshwater from rivers and rain moves in eddies or filaments.

Assessing the apparent imbalance between geochemical and biochemical indicators of meso- and bathypelagic biological activity: What the @$♯! is wrong with present calculations of carbon budgets?

Metabolic activity in the water column below the euphotic zone is ultimately fuelled by the vertical flux of organic material from the surface. Over time, the deep ocean is presumably at steady state, with sources and sinks balanced. But recently compiled global budgets and intensive local field studies suggest that estimates of metabolic activity in the dark ocean exceed the influx of organic substrates. This imbalance indicates either the existence of unaccounted sources of organic carbon or that metabolic activity in the dark ocean is being over-estimated. Budgets of organic carbon flux and metabolic activity in the dark ocean have uncertainties associated with environmental variability, measurement capabilities, conversion parameters, and processes that are not well sampled. We present these issues and quantify associated uncertainties where possible, using a Monte Carlo analysis of a published data set to determine the probability that the imbalance can be explained purely by uncertainties in measurements and conversion factors. A sensitivity analysis demonstrates that the bacterial growth efficiencies and assumed cell carbon contents have the greatest effects on the magnitude of the carbon imbalance. Two poorly quantified sources, lateral advection of particles and a population of slowly settling particles, are discussed as providing a means of closing regional carbon budgets. Finally, we make recommendations concerning future research directions to reduce important uncertainties and allow a better determination of the magnitude and causes of the unbalanced carbon budgets.

Characteristics and flux of settling particulate matter in neritic waters: The southern Yellow Sea and the East China Sea

The typology and flux of settling particulate matter (SPM) were investigated based on sediment trap sampling at six typical stations in the Yellow Sea and the East China Sea. The settling particulate matter in the neritic seas was sorted into three categories, lithogenic particles, living organisms, and particle aggregates. The mass of individual organisms is an important portion of particulate matter in the neritic waters. The aggregates contain six types, mucus aggregates, fecal pellets, diatom aggregates, silicoflagellate aggregates, tintinnids, and miscellaneous aggregates, of which the silicoflagellate aggregates and tintinnids are the most abundant in the Yellow Sea and the East China Sea. High particle fluxes, such as 215 to 874 g m(-2). day(-1) SPM in the bottom layer, were found at three stations where the water was well mixed, and the maximum flux was detected in the boundary area between the Yellow Sea and the East China Sea, where a wide nepheloid layer was present. Hence, particle flux in neritic waters can be easily shifted by water turbulence. The net vertical flux (123 to 961 mg C day(-1)), the contribution of lateral advection to resuspension flux (5 to 76%), and the particulate organic carbon export ratio (18 to 60%) were estimated for the other three stations where the water was stratified. The highest values were all found in the upwelling area off the Zhejiang coast, suggesting that the area of high productivity provides a high net vertical flux of SPM. (C) 2010 Elsevier Ltd. All rights reserved.

Seasonal and inter-annual biogeochemical variations in the Porcupine Abyssal Plain 2003–2005 associated with winter mixing and surface circulation

We present a 3-year multidisciplinary biogeochemical data set taken in situ at the Porcupine Abyssal Plain (PAP) time-series observatory in the Northeast Atlantic (49°N, 16.5°W; water depth not, vert, similar4850 m) for the period 2003 to 2005. The high-resolution year-round autonomous measurements include temperature, salinity, chlorophyll-a (derived from in situ chlorophyll-fluorescence) and inorganic nitrate, all at a nominal depth of 30 m on an Eulerian observatory mooring. This study compares these in situ time-series data with satellite chlorophyll-a data, regional data from a ship of opportunity, mixed-layer depth measurements from profiling Argo floats and lateral advection estimates from altimetry. This combined and substantial data set is used to analyse seasonal and inter-annual variability in hydrography and nitrate concentrations in relation to convective mixing and lateral advection. The PAP observatory site is in the inter-gyre region of the North Atlantic where convective mixing ranges from 25 m in the summer to over 400 m in winter when nutrients are supplied to the surface. Small inter-annual changes in the winter mixed layer can result in large changes in nitrate supply and productivity. However the decrease in maximum winter nitrate over the three-year period, from 10 to 7 mmol m?3, cannot be fully explained by convective mixing. Trajectories leading to the PAP site, computed from altimetry-derived geostrophic velocities, confirm that lateral advection cannot be ignored at this site and may be an important process along with convective mixing. Over the three years, there is an associated decrease in new production calculated from nitrate assimilation from 85.4 to 40.3±4.3 gCm?2 a?1. This confirms year-to-year variability in primary production seen in model estimates for the region. The continuous in situ dataset also shows inter-annual variation in the timing of the spring bloom due to variations in heat flux; the 2005 bloom occurred earlier than in 2004.

An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L−1 above the plateau and from 0.34 to 2.23 nmol L−1 offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329–770 nmol L−1). Apparent particulate iron (Feapp), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Feapp above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Feapp and DFe above the plateau, with a supply of 9.7±3.6×106 and 8.3±11.6×103 mol d−1, respectively. The residence times of 1.7 and 48 days estimated for Feapp and DFe respectively, indicate a rapid turnover in the surface water. A comparison between Feapp and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe)−1, which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.

Timescales of lateral sediment transport in the Panama Basin as revealed by radiocarbon ages of alkenones, total organic carbon and foraminifera

Abstract Paired radiocarbon measurements on haptophyte biomarkers (alkenones) and on co-occurring tests of planktic foraminifera (Neogloboquadrina dutertrei and Globogerinoides sacculifer) from late glacial to Holocene sediments at core locations ME0005-24JC, Y69-71P, and MC16 from the south-western and central Panama Basin indicate no significant addition of pre-aged alkenones by lateral advection. The strong temporal correspondence between alkenones, foraminifera and total organic carbon (TOC) also implies negligible contributions of aged terrigenous material. Considering controversial evidence for sediment redistribution in previous studies of these sites, our data imply that the laterally supplied material cannot stem from remobilization of substantially aged sediments. Transport, if any, requires syn-depositional nepheloid layer transport and redistribution of low-density or fine-grained components within decades of particle formation. Such rapid and local transport minimizes the potential for temporal decoupling of proxies residing in different grain-size fractions and thus facilitates comparison of various proxies for paleoceanographic reconstructions in this study area. Anomalously old foraminiferal tests from a glacial depth interval of core Y69-71P may result from episodic spillover of fast bottom currents across the Carnegie Ridge transporting foraminiferal sands towards the north.

Re-evaluation of ‘paradox of mesozooplankton’ in the eastern Arabian Sea based on ship and satellite observations

In the central and eastern Arabian Sea (EAS), biomass and production of phytoplankton are known to vary spatially and seasonally whereas, biomass of mesozooplankton (MSP) is reported to be constant. This apparent contradiction has been called ‘Arabian Sea Paradox’. However, it is important to note that the paradox is based on a very limited seasonal data (from 5 to 7 locations). Therefore, we reconsidered the paradox based on intensive in-situ observations at 37–40 locations in the EAS. In agreement with the known seasonal difference in phytoplankton standing stock in the EAS, we analysed the MSP data in two ways using two-way nested ANOVA. In ‘basin scale’ analysis, MSP data from the EAS were pooled and seasonal and inshore–offshore variations were analysed for the entire region. In ‘regional scale’ analysis, MSP data were analysed separately for (a) northern region (north of 15°N) and (b) southern region (15°N and south of it). Satellite data of chlorophyll a, SST and wind speed were also analysed to show the major differences in oceanographic features in the northern and southern EAS. The analyses showed prominently high chlorophyll a (av. 1 mg m − 3 ) in the northern region during most of the year through winter convection, open ocean upwelling and lateral advection from the Arabian coast. On the other hand, chlorophyll a was low (av. 0.2 mg m − 3 ) in the southern region during most of the year mainly due to thermohaline stratification. The MSP biomass was distributed almost in a similar way as that of phytoplankton stock with statistically significant spatial and seasonal variations in the northern and southern regions. In this paper, we review the ‘paradox of MSP’ and present clear and new evidences to show that this concept is not logically applicable for EAS.

Influence of Lateral Advection on Residual Currents in Microtidal Estuaries

Abstract The influence of nonlinear lateral advection on estuarine exchange flow is examined with a scaling analysis and eight groups of idealized numerical experiments. Nonlinear lateral advection is related to the linkage between lateral circulation and the lateral shear of the along-estuary flow. The relative contribution of lateral advection to the overall dynamics of a microtidal estuary is found to be a function of width and depth, and of vertical mixing. Lateral advection is dynamically important in narrow and deep estuaries, particularly under relatively weak vertical mixing. The relative importance of lateral advection and the earth’s rotation on estuarine dynamics can be evaluated in terms of the nondimensional Rossby and Ekman numbers (Ro and Ek). Lateral advection is most effective at large Ro and small Ek and is negligible at small Ro and large Ek. As expected, the earth’s rotation is most significant at small Ro and Ek, and is negligible at large Ro and Ek. Under the influence of lateral advection and the earth’s rotation, the lateral structure of estuarine exchange flows is a function of Ro and Ek. In some instances, the exchange flow is vertically sheared and in others it is laterally sheared. Classical estuarine dynamics, which yields vertically sheared exchange flows, occurs at intermediate Ro and large Ek. The main role of lateral advection is to reduce lateral variability of estuarine exchange flow and generate a vertically sheared, two-layer exchange flow structure.

Organic‐rich mud on the western margin of southern Africa: Nutrient source to the Southern Ocean?

The biological pump plays a major role in the transfer of CO2 from the atmosphere to the deep Southern Ocean, a transfer which is largely controlled by the supply of iron and which may partially explain glacial to interglacial variations in pCO2. Analogous to the well‐documented, smaller‐scale “island mass effect,” we propose that the lateral advection of iron by south flowing intermediate waters along the southern African margin may sustain high‐productivity blooms of the Subtropical Convergence Zone (SCZ) between 10 and 70°E. We assess the present‐day interglacial (Holocene) reservoirs and fluxes of organic carbon (OC) and terrigenous mud on the western margin of southern Africa in order to estimate the potential supply of Fe to the Southern Ocean. The highly productive Benguela Upwelling System (BUS) appears to be a relatively inefficient coastal biological pump. Repeated sediment resuspension by wave and tidal energy dissipation limits OC burial to &lt;0.2% of net primary production (NPP) in the southern BUS and to between 0.2 to 2.4% in the northern BUS. Productivity and OC‐rich mud accumulation are focused on the inner portion of the 100–200 km wide shelf which, combined with south flowing bottom currents, limits the export of OC beyond the shelf break to 1.2–8.4% of NPP. However, winnowing of 1 million tons yr−1 of clay particles and the potential early diagenetic benthic (dissolved) Fe flux may supply 10 times more Fe than is transported by dust to the open ocean biological pump of the SCZ. Lowering sea level during glacial periods disperses interglacial mud deposits off the shelf and increases particulate Fe export by as much as a factor of 4. Glacial pulses of margin export may enhance the efficiency of the subantarctic Southern Ocean biological pump and contribute to the initial as well as glacial maximum drawdown in pCO2.

Lateral advection of organic matter in cascading-dominated submarine canyons

In the Gulf of Lions (GoL), dense water overflowing off the shelf occurs seasonally and represents the main mechanism affecting the shelf-slope exchange of particulate organic matter (OM). Most of the dense water export takes place in the south-western GoL and in particular through Cap de Creus (CdC) submarine canyon. Here, benthic instruments were deployed to collect down-canyon particulate fluxes whereas surface sediments were taken after the cascading event along the sediment dispersal system on the shelf, in CdC canyon and in the nearby Lacaze-Duthiers (LD) canyon. The chemical composition of the suspended material and surface sediments were investigated using several proxies including organic and inorganic carbon, total nitrogen, biogenic silica, δ 13C, Δ 14C, and alkaline CuO oxidation products. Thermohaline anomalies and high current speed events were measured in CdC canyon since December 2004 until mid-April 2005 indicating a marked off-shelf export of dense water trough the canyon. During the cascading, mud and relatively coarse shelf and upper canyon sediments were the major component of the mass flux. Conversely, advection of fine material via nepheloid layers dominated down-slope fluxes during pre- and post-cascading. The resulting change in grain-size affected the flux of mineral-bound terrigenous OC, indicating that the down-canyon transport of land-derived OM did not occur as bulk but rather its composition is driven by sediment sorting associated with different transport mechanisms. Both surface sediments and sediment trap samples indicated that CdC canyon is well connected to the GoL terrigenous dispersal system. Conversely, our results suggest an overall limited influence of land-derived OM in LD canyon. In spite of the reduced fluvial nutrient supply, a significant pulsed input of modern marine OM was observed in April 2005 at the end of the cascading period. Both intense mixing and lack of strong water column stratification likely played a key role allowing for both diffusion of nutrients-rich waters into the euphotic zone and efficient vertical sink of marine OC. On its way toward the seafloor, this fresh pool of OC interacted with the dense plume overflowing off the shelf, becoming part of the material laterally advected to the slope.

Sediment instability on the Portuguese continental margin under abrupt glacial climate changes (last 60 kyr)

It is well established that orbital scale sea-level changes generated larger transport of sediments into the deep-sea during the last glacial maximum than the Holocene. However, the response of sedimentary processes to abrupt millennial-scale climate variability is rather unknown. Frequency of distal turbidites and amounts of advected detrital carbonate are estimated off the Lisbon–Setúbal canyons (core MD03-2698, at 4602 mwd), within a chronostratigraphy based on radiometric ages, oxygen isotopes and paleomagnetic key global anomalies. We found that: 1) higher frequency of turbidites concurred with Northern Hemisphere coldest temperatures (Greenland Stadials [GS], including Heinrich [H] events). But more than that, an escalating frequency of turbidites starts with the onset of global sea-level rising (and warming in Antarctica) and culminates during H events, at the time when rising is still in its early-mid stage, and the Atlantic Meridional Overturning Circulation (AMOC) is re-starting. This short time span coincides with maximum gradients of ocean surface and bottom temperatures between GS and Antarctic warmings (Antarctic Isotope Maximum; AIM 17, 14, 12, 8, 4, 2) and rapid sea-level rises. 2) Trigger of turbidity currents is not the only sedimentary process responding to millennial variability; land-detrital carbonate (with a very negative bulk δ 18O signature) enters the deep-sea by density-driven slope lateral advection, accordingly during GS. 3) Possible mechanisms to create slope instability on the Portuguese continental margin are sea-level variations as small as 20 m, and slope friction by rapid deep and intermediate re-accommodation of water masses circulation. 4) Common forcing mechanisms appear to drive slope instability at both millennial and orbital scales.

Seasonal sea surface temperature variability in the Indonesian Seas

The response of the Indonesian Seas to the monsoonal winds is investigated using a regional ocean model with an emphasis on understanding the mechanisms behind the observed seasonal sea surface temperature (SST) variability. Understanding the seasonal SST variability in the Indonesian Seas is crucial for understanding tropical climate variability since atmospheric deep convection is located directly above these seas. Our model results suggest that the monsoonal winds play a dominant role in creating the spatial SST variability and are responsible for inducing about half of the basin‐averaged SST variability. The remaining half is found to be caused by conditions in surface heating, which is brought about by radiation and atmospheric temperature and humidity. The northwesterly wind during Austral summer is roughly uniform over the basin so wind mixing occurs roughly uniformly and no significant spatial SST variability is forced. The southeasterly wind during Austral winter, on the other hand, has a significant spatial variation with a maximum wind speed located in the middle of the basin. This spatial variability causes Ekman upwelling and downwelling in the north and south of the Indonesian Seas, respectively, and establishes a cold SST region in the northeast. The presence of the shallow shelf region in the east is also found critical to the formation of this cold SST event. Without the shelf, lateral advection of warm water from the west will trap the cold SST region to the eastern boundary.

Western Arabian Sea SST during the penultimate interglacial: A comparison of U37K′ and Mg/Ca paleothermometry

Millennial‐scale records of planktonic foraminiferal Mg/Ca, bulk sediment U37K′, and planktonic foraminiferal δ18O are presented across the last two deglaciations in sediment core NIOP929 from the Arabian Sea. Mg/Ca‐derived temperature variability during the penultimate and last deglacial periods falls within the range of modern day Arabian Sea temperatures, which are influenced by monsoon‐driven upwelling. The U37K′‐derived temperatures in MIS 5e are similar to modern intermonsoon values and are on average 3.5°C higher than the Mg/Ca temperatures in the same period. MIS 5e U37K′ and Mg/Ca temperatures are 1.5°C warmer than during the Holocene, while the U37K′‐Mg/Ca temperature difference was about twice as large during MIS 5e. This is surprising as, nowadays, both proxy carriers have a very similar seasonal and depth distribution. Partial explanations for the MIS 5e U37K′‐Mg/Ca temperature offset include carbonate dissolution, the change in dominant alkenone‐producing species, and possibly lateral advection of alkenone‐bearing material and a change in seasonal or depth distribution of proxy carriers. Our findings suggest that (1) Mg/Ca of G. ruber documents seawater temperature in the same way during both studied deglaciations as in the present, with respect to, e.g., season and depth, and (2) U37K′‐based temperatures from MIS 5 (or older) represent neither upwelling SST nor annual average SST (as it does in the present and the Holocene) but a higher temperature, despite alkenone production mainly occurring in the upwelling season. Further we report that at the onset of the deglacial warming, the Mg/Ca record leads the U37K′ record by 4 ka, of which a maximum of 2 ka may be explained by postdepositional processes. Deglacial warming in both temperature records leads the deglacial decrease in the δ18O profile, and Mg/Ca‐based temperature returns to lower values before δ18O has reached minimum interglacial values. This indicates a substantial lead in Arabian Sea warming relative to global ice melting.

Abrupt climate change, sea surface salinity and paleoproductivity in the western Mediterranean Sea (Gulf of Lion) during the last 28 kyr

The planktonic foraminiferal record of core MD99-2346 (Gulf of Lion) shows a decrease of sea surface temperature (SST) at the time of Heinrich event H2 (24–23.5 cal kyr BP), Younger Dryas and during the early to middle Holocene. These events were associated with a decrease of surface water salinity. The salinity lowering events were similar and synchronous to those recorded in the North Atlantic Ocean during H1, H2 and the Younger Dryas. They can be explained by a decrease in the salinity of the Atlantic surface water entering the Mediterranean through the Strait of Gibraltar. This result indicates that the Mediterranean Sea remained a concentration basin and that its freshwater budget was not significantly different from the modern conditions at the time of these low salinity events. Higher abundances of the benthic foraminifer Cibicidoides pachydermus at the time of Heinrich events (H2 and H1) may result from a seasonal, and more pulsed supply of labile organic matter. A high abundance of intermediate and deep infaunal taxa ( Globobulimina spp.) during the warm Bölling/Alleröd period is interpreted as a result of an increased lateral advection of organic matter to the deeper environments of the Gulf of Lion. Surface water productivity, as reflected by the organic flux to the benthic ecosystem, reaches a maximum during the cold Younger Dryas period. It is possible that cold conditions at the time of the Younger Dryas have allowed an enhanced vertical mixing and deep-water cascading in the Gulf of Lion. This may have resulted in an increase of primary and export production. This putative intensification of deep-water cascading would have occurred after the sustained rise of the global sea-level from 16 to 12.5 cal kyr BP. The deglacial sea-level rise may also have played an important role in favouring the downslope movement of dense shelf water enriched in organic matter originating from the early Rhône delta complex. The conditions during the Holocene were characterized by a low surface productivity and oligotrophic environment similar to the modern situation.

Transfer of organic carbon on the Moroccan Atlantic continental margin (NW Africa): productivity and lateral advection

Land—ocean transfer of sediment and organic matter along the Moroccan Atlantic margin (NW Africa) seems to have been very effective during the last 130 ka. In a marine core from this region, we found total organic carbon (TOC) values ranging from 0.3 to 1.7 dry wt% of bulk sediments. These relatively high values are fairly unusual, as the core was recovered from an open-ocean environment that is currently oligotrophic. In order to explain this trend, more typical of an upwelling eutrophic setting, three processes were evaluated: (1) in situ primary production associated with the extension of the Cape Ghir upwelling filament, (2) bottom water conditions that may favour organic carbon preservation and (3) lateral organic carbon advection. The site occasionally experienced more eutrophic conditions, especially during termination I; here, we recorded a relative high abundance of the planktonic foraminifer Globigerina bulloides, suggesting high primary production. However, given the absence of correlation between TOC and G. bulloides records, high TOC storage cannot be attributed exclusively to primary production. Preservation factors such as bottom water ventilation are also ruled out. Lateral TOC advection seems to be the most plausible process. Today, lateral advection and offshore transport of nutrients and organic matter characterize the study region. However, the triggering mechanisms deserve further investigation. Different controlling factors influencing the mobilization and advection of organic carbon from coastal upwelling sites to the deep basin are discussed. The correlation found between down-core TOC and sea-level changes suggests sea-level fluctuations as the most effective mechanism driving nepheloid layer detachment and seaward material transport.

Recent accumulation of trace metals in sediments at the DYFAMED site (Northwestern Mediterranean Sea)

The DYFAMED site (Ligurian Sea, 43°25′N; 7°52′E) has been the subject of multidisciplinary research since 1987, including the study of atmospheric deposition of metals and their association with marine particles in the water column. On the other hand, the attention paid to the underlying sediments as the ultimate repository of elements, particularly pollutants, has been considerably lesser. Through the combined use of 210Pb chronologies and elemental analysis, we studied the concentrations and fluxes of trace metals (Zn, Pb, Ni, Cu, Cr) in a sediment core collected from ∼2300 m water depth at the seafloor beneath DYFAMED. Surface metal enrichment during the 20th century is apparent in the case of Pb, Zn and Cu (maximum enrichment factors 1.5, 1.2 and 1.4, respectively). A comparison with published data suggests that atmospheric deposition is the main entrance route of anthropogenic trace metals accumulated in the recent sediments, while lateral advection of sediments by currents and sediment gravity flows is proposed to account for the pre-industrial concentrations of metals. Recent atmospheric inputs of Pb and Cu are more efficiently stored in sediments with respect to Zn, most of which is apparently advected elsewhere.

The Influence of Lateral Advection on the Residual Estuarine Circulation: A Numerical Modeling Study of the Hudson River Estuary

Abstract In most estuarine systems it is assumed that the dominant along-channel momentum balance is between the integrated pressure gradient and bed stress. Scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill. However, a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order. In this study, a previously validated numerical model of the Hudson River estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships. Results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along-channel momentum balance at leading order. The contribution of these nonlinear terms is driven largely by secondary lateral flows. Under a range of forcing conditions in the model runs, the advective acceleration terms nearly always act in concert with the baroclinic pressure gradient, reinforcing the residual circulation. Despite the strong contribution of the nonlinear advective terms to the subtidal dynamical balance, a linear scaling accurately predicts the strength of the observed residual circulation in the model. However, this result is largely fortuitous, as this scaling does not account for two processes that are fundamental to the estuarine circulation. The skill of this scaling results because of the compensatory relationship between the contribution of the advective acceleration terms and the suppression of turbulence due to density stratification. Both of these processes, neither of which is accounted for in the linear scaling, increase the residual estuarine circulation but have an opposite dependence on tidal amplitude and, consequently, strength of stratification.