ALBERT

Description: The Mediterranean region is a climate hot spot, sensitive not only to global warming but also to water availability. In this work we document major temperature and precipitation changes in the Iberian Peninsula and margin during the last 2000 years and propose an interplay of the North Atlantic internal variability with the three atmospheric circulation modes (ACMs), (North Atlantic Oscillation (NAO), east atlantic (EA) and Scandinavia (SCAND)) to explain the detected climate variability. We present reconstructions of sea surface temperature (SST derived from alkenones) and on-land precipitation (estimated from higher plant n-alkanes and pollen data) in sedimentary sequences recovered along the Iberian Margin between the south of Portugal (Algarve) and the northwest of Spain (Galiza) (36 to 42° N). A clear long-term cooling trend, from 0 CE to the beginning of the 20th century, emerges in all SST records and is considered to be a reflection of the decrease in the Northern Hemisphere summer insolation that began after the Holocene optimum. Multi-decadal/centennial SST variability follows other records from Spain, Europe and the Northern Hemisphere. Warm SSTs throughout the first 1300 years encompass the Roman period (RP), the Dark Ages (DA) and the Medieval Climate Anomaly (MCA). A cooling initiated at 1300 CE leads to 4 centuries of colder SSTs contemporary with the Little Ice Age (LIA), while a climate warming at 1800 CE marks the beginning of the modern/Industrial Era. Novel results include two distinct phases in the MCA: an early period (900–1100 years) characterized by intense precipitation/flooding and warm winters but a cooler spring–fall season attributed to the interplay of internal oceanic variability with a positive phase in the three modes of atmospheric circulation (NAO, EA and SCAND). The late MCA is marked by cooler and relatively drier winters and a warmer spring–fall season consistent with a shift to a negative mode of the SCAND. The Industrial Era reveals a clear difference between the NW Iberia and the Algarve records. While off NW Iberia variability is low, the Algarve shows large-amplitude decadal variability with an inverse relationship between SST and river input. Such conditions suggest a shift in the EA mode, from negative between 1900 and 1970 CE to positive after 1970, while NAO and SCAND remain in a positive phase. The particularly noticeable rise in SST at the Algarve site by the mid-20th century (±1970), provides evidence for a regional response to the ongoing climate warming. The reported findings have implications for decadal-scale predictions of future climate change in the Iberian Peninsula.

Description: 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) 106 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. Furthermore, 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.

Description: For the first time a systematic investigation of the coccolithophore ecology based on the spatial and temporal variability in their abundance and distribution through the water column was performed for the NW Iberian coastal upwelling system. From July 2011 to June 2012 monthly sampling at different water depths was conducted at two stations located along the 42º N parallel. Total coccolithophore abundances were higher at the offshore station where warmer and nutrient–depleted waters favoured coccolithophore rather than diatom blooming, the other phytoplankton group that is known to be dominant at the onshore location. In seasonal terms, coccolithophore blooms were registered at both stations during upwelling seasons, coinciding with high irradiance levels and generally in conjunction with stratified and nutrient–poor conditions in the water column, but also when colder and nutrient–rich subsurface waters upwelled onto the continental shelf. On the contrary, despite minimum abundances were generally found during downwelling periods, unexpectedly high coccolithophore abundances were registered in subsurface waters at the onshore station. This finding was only explained if strong storms during downwelling periods favoured resuspension processes, thus remobilizing deposited coccoliths from surface sediments, and hence hampering the identification of the autochthonous coccolithophore community structure. Major composition of coccolithophore assemblages at both locations was dominated by Emiliania huxleyi, small Gephyrocapsa group, Gephyrocapsa oceanica, Florisphaera profunda, and Syracosphaera spp. Ecological preferences of the different taxa were assessed by exploring the relationships between environmental conditions and temporal and vertical variability in coccolithophore abundance. Our findings will provide relevant information in regards to the use of fossil coccolithophore assemblages in marine sediment records to infer environmental conditions in the past, which is of particular relevance in Paleoceanography. E. huxleyi and the small Gephyrocapsa group are proposed as proxies for the upwelling regime and high primary production. Conversely, F. profunda is suggested as a proxy for the downwelling regime and low productivity conditions. Finally, the assemblage composed by Syracosphaera pulchra, Coronosphaera mediterranea, and Rhabdosphaera clavigera may be used as a useful indicator of the presence of warm and nutrient–poor southerly waters conveyed by the Iberian Poleward Current.

Description: The Iberian Peninsula, at North Atlantic mid-latitude and the western extreme of the European continent, is a key point for climate reconstructions. This work provides multi-proxy records measured in 8 inner-shelf sediment cores from 5 sites located between South Portugal (Algarve) and Northwest Spain (Galiza) (36 to 41º N) and target a regional reconstruction of climate variability during the Historic period (last 2 ky). The SST records reveal a long-term scale cooling (±1 ºC/2 ky) that ends at the beginning of the 20th century at all latitudes. This cooling is a follow up of the cooling process that started in the early Holocene driven by a decrease in summer insolation in the Northern Hemisphere. Within this long term SST variability multi-decadal/centennial scale variability is detected along Iberia. The different latitudinal SST reconstructions jointly with a determined regional SST stack were compared to on-land precipitation from higher plant n-alkanes and pollen data, to assess the relationship between hydroclimate (drought and/or precipitation) and SST. Regional variability is overall in consonance with NE Spain, and other European and north Hemisphere reconstructions. Warm conditions prevailed throughout 1300 yr, encompassing the Roman Period (RP), the Dark Ages (DA) and the Medieval Warm Period (MWP). The initial cooling at 1300 CE leads to 4 centuries of ±1 ºC colder mean SSTs contemporaneous with the Little Ice Age (LIA). The transition towards the Industrial Era starts by 1800 CE with a rise to pre-LIA SSTs. Climate specificities have been detected in western Iberian margin records and reveal the existence of two distinct phases within the MWP and a two-step SST increase towards the Industrial Era. The intense precipitation/flooding and warm winters but cooler intermediate seasons observed for the early MWP imply the interplay of internal oceanic variability with the three known atmospheric circulation modes, AMO, EA and SCAND in a positive phase. The late MWP, typified by drier and cooler winters and warmer intermediate seasons calls for a change in sign of the SCAND. A stronger mark of oceanic influences on western Iberian Peninsula (IP) starts with the transition to the Industrial Era.

Description: The Iberian Peninsula, at North Atlantic mid-latitude and the western extreme of the European continent, is a relevant area for climate reconstructions. This work provides multi-proxy records measured in 7 inner-shelf sediment sequences from 5 sites located between South Portugal (Algarve) and Northwest Spain (Galiza) (36 to 42º N) and targets a regional reconstruction of climate variability during the last 2000 yr. Alkenone derived Sea Surface Temperature (SST) reconstructions were compared to on-land precipitation given by higher plant n-alkanes and pollen data, to assess the relationship between hydroclimate (drought and/or precipitation) and oceanic SST. The SST records reveal a long-term scale cooling (±1 ºC/2000 yr) that ends at the beginning of the 20th century at all sites. This cooling is a follow up of the cooling process started after the Holocene optimum and driven by a decrease in summer insolation in the Northern Hemisphere. A multi-decadal/ centennial variability is detected within this long-term cooling in convergence with other records from Spain, Europe and the Northern Hemisphere. Warm SST conditions prevailed throughout the first 1300 yr, encompassing the Roman Period (RP), the Dark Ages (DA) and the Medieval Warm Period (MWP). A cooling initiated at 1300 CE, led to 4 centuries of ±1 ºC colder mean SSTs contemporary with the Little Ice Age (LIA). The transition towards the Industrial Era starts by 1800 CE with a SST rise to pre-LIA levels. Particular climate conditions have been detected in Western Iberian margin records and reveal two distinct phases within the MWP and a two-step SST increase towards the Industrial Era. The intense precipitation/flooding and warm winters but cooler intermediate seasons (spring and fall) observed for the early MWP imply the interplay of internal oceanic variability with the three atmospheric circulation modes, North Atlantic Oscillation (NAO), East Atlantic (EA) and Sandinavia (SCAND) in a positive phase. The late MWP, marked by drier and cooler winters and warmer intermediate seasons calls for a change in sign of the SCAND. A stronger mark of oceanic influences on Western Iberian Peninsula starts with the transition to the Industrial Era.

Description: The objective of the current work is to better understand how diatoms species determine primary production signal in exported and buried particles. We evaluated how the diatom’s abundance and assemblage composition is transferred from the photic zone into the seafloor sediments. A combined analysis of water column, sediment trap and surface sediment samples recovered in the NW Iberian coastal upwelling system was used. Diatom fluxes (2.2 ± 5.6 106 # valves m−2 d−1) represented the majority of the siliceous microorganisms sinking out from the photic zone and showed strong seasonal variability. During downwelling seasons, diatoms export signal was strongly affected by resuspension of bottom sediments and intense Minho and Douro riverine inputs, with benthic and freshwater diatoms (17–24 %) becoming relevant in the sediment trap assemblage. Nevertheless, during upwelling productive seasons, the diatoms exported out from surface layer reflected water column diatom assemblage. They were principally represented by Chaetoceros spp. (mean 46 ± 25 %) and Leptocylindrus spp. (mean 20 ± 22 %) resting spores, demonstrating that both groups are a good sedimentary imprint during highly productive periods. Moreover, our data showed that the sink of Chaetoceros spp. resting spores dominated under persistent upwelling winds, high irradiance levels and cold and nutrient-rich waters. Otherwise, Leptocylindrus spp. spore fluxes were favoured when northerly winds relax, and surface waters warming promotes water column stratification. Further, this finding will provide a proxy of persistent vs. intermittent upwelling conditions, which is of particular relevance in palaeoceanography.

Description: To assess the regional multi-decadal to multi-centennial climate variability along the southern Iberian Peninsula during the mid- to late-Holocene record of paleo-environmental indicators from marine sediments were established for two sites in the Alboran Sea (ODP-161-976A) and the Gulf of Cádiz (GeoB5901-2). High-resolution records of organic geochemical properties and planktic foraminiferal assemblages are used to decipher precipitation and vegetation changes as well as hydrological conditions with respect to sea surface temperature (SST) and marine primary productivity (MPP). As a proxy for precipitation change, records of plant-derived n-alkane composition suggest a series of five distinct dry episodes in southern Iberia at 5.4±0.3 ka cal BP, from 5.1 to 4.9±0.1 ka cal BP, from 4.8 to 4.7±0.1 ka cal BP, from 4.4 to 4.3±0.1 ka cal BP, and at 3.7±0.1 ka cal BP. During each dry episode the vegetation suffered from reduced water availability. Interestingly, the dry phase from 4.4 to 4.3±0.1 ka cal BP is followed by a rapid shift towards wetter conditions revealing a more complex pattern in terms of its timing and duration than was described for the 4.2 ka event in other regions. The series of dry episodes as well as closely connected hydrological variability in the Alboran Sea were probably driven by NAO-like (North Atlantic Oscillation) variability. In contrast, surface waters in the Gulf of Cádiz appear to have responded more directly to North Atlantic cooling associated with Bond events. In particular, during Bond events 3 and 4, a pronounced increase in seasonality with summer warming and winter cooling is found.

Description: 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.

Description: To assess the regional multi-decadal to multi-centennial climate variability at the southern Iberian Peninsula during the mid- to late- Holocene transition multi-proxy records of two marine sediment cores were established for two sites in the Alboran Sea (ODP-161-976A) and the Gulf of Cadiz (GeoB5901-2). High-resolution records of organic geochemical proxies and planktic foraminiferal assemblages are used to decipher precipitation and vegetation changes as well as the sea surface conditions with respect to Sea Surface Temperature (SST) and marine primary productivity (MPP). n-Alkane records as a proxy for precipitation changes suggest a series of six distinct drought events at 5.4 ka BP, from ca. 5.1 ka BP to 4.9 ka BP, from 4.8 to 4.7 ka BP, at 4.6  ka BP, from 4.4 to 4.3 ka BP and, from 3.8 to 3.7 ka BP. Each drought event is associated with a major vegetation change towards higher proportions of C4 vegetation. The drought events are further accompanied by annual and spring/ winter SST warming as well as decreasing MPP in the Alboran Sea. Altogether, the close correlation of the observed droughts with North Atlantic Oscillation (NAO)-like variability suggests changes in the atmospheric circulation as important driving mechanism of terrestrial and oceanic variability at southern Iberia and the Alboran Sea, respectively. Sea surface variability in the Gulf of Cadiz, instead, is intimately linked to the North Atlantic Bond Events. In particular, during Bond Events 3 and 4 a pronounced increase in seasonality is found.