Diatoms as a paleoproductivity proxy in the NW Iberian coastal upwelling system (NE Atlantic)

The objective of the current work is to improve our understanding of how water column diatom's abundance and assemblage composition is seasonally transferred from the photic zone to seafloor sediments. To address this, we used a dataset derived from water column, sediment trap and surface sediment samples recovered in the NW Iberian coastal upwelling system. Diatom fluxes (2.2 (+/- 5.6) 10(6) valves m(-2) d(-1)) represented the majority of the siliceous microorganisms sinking out from the photic zone during all studied years and showed seasonal variability. Contrasting results between water column and sediment trap diatom abundances were found during downwelling periods, as shown by the unexpectedly high diatom export signals when diatom- derived primary production achieved their minimum levels. They were principally related to surface sediment remobilization and intense Minho and Douro river discharge that constitute an additional source of particulate matter to the inner continental shelf. In fact, contributions of allochthonous particles to the sinking material were confirmed by the significant increase of both benthic and freshwater diatoms in the sediment trap assemblage. In contrast, we found that most of the living diatom species blooming during highly productive upwelling periods were dissolved during sinking, and only those resistant to dissolution and the Chaetoceros and Leptocylindrus spp. resting spores were susceptible to being exported and buried. Fur-thermore, Chaetoceros spp. dominate during spring-early summer, when persistent northerly winds lead to the upwelling of nutrient-rich waters on the shelf, while Leptocylindrus spp. appear associated with late-summer upwelling relaxation, characterized by water column stratification and nutrient depletion. These findings evidence that the contributions of these diatom genera to the sediment's total marine diatom assemblage should allow for the reconstruction of different past upwelling regimes.CAIBEX [CTM2007-66408-C02-01/MAR]; REIMAGE [CTM2011-30155-C03-03]; Spanish Government, EXCAPA project [10MDS402013PR]; Xunta de Galicia; EU FEDER [INTERREG 2009/2011-0313/RAIA/E]; RAIA. co [INTERREG2011/2013-052/RAIA.co/1E]; CALIBERIA project [PTDC/MAR/102045/2008]; Fundacao para a Ciencia e a Tecnologia (FCT-Portugal) [COMPETE/FEDER-FCOMP01-0124-FEDER-010599]; Xunta de Galicia (Spain) [SFRH/BPD/111433/2015]; FCT (Portugal) [SFRH/BPD/111433/2015]; [SFRH/BD/88439/2012

Deglacial diatom productivity and surface ocean properties over the Bermuda Rise, northeast Sargasso Sea

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA4101, doi:10.1029/2008PA001729.Diatom assemblages document surface hydrographic changes over the Bermuda Rise. Between 19.2 and 14.5 ka, subtropical diatom species and Chaetoceros resting spores dominate the flora, as in North Atlantic productive regions today. From 16.9 to 14.6 ka, brackish and fresh water diatoms are common and their contribution is generally coupled with total diatom abundance. This same interval also contains rare grains of ice-rafted debris. Coupling between those proxies suggests that successive discharge of icebergs might have stimulated productivity during Heinrich event 1 (H1). Iceberg migration to the subtropics likely created an isolated environment involving turbulent mixing, upwelled water, and nutrient-rich meltwater, supporting diatom productivity in an otherwise oligotrophic setting. In addition, the occurrence of mode water eddies likely brought silica-rich waters of Southern Ocean origin to the euphotic zone. The persistence of lower-salinity surface water beyond the last ice rafting suggests continued injection of fresh water by cold-core rings and advection around the subtropical gyre. These results indicate that opal productivity may have biased estimates of meridional overturning based on 231Pa/230Th ratios in Bermuda Rise sediments during H1.Support for this research was provided by the Fundação para a Ciência e Tecnologia through the grant BPD/27214/2006 to I. M. Gil

Particle fluxes in the NW Iberian coastal upwelling system: Hydrodynamical and biological control

To better understand sources and transport of particulate material in the NW Iberian coastal upwelling system, a mooring line dotted with an automated PPS 4/3 sediment trap was deployed off Cape Silleiro at the base of the photic zone. The samples were collected from November 2008 through June 2012 over sampling periods of 4-12 days.Our study represents the first automated sediment trap database for the NW Iberian margin. The magnitude and composition of the settling material showed strong seasonal variability with the highest fluxes during the poleward and winter mixing periods (averages of 12.9 +/- 9.6 g m(-2) d(-1) and 5.6 +/- 5.6 g m(-2) d(-1) respectively), and comparatively lower fluxes (3.6 +/- 4.1 gm(-2) d(-1)) for the upwelling season. Intensive deposition events registered during poleward and winter mixing periods were dominated by the lithogenic fraction (80 +/- 3%). They were associated to high energy wave-driven re suspension processes, due to the occurrence of south-westerly storms, and intense riverine inputs of terrestrial material from Minho and Douro rivers.On the other hand, during the spring- summer upwelling season, the share of biogenic compounds (organic matter, calcium carbonate (CaCO3), biogenic silica (bSiO2)) to downward fluxes was higher, reflecting an increase in pelagic sedimentation due to the seasonal intensification of primary production and negligible river inputs and wave-driven resuspended material. Otherwise, the large variations of biogenic settling particles were mainly modulated by upwelling intensity, which by means of upwelling filaments ultimately controlled the offshore transport of the organic carbon fixed by primary producers towards the adjacent ocean. Based on the average downward flux of organic carbon (212 mg C m(-2) d(-1)) and considering an average primary production of 1013 mg C m(-2) d(-1) from literature, we estimated that about 21% of the fixed carbon is vertically exported during the upwelling season. (C) 2016 Elsevier Ltd. All rights reserved

Variability of the North Atlantic Current during the last 2000 years based on shelf bottom water and sea surface temperatures along an open ocean/shallow marine transect in western Europe

Marine localities on the west European shelf have been studied to reconstruct the nearshore palaeoceanography of the last two millennia. The sites form a transect from the Iberian margin northeastward via Scotland to western Norway and Iceland. Proxies used for palaeoclimatic reconstructions include stable isotopes, benthic and planktonic foraminfera, diatoms, dinoflagellates, as well as geochemical and sedimentological parameters. Major changes as well as long-term trends in oceanographic conditions are observed in the records, including a general cooling trend through much of the last millennium. There is a clear linkage between the atmospheric processes and the oceanic circulation, and the ocean temperature variability in the records can be correlated with the so-called 'Mediaeval Warm Period' and 'Little Ice Age'. These oscillations are, however, by no means unique within the last two millennia. As an example, sea surface temperatures to the north of Iceland and on the Iberian margin were higher in the Roman Warm Period than at any time during the 'Mediaeval Warm Period'. However, the palaeoceanographic record generally supports a distinct cooling at the transition between the 'Mediaeval Warm Period' and the 'Little Ice Age'. While a number of records indicate a warming of coastal and shelf waters during the last 200 years, the twentieth century does not appear to be unusual when the proxy records spanning the last two millennia are examined.</p

Extant coccolithophores in the water column of the NW Iberian margin

A systematic investigation of the spatial and temporal variability in coccolithophore abundance and distribution through the water column of the NW Iberian coastal upwelling system was performed. From July 2011 to June 2012, monthly sampling at various water depths was conducted at two parallel stations located at 42∘ N. Total coccosphere abundance was higher at the outer-shelf station, where warmer, nutrient-depleted waters favoured coccolithophore rather than phytoplanktonic diatom blooms, which are known to dominate the inner-shelf location. In seasonal terms, higher coccosphere and coccolith abundances were registered at both stations during upwelling seasons, coinciding with high irradiance levels. This was typically in conjunction with stratified, nutrient-poor conditions (i.e. relaxing upwelling conditions). However, it also occurred during some upwelling events of colder, nutrient-rich subsurface waters onto the continental shelf. Minimum abundances were generally found during downwelling periods, with unexpectedly high coccolith abundance registered in subsurface waters at the inner-shelf station. This finding can only be explained if strong storms during these downwelling periods favoured resuspension processes, thus remobilizing deposited coccoliths from surface sediments, and hence hampering the identification of autochthonous coccolithophore community structure. At both locations, the major coccolithophore assemblages were dominated by Emiliania huxleyi, small Gephyrocapsa group, Gephyrocapsa oceanica, Florisphaera profunda, Syracosphaera spp., Coronosphaera mediterranea, and Calcidiscus leptoporus. Ecological preferences of the different taxa were assessed by exploring the relationships between environmental conditions and temporal and vertical variability in coccosphere abundance. These findings provide relevant information for the use of fossil coccolith assemblages in marine sediment records, in order to infer past environmental conditions, of particular importance for Paleoceanography. Both E. huxleyi and the small Gephyrocapsa group are proposed as proxies for the upwelling regime with a distinct affinity for different stages of the upwelling event: E. huxleyi was associated with warmer, nutrient-poor and more stable water column (i.e. upwelling relaxation stage) while the small Gephyrocapsa group was linked to colder waters and higher nutrient availability (i.e. early stages of the upwelling event), similarly to G. oceanica. Conversely, F. profunda is suggested as a proxy for the downwelling regime and low-productivity conditions. The assemblage composed by Syracosphaera pulchra, Coronosphaera mediterranea, and Rhabdosphaera clavigera may be a useful indicator of the presence of subtropical waters conveyed northward by the Iberian Poleward Current. Finally, C. leptoporus is proposed as an indicator of warmer, saltier, and oligotrophic waters during the downwelling/winter regime

delta O-18 and Mg/Ca thermometry in planktonic foraminifera: a multiproxy approach toward tracing oastal upwelling dynamics

Planktonic foraminifera delta O-18 and Mg/Ca ratios are widely considered as a powerful proxy to reconstruct past seawater-column temperature. Due to the complex interpretation of planktonic foraminifera delta O-18 data in regard to past seawater temperatures, temperature determination based on the foraminifera shell Mg/Ca ratio is believed to be more accurate. Scarce Mg/Ca calibration data exists for coastal upwelling regions, resulting in incoherent results of past seawater reconstructions. The current study along the NW Iberia coastal upwelling system intends to define the best Mg/Ca temperature equation for the most representative species of this region (Neogloboquadrina incompta, Globigerina bulloides, and Globorotalia inflata). Seawater temperature from delta O-18 and Mg/Ca of these three planktonic foraminifera species was compared with the surface sediments alkenone derived SST and with the in situ temperatures measured at the depths where these foraminifera species currently live and calcify. The equations that better reflect each species calcification depth were selected as our regional equations for delta O-18 and Mg/Ca temperature reconstructions. The delta O-18-estimated temperatures for surface sediment specimens were comparable with in situ seawater-column temperature measurements, whereas the Mg/Ca derived temperatures seem to underestimate in situ values, in special for G. bulloides from samples affected by stronger coastal upwelling. The G. bulloides delta O-18 and Mg/Ca estimated temperatures from samples located offshore, further from coastal upwelling influence, are comparable to surface sediment alkenone derived temperatures. Our study shows that in upwelling areas, regional calibration of planktonic foraminifera Mg/Ca temperature equations is necessary for reliable interpretations of high-resolution past temperature variability in these important environments.CALIBERIA project [BI/PTDC/MAR/102045/2008/2010-016, PTDC/MAR/102045/2008/BI/2011-002]Fundacao para a Ciencia e a Tecnologia (FCT)-PortugalPortuguese Foundation for Science and Technology [PTDC/MAR/102045/2008]Xunta de GaliciaXunta de GaliciaEuropean Commission [COMPETE/FEDER-FCOMP-01-0124FEDER-010599, 10MDS402013PR]CUPEX project [PDCT/MAR/56963/2004]FCTPortuguese Foundation for Science and TechnologyEuropean Commission [SFRH/BPD/111433/2015]Xunta de Galicia (Spain) - RAIA.coXunta de Galicia [SFRH/BPD/108600/2015]Isabel Barreto Program (Xunta de Galicia, Spain) [0520_RAIA]CSIC data server [IPMA-2018-05-BTI, UID/Multi/04326/2019]info:eu-repo/semantics/publishedVersio

δ18O and Mg/Ca thermometry in planktonic foraminifera: a multi-proxy approach towards tracing coastal upwelling dynamics

Planktonic foraminifera δ18O and Mg/Ca ratios are widely considered as a powerful proxy to reconstruct past seawater-column temperature in coastal upwelling regions. Due to the complexity in the interpretation of planktonic foraminifera δ18O data with respect to past seawater temperatures, temperature determination based on the foraminifera shell Mg/Ca ratio is believed to be more accurate. However, scarce Mg/Ca calibration data exists for coastal upwelling regions, resulting in incoherent results of past seawater reconstructions. The current study along the NW Iberia coastal upwelling system intends to define the best Mg/Ca temperature equation for the most representative species of this region (Neogloboquadrina incompta, Globigerina bulloides and Globorotalia inflata). Seawater temperature from δ18O and Mg/Ca of these three planktonic foraminifera species were compared with the surface sediments alkenone derived SST, and with the in-situ temperatures measured at the depths where these foraminifera species currently live and calcify. The equations that better reflect each species calcification depth were selected as our regional equations for δ18O and Mg/Ca temperature reconstructions. The δ18O estimated temperatures for surface sediment specimens were comparable with in-situ seawater-column temperature measurements, whereas the Mg/Ca derived temperatures seem to underestimate in-situ values, in special for G. bulloides from samples affected by stronger coastal upwelling. The G. bulloides δ18O and Mg/Ca estimated temperatures from samples located offshore, further from coastal upwelling influence, are comparable to surface sediment alkenone derived temperatures. Our study shows that in upwelling areas, regional calibration of planktonic foraminifera Mg/Ca temperature equations is necessary for reliable interpretations of high-resolution past temperature variability in these important environments