Oscar E Romero

Seasonal productivity dynamics in the pelagic central Benguela System inferred from the flux of carbonate and silicate organisms

Flux of bulk components, carbonate- and silicate-bearing skeleton organisms, and the d 15 N-isotopic signal were investigated on a 1-year time-series sediment trap deployed at the pelagic NU mooring site (Namibia Upwelling, ca. 29jS, 13jE) in the central Benguela System. The flux of bulk components mostly shows bimodal seasonality with major peaks in austral summer and winter, and moderate to low export in austral fall and spring. The calcium carbonate fraction dominates the export of particulates throughout the year, followed by lithogenic and biogenic opal. Planktonic foraminifera and coccolithophorids are major components of the carbonate fraction, while diatoms clearly dominate the biogenic opal fraction. Bulk d 15 N isotopic composition of particulate matter is positively correlated with the total mass flux during summer and fall, while negatively correlated during winter and spring. Seasonal changes in the intensity of the main oceanographic processes affecting the NU site are inferred from variations in bulk component flux, and in the flux and diversity patterns of individual species or group of species. Influence from the Namaqua (Hondeklip) upwelling cell through offshore migration of chlorophyll filaments is stronger in summer, while the winter flux maximum seems to reflect mainly in situ production, with less influence from the coastal and shelf upwelling areas. On a yearly basis, dominant microorganisms correspond well with the flora and fauna of tropical/ subtropical waters, with minor contribution of near-shore organisms. The simultaneous occurrence of species with different ecological affinities mirrors the fact that the mooring site was located in a transitional region with large hydrographic variability over short-time intervals. D 2002 Elsevier Science B.V. All rights reserved.

Oceanographic control of biogenic opal and diatoms in surface sediments of the Southwestern Atlantic

Based on 66 surface sediment samples collected in the SW Atlantic Ocean between 27 and 50°S, this study presents an overview of the spatial distribution of biogenic opal and diatom concentrations, and diatom assemblages. Biogenic opal has highest values in the deepest, pelagic stations and decreases toward the slope. Diatoms closely follow the spatial trend of opal. Diatom assemblages reflect the present-day dominant hydrographical features. Antarctic diatoms are the main contributors to the preserved diatom community in core top sediments, with coastal planktonic and tropical/subtropical diatoms as secondary components. Dominance of Antarctic diatoms between 35 and 50°S in the pelagic realm mirrors the northward displacement of Antarctic-source water masses, characterized by high nutrient content and low salinity. Northward of ca. 35°S, the highest contribution of tropical/subtropical, pelagic diatoms, typical for nutrient-poor and high salinity waters, matches the main southward path of the Brazil Current. Mixing of Antarctic and tropical waters down up to 45°S is clearly illustrated by the diatom assemblage. Concentrations of biogenic opal and diatoms rather reflect the path of predominant water masses, but are less correlated with surface water productivity in the SW Atlantic.

Variability in Export Production Documented by Downward Fluxes and Species Composition of Marine Planktic Diatoms: Observations from the Tropical and Equatorial Atlantic

We analyzed the value of diatoms as a proxy of export production from the surface waters in the tropical and equatorial Atlantic. Species composition and downward flux of diatom assemblages was determined from time-series sediment traps off Cape Blanc in the Mauretanian upwelling zone; south of Cape Verde Island within the Guinea Dome; along a N-S transect in the high productivity belt of the eastern equatorial Atlantic in the Guinea Basin; in the less productive western equatorial Atlantic around 25°W; and at the Walvis Ridge within the Benguela upwelling system. Diatom fluxes were highest at the sites located adjacent to or influenced by coastal upwelling (off Cape Blanc and at the Walvis Ridge), south of Cape Verde Islands, and north of the equator in the Guinea Basin. Lowest values were recorded in the oligotrophic western Atlantic. Regardless of the percent contribution to the total flux, diatoms dominated (in numbers per m-2 d-1) the biogenic opal fraction at all sites and, in general, showed significant correlation with organic carbon flux. At all sites, downward fluxes revealed strong coupling with surface water production, and with the atmospheric and oceanic circulation systems related to the Trade Winds and the seasonal migration of the Intertropical Convergence Zone (ITCZ): Diatom flux maxima were observed in early spring and early summer off Cape Blanc; in fall and winter at Cape Verde; in spring and summer in the N-Guinea Basin, and only in spring in the S-Guinea Basin; and in austral fall and spring at the Walvis Ridge. No distinct seasonal pattern was seen in the western Atlantic. Diatom assemblages varied with the water mass characteristics influencing each trap location. Throughout the sampling periods, small specimens of Nitzschia bicapitata accompanied by other taxa typical of open-ocean conditions dominated the diatom assemblage in the eastern and western equatorial Atlantic, and at Cape Verde. At Cape Blanc and Walvis Ridge, on the other hand, a coastal upwelling assemblage was characterized by the colonial diatom Thalassionema nitzschioides var. nitzschioides and members of the chain-forming genus Chaetoceros, with a minor contribution of pelagic forms. In addition to the marine assemblage, freshwater diatoms and phytoliths (of continental origin) were present in the traps. Their fluxes coincided with the seasonal changes in Saharan dust transport patterns, and the geographical extension of the dust plume across the Atlantic. Flux values decreased as a function of distance from the African continent. Discrepancies between diatoms trapped and diatoms accumulating on the seafloor were seen in the eastern and western equatorial Atlantic, and at Walvis Ridge, and are attributed to strong dissolution at the sediment/water interface. At these sites, surface sediment assemblages are enriched in robust diatom species representative of the low productivity season of the surface waters. In contrast, diatoms in the trap off Cape Blanc, indicators of persistent offshore spreading of the coastal upwelling, possess moderately robust frustules readily preserved in the underlying sediments. Here, diatom species-specific selective dissolution was minor in sediment-trap samples. Despite this loss, the preserved diatom assemblage carries generalized information which can be related to the hydrographic conditions of the surface waters.

Eolian-transported freshwater diatoms and phytoliths across the equatorial Atlantic record: Temporal changes in Saharan dust transport patterns

The deposition of freshwater diatoms (FD) and phytotliths (P) was determined from sediment traps for a wide region (20°N–7°S) in the tropical and equatorial Atlantic, along a N-S transect in the eastern equatorial Atlantic around 10°W and in the western equatorial Atlantic around 25°W. These siliceous organisms are derived from the Sahara and Sahel regions of Africa, and eolian transport with direct settling over the open ocean is assumed to be the transport agent. Depositional rates of FD and P revealed strong coupling with seasonal changes in Saharan dust transport that are associated with seasonal precipitation patterns, major wind systems, and the geographical extension of the dust plume across the Atlantic. Mean daily fluxes were highest south of Cape Verde (FD = 9 × 104 valves m−2 d−1; P = 2 × 104 bodies m−2 d−1), moderately high off Cape Blanc and in the Guinea Basin north of the equator (of the order of 3 × 104 valves m−2 d−1 for FD, and 0.7 × 104 bodies m−2 d−1 for P), and consistently low south of the equator and in the western equatorial Atlantic. In traps north of the equator, seasonal changes were marked. Aulacoseira granulata and A. islandica were the most abundant FD in the traps, regardless of trap location and season. However, the number of FD species was higher in the Cape Blanc and Cape Verde areas. The morphological diversity (shape and size) of the P assemblage decreased with increasing distance from the African continent. Patterns of FD and P accumulation rates in surface sediments coincided with those in the traps. Robust freshwater diatom and phytolith records associated with seasonal eolian transport from Saharan and Sahelian regions into the Atlantic furnish clues that can help in our present understanding of the processes linking transport between land, atmosphere, and ocean.

Temporal and spatial variability in export production in the SE Pacific Ocean : evidence from siliceous plankton fluxes and surface sediment assemblages

Flux of siliceous plankton and taxonomic composition of diatom and silicoflagellate assemblages were determined from sediment trap samples collected in coastal upwelling-influenced waters off northern Chile (30°S, CH site) under “normal” or non-El Nino (1993–94) and El Nino conditions (1997–98). In addition, concentration of biogenic opal and siliceous plankton, and diatom and silicoflagellate assemblages preserved in surface sediments are provided for a wide area between 27° and 43°S off Chile. Regardless of the year, winter upwelling determines the maximum production pattern of siliceous microorganisms, with diatoms numerically dominating the biogenic opal flux. During the El Nino year the export is markedly lower: on an annual basis, total mass flux diminished by 60%, and diatom and silicoflagellate export by 75%. Major components of the diatom flora maintain much of their regular seasonal cycle of flux maxima and minima during both sampling periods. Neritic resting spores (RS) of Chaetoceros dominate the diatom flux, mirroring the influence of coastal-upwelled waters at the CH trap site. Occurrence of pelagic diatoms species Fragilariopsis doliolus , members of the Rhizosoleniaceae, Azpeitia spp. and Nitzschia interruptestriata , secondary components of the assemblage, reflects the intermingling of warmer waters of the Subtropical Gyre. Dictyocha messanensis dominates the silicoflagellate association almost year-around, but Distephanus pulchra delivers ca. 60% of its annual production in less than three weeks during the winter peak. The siliceous thanatocoenosis is largely dominated by diatoms, whose assemblage shows significant qualitative and quantitative variations from north to south. Between 27° and 35°S, the dominance of RS Chaetoceros , Thalassionema nitzschioides var. nitzschioides and Skeletonema costatum reflects strong export production associated with occurrence of coastal upwelling. Both highest biogenic opal content and diatom concentration at 35° and 41°–43°S coincide with highest pigment concentrations along the Chilean coast. Predominance of the diatom species Thalassiosira pacifica and T . poro - irregulata , and higher relative contribution of the silicoflagellate Distephanus speculum at 41°–43°S suggest the influence of more nutrient-rich waters and low sea surface temperatures, probably associated with the Antarctic Circumpolar Water.

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska.

Significance In coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This finding underscores the power of climate in driving erosion rates, potential feedback mechanisms linking climate, erosion, and tectonics, and the complex nature of climate−tectonic coupling in transient responses toward longer-term dynamic equilibration of landscapes with ever-changing environments. Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Siliceous phytoplankton of the western equatorial Atlantic: sediment traps and surface sediments

Abstract We present information on seasonal, interannual, and latitudinal variations of total mass, CaCO 3 , and biogenic opal in the western equatorial Atlantic from time-series sediment traps deployed between 550 and 850 m water depth at the equator ( WA 0°), 4° S ( WA 4° S ) and 7° S ( WA 7° S ) . In addition, species composition and downward fluxes of diatoms and silicoflagellates were determined. Significant seasonal variations in export production were observed between the equator and WA4°S, and were substantially diminished at WA7°S (oligotrophic site). CaCO 3 dominates sedimentation, followed by lithogenic particle and biogenic opal. Diatom and silicoflagellate maxima coincide with the southernmost position of the Intertropical Convergence Zone (January–March), and with the equatorial upwelling season of July–September, mainly reflected at WA4°S, where mean diatom and silicoflagellate fluxes are highest. Interannual variability may be more common than generally assumed for oligotrophic central-ocean gyres. On a yearly basis, total particle and CaCO 3 fluxes covary and show opposite latitudinal trends: high annual fluxes at WA4°S in 1994 correspond with low values at WA7°S, and vice versa for 1993 and 1995. Annual diatom and silicoflagellate fluxes, observed only at the oligotrophic site WA7°S, were one order of magnitude higher in 1993 than those of 1994 and 1995. Lightly silicified, small diatoms of the Nitzschia bicapitata group are the most abundant contributors to the diatom flux. Because these organisms are not preserved in the sedimentary record, significant discrepancies between trapped and preserved assemblages were seen. A substantial enrichment of the moderately robust species, representative of the period of low diatom export, characterizes the assemblages of the surface sediments. Differences in total particle and diatom fluxes between the western and eastern basins of the equatorial Atlantic reflect basin to basin asymmetry with W–E increase in primary productivity estimates, organic carbon fluxes, and silicate content in the subsurface waters, and the W–E shallowing of the thermocline depth. Despite flux differences, siliceous phytoplankton species composition remained constant along the equatorial Atlantic.

Continental erosion and the Cenozoic rise of marine diatoms

Significance Diatoms are silica-precipitating microalgae responsible for roughly one-fifth of global primary production. The mechanisms that led these microorganisms to become one of the most prominent primary producers on Earth remain unclear. We explore the linkage between the erosion of continental silicates and the ecological success of marine diatoms over the last 40 My. We show that the diversification and geographic expansion of diatoms coincide with periods of increased continental weathering fluxes and silicic acid input to the oceans. On geological time scales, the ocean’s biologically driven sequestration of organic carbon (the biological pump) is proportional to the input flux of inorganic nutrients to the oceans. Our results suggest that the strength and efficiency of the biological pump increased over geological time. Marine diatoms are silica-precipitating microalgae that account for over half of organic carbon burial in marine sediments and thus they play a key role in the global carbon cycle. Their evolutionary expansion during the Cenozoic era (66 Ma to present) has been associated with a superior competitive ability for silicic acid relative to other siliceous plankton such as radiolarians, which evolved by reducing the weight of their silica test. Here we use a mathematical model in which diatoms and radiolarians compete for silicic acid to show that the observed reduction in the weight of radiolarian tests is insufficient to explain the rise of diatoms. Using the lithium isotope record of seawater as a proxy of silicate rock weathering and erosion, we calculate changes in the input flux of silicic acid to the oceans. Our results indicate that the long-term massive erosion of continental silicates was critical to the subsequent success of diatoms in marine ecosystems over the last 40 My and suggest an increase in the strength and efficiency of the oceanic biological pump over this period.

Cocconeis Ehrenberg and Psammococconeis García (Bacillariophyta) from the Gulf of San Matías, Patagonia, Argentina

The morphology and taxonomy of species of the genera Cocconeis Ehrenberg and Psammococconeis Garcia from the Gulf of San Matías (Patagonia, Argentina) is documented in the present study. A total of 16 taxa of Cocconeis and one species of Psammococconeis were found in surface net hauls taken in shallow coastal waters of the Gulf. Cocconeis fluminensis var. fluminensis, C. fluminensis var. subimpleta, C. guttata, C. peltoides and C. neothumensis var. marina are new records for the southwestern Atlantic Ocean.

Transfer of Particles into the Deep Atlantic and the Global Ocean: Control of Nutrient Supply and Ballast Production

Particle fluxes from 20 trap sites in the Atlantic/Southern Ocean have been compiled to study the regional variations in comparison with important environmental variables. In turn, these results have been compared to other study sites from the world ocean, mainly regarding the relations-hip between bulk fluxes/various flux ratios to nutrient supply. It is shown that the supply of dissolved silicic acid to the surface waters (the ‘silicate pump’, Dugdale et al. 1995) plays a central role in opal fluxes, BSi:Corg ratios, BSi:carbonate ratios, and thus carbon rain ratios. The mean annual BSi:Corg ratio (mol/mol) normalized to 1000 m was 0.05 in the Atlantic, 0.4 in the Indian, 0.5 in the Pacific, and 0.1–3 in the Southern Ocean and follow s the general path of the conveyor belt (Ragueneau et al. 2000). A shift in the primary producer community from coccolithophorids to diatoms, reflected by an exponential increase of the annual BSi:carbonate flux ratios, occurs above a molar Si:N(250m) nutrient threshold of about 1.7. The surface sediment opal:carbonate ratios (%) versus the Si:N(250m) nutrient values produce a threshold of 2–2.5, however, this value may be biased by opal dissolution during early diagenesis. We also tested the most recent findings about particle ballast which presume that carbonate is most important for the rapid downward transport of organic particles to bathypelagic depths. Our compilation of global flux data confirms such a general relationship. However, at certain sites and in particular years /seasons, other minerals may serve as ballast for organic carbon. Off NW Africa, for instance, lithogenic components were the major particle carriers. There, relationships between carbonate/lithogenic/total ballast fluxes versus daily organic carbon fluxes may even vary from year to year. Off Cape Blanc, the carbonate-Corg-relationship is highly significant during a strong coccolithophorid bloom in 1991, probably resulting in an efficient downward transfer of organic carbon. Interannual variation of fluxes was highest in high production systems combined with high seasonality of fluxes. We obtained ca. 20% variability in oligotrophic regions and up to 100% in the Southern Ocean where seasonality is most pronounced.