Paleoproductivity changes during the Holocene in the inner shelf of Cabo Frio, southeastern Brazilian continental margin: Benthic foraminifera and sedimentological proxies

Sedimentological and benthic foraminifera analyses carried out on a core (length 4.15 in, collected at 22 degrees 56`31 `` S and 41 degrees 58`48 `` W, at a water depth of 43 in) sampled from the inner shelf of Cabo Frio, southeastern Brazilian continental margin, allowed identification of different hydrodynamic and productivity regimes related to sea-level fluctuations and/or climatic changes, during the last 9.4 ka cal BP. Prior to 7.0 ka cal BP, a less intense hydrodynamic and lower productivity regime occurred at lower sea levels and under drier climatic conditions. Between 7.0 and 5.0 ka cal BP, relatively stronger local oceanic circulation and relatively high productivity were observed, in a scenario of rising sea levels and more humid conditions. From 5.0 to 3.0 ka cal BP, bottom currents weakened and input of nutrients increased, with productivity levels similar to the previous phase at lower sea level and in a drier climate. From 3.0 ka cal BP up to the present, stronger hydrodynamic conditions and a higher productivity regime are linked to the establishment of the upwelling process in Cabo Frio. From 2.5 ka cal BP to the present, upwelling enhancement has been recognized, resulting from the combined action of NE winds and the intensification of the meandering pattern of the Brazil Current (BC). (C) 2008 Elsevier Ltd and INQUA. All rights reserved.

Planktonic foraminifera fluxes and stable isotope ratios in the Irminger Sea

Past water column stratification can be assessed through comparison of the d18O of different planktonic foraminiferal species. The underlying assumption is that different species form their shells simultaneously, but at different depths in the water column. We evaluate this assumption using a sediment trap time-series of Neogloboquadrina pachyderma (s) and Globigerina bulloides from the NW North Atlantic. We determined fluxes, d18O and d13C of shells from two size fractions to assess size-related effects on shell chemistry and to better constrain the underlying causes of isotopic differences between foraminifera in deep-sea sediments. Our data indicate that in the subpolar North Atlantic differences in the seasonality of the shell flux, and not in depth habitat or test size, determine the interspecies Delta d18O. N. pachyderma (s) preferentially forms from early spring to late summer, whereas the flux ofG. bulloides peaks later in the season and is sustained until autumn. Likewise, seasonality influences large and small specimens differently, with large shells settling earlier in the season. The similarity of the seasonal d18O patterns between the two species indicates that they calcify in an overlapping depth zone close to the surface. However, their d13C patterns are markedly different (>1 per mil). Both species have a seasonally variable offset from d13CDIC that appears to be governed primarily by temperature, with larger offsets associated with higher temperatures. The variable offset from d13CDIC implies that seasonality of the flux affects the fossil d13C signal, which has implications for reconstruction of the past oceanic carbon cycle.

Oxygen and carbon isotope stratigraphy of ODP Holes 107-653A and 107-654A

Stable isotope analysis of two species (or groups of species) of planktonic foraminifers: Globigerinoides ruber (or G. obliquus and G. obliquus extremus) and Globigerina bulloides (or G. falconensis and G. obesa) from ODP Hole 653A and Site 654 in the Tyrrhenian basin, records the Pliocene-Pleistocene glacial history of the Northern Hemisphere. The overall increase in mean d18O values through the interval 4.6-0.08 Ma is 1.7 per mil for G. bulloides and 1.5 per mil for G. ruber. The time interval 3.1-2.5 Ma corresponds to an important phase of 18O enrichment for planktonic foraminifers. In this interval, glacial d18O values of both species G. bulloides and G. ruber increase by about l per mil, this increase being more progressive for G. ruber than for G. bulloides. The increase of interglacial d18O values is higher for G. bulloides (1.5 per mil) than for the Gruber group (1 per mil). These data suggest a more pronounced seasonal stratification of the water masses during interglacial phases. Large positive d18O fluctuations of increasing magnitude are also recorded at 2.25 and 2.15 Ma by G bulloides and appear to be diachronous with those of Site 606 in the Atlantic Ocean. Other events of increasing d18O values are recorded between 1.55 and 1.3 Ma, at 0.9 Ma, 0.8 Ma, and near 0.34 Ma. In the early Pliocene the d18O variability recorded by the planktonic species G. bulloides was higher in the Mediterranean than in the Atlantic at the same latitude. This suggests that important cyclic variations in the water budget of the Mediterranean occurred since that time. Step increases in the d18O variability are synchronous with those of the open ocean at 0.9 and 0.34 Ma. The higher variability as well as the higher amplitude of the peaks of 18O enrichment may be partly accounted for by increase of dryness over the Mediterranean area. In particular the high amplitude d18O fluctuations recorded between 3.1 and 2.1 Ma are correlated with the onset of a marked seasonal contrast and a summer dryness, revealed by pollen analyses. Strong fluctuations towards d13C values higher than modern ones are recorded by the G. ruber group species before 1.7 Ma and suggest a high production of phytoplankton. When such episodes of high primary production are correlated with episodes of decreasing 13C content of G. bulloides, they are interpreted as the consequence of a higher stratification of the upper water masses resulting itself from a marked seasonality. Such episodes occur between 4.6 and 4.05 Ma, 3.9 and 3.6 Ma, and 3.25 and 2.66 Ma. The interval 2.66-1.65 Ma corresponds to a weakening of the stratification of the upper water layers. This may be related to episodes of cooling and increasing dryness induced by the Northern Hemisphere Glaciations. The Pleistocene may have been a less productive period. The transition from highly productive to less productive surface waters also coincides with a new step increase in dryness and cooling, between 1.5 and 1.3 Ma. The comparison of the 13C records of G ruber and G. bulloides in fact suggests that a high vertical convection became a dominant feature after 2.6 Ma. Increases in the nutrient input and the stratification of the upper water masses may be suspected, however, during short episodes near 0.86 Ma (isotopic stage 25), 0.57-0.59 Ma (isotopic stage 16), 0.49 Ma (isotopic stage 13), 0.4-0.43 Ma (isotopic stage 11), and 0.22 and 0.26 Ma (part of isotopic stage 7 and transition 7/8). In fact, changes in the C02 balance within the different water masses of the Tyrrhenian basin as well as in the local primary production did not follow the general patterns of the open ocean.

Mg/Ca ratios of Neogloboquadrina pachyderma (s) and Globigerina bulloides in Irminger Sea sediment trap time series

iven the importance of high-latitude areas in the ocean-climate system, there is need for a paleothermometer that is reliable at low temperatures. Here we assess the applicability of the Mg/Ca-temperature proxy in colder waters (5-10?°C) by comparing for the first time the seasonal Mg/Ca and d18O cycles of N. pachyderma (s) and G. bulloides using a sediment trap time-series from the northern North Atlantic. While both species show indistinguishable seasonal d18O patterns that clearly track the near surface temperature cycle, their Mg/Ca are very different. G. bulloides Mg/Ca is high (2.0-3.1 mmol/mol), but varies in concert with the seasonal temperature cycle. The Mg/Ca of N. pachyderma (s), on the other hand, is low (1.1-1.5 mmol/mol) and shows only a very weak seasonal cycle. The d18O patterns indicate that both species calcify in the same depth zone. Consequently, depth habitat differences cannot explain the contrasting Mg/Ca patterns. The elevated Mg/Ca in pristine G. bulloides might be due to the presence of high Mg phases that are not preserved in fossil shells. The contrasting absence of a seasonal trend in the Mg/Ca of N. pachyderma (s) confirms other studies where calcification temperatures were less well constrained. The reason for this absence is not fully known, but may include species-specific vital effects. The very different seasonal patterns of both species' Mg/Ca underscore the importance of parameters other than temperature in controlling planktonic foraminiferal Mg/Ca. Our results therefore lend further caution in the interpretation of Mg/Ca-temperature reconstructions from high northern latitudes.

(Table 2) Age determination of Aegean Sea sediments

The modern Aegean Sea is an important source of deep water for the eastern Mediterranean. Its contribution to deep water ventilation is known to fluctuate in response to climatic variation on a decadal timescale. This study uses marine micropalaeontological and stable isotope data to investigate longer-term variability during the late glacial and Holocene, in particular that associated with the deposition of the early Holocene dysoxic/anoxic sapropel S1. Concentrating on the onset of sapropel-forming conditions, we identify the start of 'seasonal' stratification and highlight a lag in d18O response of the planktonic foraminifer N. pachyderma to termination T1b as identified in the d18O record of G. ruber. By use of a simple model we determine that this offset cannot be a function of bioturbation effects. The lag is of the order of 1 kyr and suggests that isolation of intermediate/deep water preceded the start of sapropel formation by up to 1.5 kyr. Using this discovery, we propose an explanation for the major unresolved problem in sapropel studies, namely, the source of nutrient supply required for export productivity to reach levels needed for sustained sapropel deposition. We suggest that nutrients had been accumulating in a stagnant basin for 1-1.5 kyr and that these accumulated resources were utilized during the deposition of S1. In addition, we provide a first quantitative estimate of the diffusive (1/e) mixing timescale for the eastern Mediterranean in its "stratified" sapropel mode, which is of the order of 450 years.

Shell flux and oxygen isotope data of North Atlantic foraminifera

We present an almost 3 year long time series of shell fluxes and oxygen isotopes of left-coiling Neogloboquadrina pachyderma and Turborotalita quinqueloba from sediment traps moored in the deep central Irminger Sea. We determined their response to the seasonal change from a deeply mixed water column with occasional deep convection in winter to a thermally stratified water column with a surface mixed layer (SML) of around 50 m in summer. Both species display very low fluxes during winter with a remnant summer population holding out until replaced by a vital population that seeds the subsequent blooms. This annual population overturning is marked by a 0.7 per mill increase in d18O in both species. The shell flux of N. pachyderma peaks during the spring bloom and in late summer, when stratification is close to its minimum and maximum, respectively. Both export periods contribute about equally and account for >95% of the total annual flux. Shell fluxes of T. quinqueloba show only a single broad pulse in summer, thus following the seasonal stratification cycle. The d18O of N. pachyderma reflects temperatures just below the base of the seasonal SML without offset from isotopic equilibrium. The d18O pattern of T. quinqueloba shows a nearly identical amplitude and correlates highly with the d18O of N. pachyderma. Therefore T. quinqueloba also reflects temperature near the base of the SML but with a positive offset from isotopic equilibrium. These offsets contrast with observations elsewhere and suggest a variable offset from equilibrium calcification for both species. In the Irminger Sea the species consistently show a contrast in their flux timings. Their flux-weighted delta d18O will thus dominantly be determined by seasonal temperature differences at the base of the SML rather than by differences in their depth habitat. Consequently, their sedimentary delta d18O may be used to infer the seasonal contrast in temperature at the base of the SML.

Warmer, deeper and greener mixed layers in the north Atlantic subpolar gyre over the last 50 years.

Shifts in global climate resonate in plankton dynamics, biogeochemical cycles, and marine food webs. We studied these linkages in the North Atlantic subpolar gyre (NASG), which hosts extensive phytoplankton blooms. We show that phytoplankton abundance increased since the 1960s in parallel to a deepening of the mixed layer and a strengthening of winds and heat losses from the ocean, as driven by the low frequency of the North Atlantic Oscillation (NAO). In parallel to these bottom-up processes, the top-down control of phytoplankton by copepods decreased over the same time period in the western NASG, following sea surface temperature changes typical of the Atlantic Multi-decadal Oscillation (AMO). While previous studies have hypothesized that climate-driven warming would facilitate seasonal stratification of surface waters and long-term phytoplankton increase in subpolar regions, here we show that deeper mixed layers in the NASG can be warmer and host a higher phytoplankton biomass. These results emphasize that different modes of climate variability regulate bottom-up (NAO control) and top-down (AMO control) forcing on phytoplankton at decadal timescales. As a consequence, different relationships between phytoplankton, zooplankton, and their physical environment appear subject to the disparate temporal scale of the observations (seasonal, interannual, or decadal). The prediction of phytoplankton response to climate change should be built upon what is learnt from observations at the longest timescales.

Warmer, deeper and greener mixed layers in the north Atlantic subpolar gyre over the last 50 years.

Shifts in global climate resonate in plankton dynamics, biogeochemical cycles, and marine food webs. We studied these linkages in the North Atlantic subpolar gyre (NASG), which hosts extensive phytoplankton blooms. We show that phytoplankton abundance increased since the 1960s in parallel to a deepening of the mixed layer and a strengthening of winds and heat losses from the ocean, as driven by the low frequency of the North Atlantic Oscillation (NAO). In parallel to these bottom-up processes, the top-down control of phytoplankton by copepods decreased over the same time period in the western NASG, following sea surface temperature changes typical of the Atlantic Multi-decadal Oscillation (AMO). While previous studies have hypothesized that climate-driven warming would facilitate seasonal stratification of surface waters and long-term phytoplankton increase in subpolar regions, here we show that deeper mixed layers in the NASG can be warmer and host a higher phytoplankton biomass. These results emphasize that different modes of climate variability regulate bottom-up (NAO control) and top-down (AMO control) forcing on phytoplankton at decadal timescales. As a consequence, different relationships between phytoplankton, zooplankton, and their physical environment appear subject to the disparate temporal scale of the observations (seasonal, interannual, or decadal). The prediction of phytoplankton response to climate change should be built upon what is learnt from observations at the longest timescales.

Unstructured grid modelling of offshore wind farm impacts on seasonally stratified shelf seas

Shelf seas comprise approximately 7% of the world’s oceans and host enormous economic activity. Development of energy installations (e.g. Offshore Wind Farms (OWFs), tidal turbines) in response to increased demand for renewable energy requires a careful analysis of potential impacts. Recent remote sensing observations have identified kilometrescale impacts from OWFs. Existing modelling evaluating monopile impacts has fallen into two camps: small-scale models with individually resolved turbines looking at local effects; and large-scale analyses but with sub-grid scale turbine parameterisations. This work straddles both scales through a 3D unstructured grid model (FVCOM): wind turbine monopiles in the eastern Irish Sea are explicitly described in the grid whilst the overall grid domain covers the south-western UK shelf. Localised regions of decreased velocity extend up to 250 times the monopile diameter away from the monopile. Shelf-wide, the amplitude of the M2 tidal constituent increases by up to 7%. The turbines enhance localised vertical mixing which decreases seasonal stratification. The spatial extent of this extends well beyond the turbines into the surrounding seas. With significant expansion of OWFs on continental shelves, this work highlights the importance of how OWFs may impact coastal (e.g. increased flooding risk) and offshore (e.g. stratification and nutrient cycling) areas.

Unstructured grid modelling of offshore wind farm impacts on seasonally stratified shelf seas

Shelf seas comprise approximately 7% of the world’s oceans and host enormous economic activity. Development of energy installations (e.g. Offshore Wind Farms (OWFs), tidal turbines) in response to increased demand for renewable energy requires a careful analysis of potential impacts. Recent remote sensing observations have identified kilometrescale impacts from OWFs. Existing modelling evaluating monopile impacts has fallen into two camps: small-scale models with individually resolved turbines looking at local effects; and large-scale analyses but with sub-grid scale turbine parameterisations. This work straddles both scales through a 3D unstructured grid model (FVCOM): wind turbine monopiles in the eastern Irish Sea are explicitly described in the grid whilst the overall grid domain covers the south-western UK shelf. Localised regions of decreased velocity extend up to 250 times the monopile diameter away from the monopile. Shelf-wide, the amplitude of the M2 tidal constituent increases by up to 7%. The turbines enhance localised vertical mixing which decreases seasonal stratification. The spatial extent of this extends well beyond the turbines into the surrounding seas. With significant expansion of OWFs on continental shelves, this work highlights the importance of how OWFs may impact coastal (e.g. increased flooding risk) and offshore (e.g. stratification and nutrient cycling) areas.

The seasonal evolution of high vertical–mode internal waves in a deep reservoir

The causal mechanism and seasonal evolution of the internal wave field in a deep, warm, monomictic reservoirare examined through the analysis of field observations and numerical techniques. The study period extends fromthe onset of thermal stratification in the spring until midsummer in 2005. During this time, wind forcing wasperiodic, with a period of 24 h (typical of land–sea breezes), and the thermal structure in the lake wascharacterized by the presence of a shallow surface layer overlying a thick metalimnion, typical of small to mediumsized reservoirs with deep outtakes. Basin-scale internal seiches of high vertical mode (ranging from mode V3 toV5) were observed in the metalimnion. The structure of the dominant modes of oscillation changed asstratification evolved on seasonal timescales, but in all cases, their periods were close to that of the local windforcing (i.e., 24 h), suggesting a resonant response. Nonresonant oscillatory modes of type V1 and V2 becamedominant after large frontal events, which disrupted the diurnal periodicity of the wind forcing

SEASONAL MICROHABITAT SELECTION BY TERRESTRIAL RODENTS IN AN OLD-GROWTH ATLANTIC FOREST

We tested the hypothesis that microhabitat variables, abundance of terrestrial rodents, and microhabitat selection patterns of terrestrial rodents vary between the cool-dry and warm-wet season in the Atlantic forest of Brazil. We selected variables associated with ecological factors potentially important to terrestrial rodents (physical structure of litter and woody debris, and arthropod availability) and established 25 small, independent sampling units covering 36 ha of a homogenous, mature Atlantic forest patch. Litter humidity and height, amount of small woody debris, arthropod availability, and terrestrial rodent abundance increased, whereas the quantity of large woody debris decreased in the warm-wet season. Greater spatial segregation among terrestrial rodents also was observed in this season, especially between morphologically similar species. The distribution of 3 of the 4 most common terrestrial rodents was influenced by microhabitat variables in at least I of the seasons, and these species also differed in their pattern of microhabitat selection between seasons. In general, the amount of small woody debris and litter humidity were more important for the microscale distribution of terrestrial rodents in the cool-dry season, whereas in the mild warm-wet season species distributions were associated with food availability or were not clearly influenced by the measured variables. The patterns of microhabitat selection by 3 common terrestrial rodents, which were associated with features that characterize old-growth forest, may be responsible for their vulnerability to forest fragmentation.

The effects of river discharge and seasonal winds on the shelf off southeastern South America

The Rio de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28,S) during austral winter and 32 degrees S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds. (C) 2008 Elsevier Ltd. All rights reserved.

Seasonal dynamics of cyanobacteria in a eutrophic reservoir (Arcoverde) in a semi-arid region of Brazil

Environmental conditions favor the predominance of dense populations of cyanobacteria in reservoirs in northeastern Brazil. The aim of this study was to understand cyanobacterial population dynamics in the rainy and dry seasons at two depths in the Arcoverde reservoir. Microalgae and cyanobacteria samples were collected during 24 hours with intervals of 4 hours (nycthemeral) at sub-surface and 10 m using a van Dorn bottle and a determined biomass. Physical and chemical variables were obtained and the data were analyzed using the principal component analysis (PCA). No nycthemeral variations in the taxonomic composition or distribution of the populations of cyanobacteria were found between the different times of day in either the rainy or dry season. In both seasons, the greatest biomass of the phytoplankton community was made up of cyanobacteria at two depths and all times of the day. Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju was dominant at all times of the day on both the surface and at the bottom. In the rainy season, the differences in cyanobacterial biomass between the surface and bottom were less significant than in the dry season. The differences in cyanobacterial biomass between surface and bottom were less pronounced than those found in the dry season. We concluded that a) physical variables better explain the alterations of species in the phytoplankton community in an environment dominated by cyanobacteria throughout the year; b) seasonal climatic factors associated to periods of stratification and de-stratification are important for alterations in the community and variations in biomass and, c) the turbidity caused by rainfall favored the emergence and establishment of other cyanobacteria, especially Planktothrix agardhii (Gomont) Anagnostidis & Komarek.

A Model Study of the Seasonal Circulation in the Gulf of Maine

The Princeton Ocean Model is used to study the circulation in the Gulf of Maine and its seasonal transition in response to wind, surface heat flux, river discharge, and the M-2 tide. The model has an orthogonal-curvature linear grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore and 19 levels in the vertical. It is initialized and forced at the open boundary with model results from the East Coast Forecast System. The first experiment is forced by monthly climatological wind and heat flux from the Comprehensive Ocean Atmosphere Data Set; discharges from the Saint John, Penobscot, Kennebec, and Merrimack Rivers are added in the second experiment; the semidiurnal lunar tide (M-2) is included as part of the open boundary forcing in the third experiment. It is found that the surface heat flux plays an important role in regulating the annual cycle of the circulation in the Gulf of Maine. The spinup of the cyclonic circulation between April and June is likely caused by the differential heating between the interior gulf and the exterior shelf/slope region. From June to December the cyclonic circulation continues to strengthen, but gradually shrinks in size. When winter cooling erodes the stratification, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the energy is consumed by bottom friction. While inclusion of river discharge changes details of the circulation pattern, the annual evolution of the circulation is largely unaffected. On the other hand, inclusion of the tide results in not only the anticyclonic circulation on Georges Bank but also modifications to the seasonal circulation.