We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group data... more We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group database, of oxygen and carbon stable isotope ratios from benthic foraminifera in deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-19 ky) to the Holocene (<10 ky) with a particular focus on the early last deglaciation (19-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models, and sites with at least millennial resolution were preferred. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data, even though its coverage is still sparse in many regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation.
The marine topshell Phorcus sauciatus is currently found along the temperate–subtropical shores o... more The marine topshell Phorcus sauciatus is currently found along the temperate–subtropical shores of the Northeast Atlantic Ocean. Although present in the Iberian Peninsula, Madeira, and Canaries for centuries, P. sauciatus has only recently reached another oceanic volcanic archipelago in the region. In 2013, a small population was recorded for the first time in Santa Maria Island (Azores), widening its distribution around the entire island and to the neighbouring island of São Miguel in a short period of time. The success of such colonization of the remote archipelago by P. sauciatus still awaits an explanation. To better understand the populational dynamics of the species in the NE Atlantic Ocean, we used a molecular approach to evaluate the genetic structure of P. sauciatus aiming at the determination of a potential origin for the first individuals that reached the Azores. On the foundations of detailed oceanographic, palaeontological, and ecological data, we discuss the impact of climate change as a trigger for colonization of remote oceanic islands and suggest a mechanism that might explain the long-distance dispersal of the non-planktotrophic gastropod P. sauciatus across important biogeographical barriers in the NE Atlantic.
&amp;amp;lt;p&amp;amp;gt;The Holocene, the current interglacial period covering the last ... more &amp;amp;lt;p&amp;amp;gt;The Holocene, the current interglacial period covering the last ~11.5 ky, is generally characterized by warm sea surface temperatures (SSTs), high atmospheric CO&amp;amp;lt;sub&amp;amp;gt;2 &amp;amp;lt;/sub&amp;amp;gt;levels, and low ocean productivity in some regions. However, previous studies showed that the oceanographic conditions across the Holocene are highly variable, whereby regional patterns, such as in upwelling regions, are still poorly known.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;The Portuguese margin is under influence of the Canary Eastern Boundary Upwelling System and is characterized by seasonal upwelling associated with cold waters and high primary productivity during spring-summer. During this period, are present the southward flowing, relatively cool and nutrient rich offshore Portugal Current (PC), and the nearshore Portuguese Coastal Current (PCC). During the fall-winter months, the PCC is substituted by the warm and nutrient poor Iberian Polar Current.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;Considering that the planktonic foraminifera (PF) fauna is affected by water temperature and food availability, the present study reconstructs the Portuguese margin temperature and productivity, through a transfer function, across the Holocene using the PF fauna, at two sites under different oceanographic conditions: Shak-03-6K (37&amp;amp;amp;#8304;42.45&amp;amp;amp;#8217;N, 10&amp;amp;amp;#8304;29.542&amp;amp;amp;#8217;W, 3735m) recovered on the Sines margin and core MD03-2699 (39&amp;amp;amp;#176;02.20&amp;amp;amp;#8242;N, 10&amp;amp;amp;#176;39.63&amp;amp;amp;#8242;W, 1895m) from the Estremadura spur.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;The fauna analysis at the Estremadura core revealed a total of 27 species and an estimated average summer SST of 18.9 &amp;amp;amp;#176;C. At the Sines core, we found 25 species, and a Holocene average SST of 21.2 &amp;amp;amp;#176;C. Our Holocene record starts with warm SSTs (above 20 &amp;amp;amp;#176;C) at both sites, followed by a cold event (starting at 10.5 ky), when the SST record at the Estremadura site abruptly dropped by 13 &amp;amp;amp;#186;C, while at the Sines site the observed cooling was only 2 &amp;amp;amp;#186;C. Two additional cold events, at 8.2 ky (~16 &amp;amp;amp;#176;C) and 5.2 ky (17.5 &amp;amp;amp;#186;C) occurred during the Holocene Thermal Optimum (HTO, from 9.5 to 5.5 ky) at the Estremadura Site. According to previous studies, these short cold events are well marked in the North Atlantic, possibly caused by freshwater input from the melting Laurentide ice sheet. During these events, the relative abundance of polar and subpolar species (&amp;amp;lt;em&amp;amp;gt;N. pachyderma&amp;amp;lt;/em&amp;amp;gt; and &amp;amp;lt;em&amp;amp;gt;T. quinqueloba&amp;amp;lt;/em&amp;amp;gt;) increased, while the PC related species (&amp;amp;lt;em&amp;amp;gt;N. incompta&amp;amp;lt;/em&amp;amp;gt; and &amp;amp;lt;em&amp;amp;gt;G. inflata&amp;amp;lt;/em&amp;amp;gt;) decreased. However, this period is the warmest period of the Holocene when global glaciers reached their minimum volume extend, and in both sites the subtropical and tropical species increased.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;During the Late Holocene (~4.2 ky to present), the SSTs at both sites remained constantly warm, with a high abundance of &amp;amp;lt;em&amp;amp;gt;G. bulloides&amp;amp;lt;/em&amp;amp;gt; at the transition of HTO and this period. A feature that, most probably reflects enhanced upwelling influence, mainly at the Estremadura site. In fact, the Estremadura site records higher productivity than the Sines site, as that site is under a stronger influence of upwelling events and riverine nutrient supply from the Tagus River. Furthermore, occasional incursions of cold waters from the North are clearly marked on the Estremadura site, located close to the limit between subtropical and transitional surface and subsurface waters.&amp;amp;lt;/p&amp;amp;gt;
Paleoclimate reconstructions suggest that the complex variability within the Greenland stadial 1 ... more Paleoclimate reconstructions suggest that the complex variability within the Greenland stadial 1 (GS-1) over western Europe was governed by coupled ocean and atmospheric changes. However, few works from the North Atlantic mid-latitudes document both the GS-1 onset and its termination, which are often considered as single abrupt transition events. Here, we present a direct comparison between marine (alkenone-based sea surface temperatures) and terrestrial (pollen) data, at very high resolution (28 years mean), from the southwestern Iberian shelf record D13882. Our results reveal a rather complex climatic period with internally changing conditions. The GS-1 onset (GS-1a: 12890-12720 yr BP) is marked by a progressive cooling and drying; GS-1b (12720-12390 yr BP) is the coldest and driest phase; GS-1c (12390-12030 yr BP) is marked by a progressive warming and increase in moisture conditions; GS-1 termination (GS-1d: 12030-11770 yr BP) is marked by rapid switches between cool wet, cold dry and cool wet conditions. Although hydroclimate response was very unsteady throughout the GS-1 and in particular during its termination phase, the persistence of an open temperate and Mediterranean forest in southwestern Iberia during the entire episode suggests that at least some moisture was delivered via the Westerlies. We propose coupled ocean and atmospheric mechanisms to reproduce these scenaria. Changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) as well as variations in the North Atlantic sea-ice growth have favoured the displacement of the polar jet stream's latitudinal position and contributed to a complex spatial pattern and strength of the Westerlies across western Europe.
Climate records from IODP Site U1313 (41°N, 33°W; 3412 m w.d.) from the mid-latitude North Atlant... more Climate records from IODP Site U1313 (41°N, 33°W; 3412 m w.d.) from the mid-latitude North Atlantic were used to reconstruct surface and deep-water changes during the mid-Brunhes glacial/ interglacial cycles of Marine Isotope Stages (MIS) 10 to 16 (340-640 ka) including MIS 11c, one of the longest interglacials of the Pleistocene. The records reveal Heinrich-type ice-rafting events during the glacial MIS 16, 12 and 10 that led to a reduction in the Atlantic overturning circulation. Here we focus on the conditions in the surface waters, namely the winter mixed layer/ spring thermocline, using G. inflata stable isotope and trace element records. Today the site is influenced by the North Atlantic Drift, the northward extension of the Gulf Stream. Past surface water changes therefore reflect conditions in this current and its southward displacement due to incursions of iceberg-laden subpolar waters. Thermocline temperatures were estimated from the Mg/ Ca ratio using the equation of Elderfield and Ganssen (2000). The overall temperature range varied between 1.4 and 12.8°C with the colder temperatures related to the Heinrich-type ice-rafting events. Interglacial and often also interstadial temperatures were similar, but-with the current equation-slightly colder than modern levels. MIS 11 experienced millennial-scale oscillations with maximum interstadial temperatures declining towards MIS 10. The interglacial section, i.e. MIS 11c, was associated with an early warming lasting from 426 to 396 ka, but temperatures started to cool already after 400 ka indicating that subpolar subsurface water might have started to penetrate further south after 400 ka. Interglacial MIS 13a temperatures were in the range or even warmer than those of MIS 11c revealing that at least in the thermocline waters this interglacial was not colder than its younger counterpart. Glacial MIS 12 experienced several temperature oscillations with thermocline temperatures being colder in the first half of the glacial. During late MIS 12 maximum temperatures even reached interglacial levels raising the question if warm core rings might have reached the site or if G. inflata values then reflected a different growth season. Cd/ Ca values were highly variable and no clear relationship between thermocline temperature and nutrient levels can be seen. REE/ Ca ratios increased during glacial inceptions and glacials raising the possibility that these values reflect glacial erosion more than a water mass signal such as Antarctic Intermediate Water.
The characteristics of the CISE-LOCEAN seawater isotope dataset (δ 18 O, δ 2 H, referred to as δD... more The characteristics of the CISE-LOCEAN seawater isotope dataset (δ 18 O, δ 2 H, referred to as δD) are presented (https://doi.org/10.17882/71186; Waterisotopes-CISE-LOCEAN, 2021). This dataset covers the time period from 1998 to 2021 and currently includes close to 8000 data entries, all with δ 18 O, three-quarters of them also with δD, associated with a date stamp, space stamp, and usually a salinity measurement. Until 2010, samples were analyzed by isotopic ratio mass spectrometry and since then mostly by cavity ring-down spectroscopy (CRDS). Instrumental uncertainty in this dataset is usually as low as 0.03 ‰ for δ 18 O and 0.15 ‰ for δD. An additional uncertainty is related to the isotopic composition of the in-house standards that are used to convert data to the Vienna Standard Mean Ocean Water (VSMOW) scale. Different comparisons suggest that since 2010 the latter have remained within at most 0.03 ‰ for δ 18 O and 0.20 ‰ for δD. Therefore, combining the two uncertainties suggests a standard deviation of at most 0.05 ‰ for δ 18 O and 0.25 ‰ for δD. For some samples, we find that there has been evaporation during collection and storage, requiring adjustment of the isotopic data produced by CRDS, based on d-excess (δD − 8×δ 18 O). This adjustment adds an uncertainty in the respective data of roughly 0.05 ‰ for δ 18 O and 0.10 ‰ for δD. This issue of conservation of samples is certainly a strong source of quality loss for parts of the database, and "small" effects may have remained undetected.
The presence and high abundances of the benthic foraminiferal group "elevated epifauna"... more The presence and high abundances of the benthic foraminiferal group "elevated epifauna" has been proposed as indicator of the existence of Mediterranean Outflow Water (MOW) in the Gulf of Cadiz. Here we intend to use this potential proxy to reconstruct MOW in the early Pliocene at the Integrated Ocean Drilling Program (IODP) Hole U1387C. Cibicides lobatulus, Planulina ariminensis and Cibicides refulgens were found at this site, but high abundances of C. lobatulus and C. refulgens coincide with increases of shelf taxa and grain size, related to episodes of downslope transport. This issue calls into question the applicability of these species as MOW proxies. The present study therefore reassesses the role of these three elevated epifaunal species by means of stable isotope analyses (d18O, d13C) by contrasting their isotopic signature with that of shelf dwellers and deep-water taxa from Pliocene and Recent sediments from the Gulf of Cadiz and the western Iberian margin. Since foraminiferal calcite does not always precipitate in equilibrium with seawater, substantial efforts have been taken in order to determine species-specific offset values and to correct the isotopic signature for vital effects. Our corrected results demonstrate that C. lobatulus and C. refulgens are isotopically similar to the shelf dwellers in the Pliocene data-set, eliminating them as MOW indicators. Recent samples from the Gulf of Cadiz and the western Portugal shelf corroborate the wide bathymetric range of C. lobatulus from the shelf to the slope. Our results thus warrant for caution when considering C. lobatulus and C. refulgens as indicators of bottom current strength in unstable slope settings, particularly when co-occurring with allochthonous shelf dwellers. In contrast, P. ariminensis shows a signature close to deep-water taxa, corroborating its reliability as indicator of MOW.
The Gulf of Cadiz off southern Iberia is an ideal place to study the interaction between North At... more The Gulf of Cadiz off southern Iberia is an ideal place to study the interaction between North Atlantic and Mediterranean Sea climate variations with surface waters reflecting subtropical gyre conditions and the intermediate-depth Mediterranean Outflow Water (MOW) combining Mediterranean Sea and North Atlantic signals. Using centennialscale records from IODP Site U1387 (36.8◦N, 7.7◦W; 559 m w.d.) we evaluate interglacial surface-water and MOW conditions during the interval from Marine Isotope Stage (MIS) 16 to 48 (630-1470 ka). Surface-water changes are deduced from alkenone-derived sea-surface temperature (SST) and G. bulloides stable isotope records and MOW conditions from the benthic foraminifer stable isotope data and the wt% sand.
Understanding interglacial climate variability is a key issue in the scientific community. Here w... more Understanding interglacial climate variability is a key issue in the scientific community. Here we compared records from Marine Isotope Stage (MIS) 11 to those from MIS 1 (Holocene) as they are perceived to be possible analogs. Our study on the Iberian Margin, a key area to investigate surface dynamics in the Atlantic Ocean, incorporates coccolithophore assemblage and alkenone data of core MD03-2699 and their statistical analyses. Evaluating similarities between MIS 11 and MIS 1 depends on the way the two MIS are being aligned, i.e. at the deglaciation or based on the precession signal. During the deglaciation of either MIS 12 or MIS 2, the Iberian Margin was affected by abrupt decreases in SST and in coccolithophores' paleoproductivity caused by the arrival of subpolar surface waters. Just prior to the decline, in both the intervals, the Portugal Current affected the studied site, although a possible difference in upwelling strength is here suggested and related to more intense westerlies during the last glacial than the late MIS 12. Similar surface-ocean dynamics occurred at the onset of both MIS 11 and MIS 1 as indicated by the prevalence of the Iberian Poleward Current and sometimes the Azores Current, although the subtropical waters were more oligotrophic during the MIS 2 deglaciation than the MIS 12 one. Synchronizing our records according to the precession cycles aligns the early-to-mid Holocene with the second, warmer phase of MIS 11c. During both these intervals, the western Iberian Margin was mainly affected by the Iberian Poleward Current that transported more temperate-warm, mesotrophic surface waters during MIS 11c than during the early-to-mid Holocene. During the early to mid-Holocene the Iberian Margin endured incursions of colder surface waters that did not occur during MIS 11c allowing us to hypothesize that the studied site experienced, from a paleoceanographic point of view, a more stable period during MIS 11c than the early Holocene. Finally, spectral analysis suggests the role of full, half and fourth precession components in driving surface-ocean variability during MIS 11 and during the last 24 kyr BP.
The overall aim of the North Atlantic paleoceanography study of Integrated Ocean Drilling Program... more The overall aim of the North Atlantic paleoceanography study of Integrated Ocean Drilling Program Expedition 306 is to place late Neogene-Quaternary climate proxies in the North Atlantic into a chronology based on a combination of geomagnetic paleointensity, stable isotope, and detrital layer stratigraphies, and in so doing generate integrated North Atlantic millennial-scale stratigraphies for the last few million years. To reach this aim, complete sedimentary sections were drilled by multiple advanced piston coring directly south of the central Atlantic "ice-rafted debris belt" and on the southern Gardar Drift. In addition to the North Atlantic paleoceanography study, a borehole observatory was successfully installed in a new ~180 m deep hole close to Ocean Drilling Program Site 642, consisting of a circulation obviation retrofit kit to seal the borehole from the overlying ocean, a thermistor string, and a data logger to document and monitor bottom water temperature variations through time. Preface Integrated Ocean Drilling Program (IODP) Expedition 306 is based on two separate proposals entitled (A) "Ice sheet-ocean atmosphere interactions on millennial timescales during the late Neogene-Quaternary using a paleointensity-assisted chronology (PAC) for the North Atlantic" and (B) "Installation of a circulation obviation retrofit kit (CORK) near Hole 642E to document and monitor bottom water temperature variations through time." Since both parts are independent of each other, the introductory sections of the Expedition 306 report (introduction, background, geological setting, and objectives) have been divided into Part A and Part B. Sites U1312, U1313, and U1314 are related to the North Atlantic paleoceanography study (Part A), whereas Site U1315 and the revisited Ocean Drilling Program (ODP) Hole 642E are related to the CORK program (Part B).
New dinoflagellate cyst (dinocyst) analyses were carried out at high-resolution in core MD99-2339... more New dinoflagellate cyst (dinocyst) analyses were carried out at high-resolution in core MD99-2339, retrieved from a contouritic field in the central part of the Gulf of Cadiz, for the Marine Isotope Stage (MIS) 3 interval, allowing to discuss paleohydrological changes over the last 50 ky in the subtropical NE Atlantic Ocean. Some index dinocyst taxa, according to their (paleo) ecological significance, shed light on significant sea-surface changes. Superimposed on the general decreasing pattern of dinocyst export to the seafloor over the last 50 ky, paralleling the general context of decreasing aeolian dust fertilization, a complex variability in dinocyst assemblages was detected at millennial timescale. Enhanced fluvial discharges occurred during Greenland Interstadials (GI) and especially GI 1, 8 and 12, while enhanced upwelling cell dynamics were suggested during the Last Glacial Maximum and Heinrich Stadials. Finally, during the early Holocene, and more specifically during the Sa...
ABSTRACT Marine Isotope Stage (MIS) 11 and the Holocene are often seen as equal as the orbital pa... more ABSTRACT Marine Isotope Stage (MIS) 11 and the Holocene are often seen as equal as the orbital parameters and greenhouse gas concentrations during the prominent interglacial 400 ky ago were similar to the ones of the present interglacial period. To study these analogues on the Portuguese Margin we use core MD03-2699 retrieved from the Estremadura Spur north of Lisbon for the MIS 11 record, while the Holocene variations are based on two sediment sequences: 1) MD03-2699 in conjunction with box core PO287-44B from the same location; and 2) core D13882, D13902 and a box cores from the Tagus prodelta off Lisbon, collected at 90m water. Sea Surface Temperatures (SST) reconstructed by the Uk&#39;37 alkenone index suggest that climate conditions during MIS 11 were not significantly different from those registed during the elapsed portion of the Holocene. The SST change at the glacial to interglacial transition (at the transition preceding the interglacials) is characterized by an abrupt rise of 6 degrees C in 1750 yr during for the MIS 11 and 8 degrees C in 280 yr for the transition to the Holocene. The maximum values of Holocene SST, close to 19ºC occur between 10,5 and 9,7 ky BP, while the MIS 11 record reveals two warmer phases: the first with maximum SST close to 18ºC (427 to 412 ky BP) and a second with temperatures close to 19ºC (407 to 395 ky BP). Both periods display a SST decrease following the maximum, which in the case of MIS 11, however, is interrupted by the second SST increase that marks the peak interglacial. Amplitude of SST variation of 1.5 degrees C is similar during both interglacial periods. The results obtained prove that the first part of MIS 11 (427 to 412 ky) and the last 10,5 ky have a similar SST record implying that hydrographic conditions must have been similar during both periods. The maximal SSTs are always associated with the insolation maximum during the respective interglacial, i.e. at the beginning of the Holocene and during MIS 11.3.1.
We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group data... more We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group database, of oxygen and carbon stable isotope ratios from benthic foraminifera in deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-19 ky) to the Holocene (<10 ky) with a particular focus on the early last deglaciation (19-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models, and sites with at least millennial resolution were preferred. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data, even though its coverage is still sparse in many regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation.
The marine topshell Phorcus sauciatus is currently found along the temperate–subtropical shores o... more The marine topshell Phorcus sauciatus is currently found along the temperate–subtropical shores of the Northeast Atlantic Ocean. Although present in the Iberian Peninsula, Madeira, and Canaries for centuries, P. sauciatus has only recently reached another oceanic volcanic archipelago in the region. In 2013, a small population was recorded for the first time in Santa Maria Island (Azores), widening its distribution around the entire island and to the neighbouring island of São Miguel in a short period of time. The success of such colonization of the remote archipelago by P. sauciatus still awaits an explanation. To better understand the populational dynamics of the species in the NE Atlantic Ocean, we used a molecular approach to evaluate the genetic structure of P. sauciatus aiming at the determination of a potential origin for the first individuals that reached the Azores. On the foundations of detailed oceanographic, palaeontological, and ecological data, we discuss the impact of climate change as a trigger for colonization of remote oceanic islands and suggest a mechanism that might explain the long-distance dispersal of the non-planktotrophic gastropod P. sauciatus across important biogeographical barriers in the NE Atlantic.
&amp;amp;lt;p&amp;amp;gt;The Holocene, the current interglacial period covering the last ... more &amp;amp;lt;p&amp;amp;gt;The Holocene, the current interglacial period covering the last ~11.5 ky, is generally characterized by warm sea surface temperatures (SSTs), high atmospheric CO&amp;amp;lt;sub&amp;amp;gt;2 &amp;amp;lt;/sub&amp;amp;gt;levels, and low ocean productivity in some regions. However, previous studies showed that the oceanographic conditions across the Holocene are highly variable, whereby regional patterns, such as in upwelling regions, are still poorly known.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;The Portuguese margin is under influence of the Canary Eastern Boundary Upwelling System and is characterized by seasonal upwelling associated with cold waters and high primary productivity during spring-summer. During this period, are present the southward flowing, relatively cool and nutrient rich offshore Portugal Current (PC), and the nearshore Portuguese Coastal Current (PCC). During the fall-winter months, the PCC is substituted by the warm and nutrient poor Iberian Polar Current.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;Considering that the planktonic foraminifera (PF) fauna is affected by water temperature and food availability, the present study reconstructs the Portuguese margin temperature and productivity, through a transfer function, across the Holocene using the PF fauna, at two sites under different oceanographic conditions: Shak-03-6K (37&amp;amp;amp;#8304;42.45&amp;amp;amp;#8217;N, 10&amp;amp;amp;#8304;29.542&amp;amp;amp;#8217;W, 3735m) recovered on the Sines margin and core MD03-2699 (39&amp;amp;amp;#176;02.20&amp;amp;amp;#8242;N, 10&amp;amp;amp;#176;39.63&amp;amp;amp;#8242;W, 1895m) from the Estremadura spur.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;The fauna analysis at the Estremadura core revealed a total of 27 species and an estimated average summer SST of 18.9 &amp;amp;amp;#176;C. At the Sines core, we found 25 species, and a Holocene average SST of 21.2 &amp;amp;amp;#176;C. Our Holocene record starts with warm SSTs (above 20 &amp;amp;amp;#176;C) at both sites, followed by a cold event (starting at 10.5 ky), when the SST record at the Estremadura site abruptly dropped by 13 &amp;amp;amp;#186;C, while at the Sines site the observed cooling was only 2 &amp;amp;amp;#186;C. Two additional cold events, at 8.2 ky (~16 &amp;amp;amp;#176;C) and 5.2 ky (17.5 &amp;amp;amp;#186;C) occurred during the Holocene Thermal Optimum (HTO, from 9.5 to 5.5 ky) at the Estremadura Site. According to previous studies, these short cold events are well marked in the North Atlantic, possibly caused by freshwater input from the melting Laurentide ice sheet. During these events, the relative abundance of polar and subpolar species (&amp;amp;lt;em&amp;amp;gt;N. pachyderma&amp;amp;lt;/em&amp;amp;gt; and &amp;amp;lt;em&amp;amp;gt;T. quinqueloba&amp;amp;lt;/em&amp;amp;gt;) increased, while the PC related species (&amp;amp;lt;em&amp;amp;gt;N. incompta&amp;amp;lt;/em&amp;amp;gt; and &amp;amp;lt;em&amp;amp;gt;G. inflata&amp;amp;lt;/em&amp;amp;gt;) decreased. However, this period is the warmest period of the Holocene when global glaciers reached their minimum volume extend, and in both sites the subtropical and tropical species increased.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;During the Late Holocene (~4.2 ky to present), the SSTs at both sites remained constantly warm, with a high abundance of &amp;amp;lt;em&amp;amp;gt;G. bulloides&amp;amp;lt;/em&amp;amp;gt; at the transition of HTO and this period. A feature that, most probably reflects enhanced upwelling influence, mainly at the Estremadura site. In fact, the Estremadura site records higher productivity than the Sines site, as that site is under a stronger influence of upwelling events and riverine nutrient supply from the Tagus River. Furthermore, occasional incursions of cold waters from the North are clearly marked on the Estremadura site, located close to the limit between subtropical and transitional surface and subsurface waters.&amp;amp;lt;/p&amp;amp;gt;
Paleoclimate reconstructions suggest that the complex variability within the Greenland stadial 1 ... more Paleoclimate reconstructions suggest that the complex variability within the Greenland stadial 1 (GS-1) over western Europe was governed by coupled ocean and atmospheric changes. However, few works from the North Atlantic mid-latitudes document both the GS-1 onset and its termination, which are often considered as single abrupt transition events. Here, we present a direct comparison between marine (alkenone-based sea surface temperatures) and terrestrial (pollen) data, at very high resolution (28 years mean), from the southwestern Iberian shelf record D13882. Our results reveal a rather complex climatic period with internally changing conditions. The GS-1 onset (GS-1a: 12890-12720 yr BP) is marked by a progressive cooling and drying; GS-1b (12720-12390 yr BP) is the coldest and driest phase; GS-1c (12390-12030 yr BP) is marked by a progressive warming and increase in moisture conditions; GS-1 termination (GS-1d: 12030-11770 yr BP) is marked by rapid switches between cool wet, cold dry and cool wet conditions. Although hydroclimate response was very unsteady throughout the GS-1 and in particular during its termination phase, the persistence of an open temperate and Mediterranean forest in southwestern Iberia during the entire episode suggests that at least some moisture was delivered via the Westerlies. We propose coupled ocean and atmospheric mechanisms to reproduce these scenaria. Changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) as well as variations in the North Atlantic sea-ice growth have favoured the displacement of the polar jet stream's latitudinal position and contributed to a complex spatial pattern and strength of the Westerlies across western Europe.
Climate records from IODP Site U1313 (41°N, 33°W; 3412 m w.d.) from the mid-latitude North Atlant... more Climate records from IODP Site U1313 (41°N, 33°W; 3412 m w.d.) from the mid-latitude North Atlantic were used to reconstruct surface and deep-water changes during the mid-Brunhes glacial/ interglacial cycles of Marine Isotope Stages (MIS) 10 to 16 (340-640 ka) including MIS 11c, one of the longest interglacials of the Pleistocene. The records reveal Heinrich-type ice-rafting events during the glacial MIS 16, 12 and 10 that led to a reduction in the Atlantic overturning circulation. Here we focus on the conditions in the surface waters, namely the winter mixed layer/ spring thermocline, using G. inflata stable isotope and trace element records. Today the site is influenced by the North Atlantic Drift, the northward extension of the Gulf Stream. Past surface water changes therefore reflect conditions in this current and its southward displacement due to incursions of iceberg-laden subpolar waters. Thermocline temperatures were estimated from the Mg/ Ca ratio using the equation of Elderfield and Ganssen (2000). The overall temperature range varied between 1.4 and 12.8°C with the colder temperatures related to the Heinrich-type ice-rafting events. Interglacial and often also interstadial temperatures were similar, but-with the current equation-slightly colder than modern levels. MIS 11 experienced millennial-scale oscillations with maximum interstadial temperatures declining towards MIS 10. The interglacial section, i.e. MIS 11c, was associated with an early warming lasting from 426 to 396 ka, but temperatures started to cool already after 400 ka indicating that subpolar subsurface water might have started to penetrate further south after 400 ka. Interglacial MIS 13a temperatures were in the range or even warmer than those of MIS 11c revealing that at least in the thermocline waters this interglacial was not colder than its younger counterpart. Glacial MIS 12 experienced several temperature oscillations with thermocline temperatures being colder in the first half of the glacial. During late MIS 12 maximum temperatures even reached interglacial levels raising the question if warm core rings might have reached the site or if G. inflata values then reflected a different growth season. Cd/ Ca values were highly variable and no clear relationship between thermocline temperature and nutrient levels can be seen. REE/ Ca ratios increased during glacial inceptions and glacials raising the possibility that these values reflect glacial erosion more than a water mass signal such as Antarctic Intermediate Water.
The characteristics of the CISE-LOCEAN seawater isotope dataset (δ 18 O, δ 2 H, referred to as δD... more The characteristics of the CISE-LOCEAN seawater isotope dataset (δ 18 O, δ 2 H, referred to as δD) are presented (https://doi.org/10.17882/71186; Waterisotopes-CISE-LOCEAN, 2021). This dataset covers the time period from 1998 to 2021 and currently includes close to 8000 data entries, all with δ 18 O, three-quarters of them also with δD, associated with a date stamp, space stamp, and usually a salinity measurement. Until 2010, samples were analyzed by isotopic ratio mass spectrometry and since then mostly by cavity ring-down spectroscopy (CRDS). Instrumental uncertainty in this dataset is usually as low as 0.03 ‰ for δ 18 O and 0.15 ‰ for δD. An additional uncertainty is related to the isotopic composition of the in-house standards that are used to convert data to the Vienna Standard Mean Ocean Water (VSMOW) scale. Different comparisons suggest that since 2010 the latter have remained within at most 0.03 ‰ for δ 18 O and 0.20 ‰ for δD. Therefore, combining the two uncertainties suggests a standard deviation of at most 0.05 ‰ for δ 18 O and 0.25 ‰ for δD. For some samples, we find that there has been evaporation during collection and storage, requiring adjustment of the isotopic data produced by CRDS, based on d-excess (δD − 8×δ 18 O). This adjustment adds an uncertainty in the respective data of roughly 0.05 ‰ for δ 18 O and 0.10 ‰ for δD. This issue of conservation of samples is certainly a strong source of quality loss for parts of the database, and "small" effects may have remained undetected.
The presence and high abundances of the benthic foraminiferal group "elevated epifauna"... more The presence and high abundances of the benthic foraminiferal group "elevated epifauna" has been proposed as indicator of the existence of Mediterranean Outflow Water (MOW) in the Gulf of Cadiz. Here we intend to use this potential proxy to reconstruct MOW in the early Pliocene at the Integrated Ocean Drilling Program (IODP) Hole U1387C. Cibicides lobatulus, Planulina ariminensis and Cibicides refulgens were found at this site, but high abundances of C. lobatulus and C. refulgens coincide with increases of shelf taxa and grain size, related to episodes of downslope transport. This issue calls into question the applicability of these species as MOW proxies. The present study therefore reassesses the role of these three elevated epifaunal species by means of stable isotope analyses (d18O, d13C) by contrasting their isotopic signature with that of shelf dwellers and deep-water taxa from Pliocene and Recent sediments from the Gulf of Cadiz and the western Iberian margin. Since foraminiferal calcite does not always precipitate in equilibrium with seawater, substantial efforts have been taken in order to determine species-specific offset values and to correct the isotopic signature for vital effects. Our corrected results demonstrate that C. lobatulus and C. refulgens are isotopically similar to the shelf dwellers in the Pliocene data-set, eliminating them as MOW indicators. Recent samples from the Gulf of Cadiz and the western Portugal shelf corroborate the wide bathymetric range of C. lobatulus from the shelf to the slope. Our results thus warrant for caution when considering C. lobatulus and C. refulgens as indicators of bottom current strength in unstable slope settings, particularly when co-occurring with allochthonous shelf dwellers. In contrast, P. ariminensis shows a signature close to deep-water taxa, corroborating its reliability as indicator of MOW.
The Gulf of Cadiz off southern Iberia is an ideal place to study the interaction between North At... more The Gulf of Cadiz off southern Iberia is an ideal place to study the interaction between North Atlantic and Mediterranean Sea climate variations with surface waters reflecting subtropical gyre conditions and the intermediate-depth Mediterranean Outflow Water (MOW) combining Mediterranean Sea and North Atlantic signals. Using centennialscale records from IODP Site U1387 (36.8◦N, 7.7◦W; 559 m w.d.) we evaluate interglacial surface-water and MOW conditions during the interval from Marine Isotope Stage (MIS) 16 to 48 (630-1470 ka). Surface-water changes are deduced from alkenone-derived sea-surface temperature (SST) and G. bulloides stable isotope records and MOW conditions from the benthic foraminifer stable isotope data and the wt% sand.
Understanding interglacial climate variability is a key issue in the scientific community. Here w... more Understanding interglacial climate variability is a key issue in the scientific community. Here we compared records from Marine Isotope Stage (MIS) 11 to those from MIS 1 (Holocene) as they are perceived to be possible analogs. Our study on the Iberian Margin, a key area to investigate surface dynamics in the Atlantic Ocean, incorporates coccolithophore assemblage and alkenone data of core MD03-2699 and their statistical analyses. Evaluating similarities between MIS 11 and MIS 1 depends on the way the two MIS are being aligned, i.e. at the deglaciation or based on the precession signal. During the deglaciation of either MIS 12 or MIS 2, the Iberian Margin was affected by abrupt decreases in SST and in coccolithophores' paleoproductivity caused by the arrival of subpolar surface waters. Just prior to the decline, in both the intervals, the Portugal Current affected the studied site, although a possible difference in upwelling strength is here suggested and related to more intense westerlies during the last glacial than the late MIS 12. Similar surface-ocean dynamics occurred at the onset of both MIS 11 and MIS 1 as indicated by the prevalence of the Iberian Poleward Current and sometimes the Azores Current, although the subtropical waters were more oligotrophic during the MIS 2 deglaciation than the MIS 12 one. Synchronizing our records according to the precession cycles aligns the early-to-mid Holocene with the second, warmer phase of MIS 11c. During both these intervals, the western Iberian Margin was mainly affected by the Iberian Poleward Current that transported more temperate-warm, mesotrophic surface waters during MIS 11c than during the early-to-mid Holocene. During the early to mid-Holocene the Iberian Margin endured incursions of colder surface waters that did not occur during MIS 11c allowing us to hypothesize that the studied site experienced, from a paleoceanographic point of view, a more stable period during MIS 11c than the early Holocene. Finally, spectral analysis suggests the role of full, half and fourth precession components in driving surface-ocean variability during MIS 11 and during the last 24 kyr BP.
The overall aim of the North Atlantic paleoceanography study of Integrated Ocean Drilling Program... more The overall aim of the North Atlantic paleoceanography study of Integrated Ocean Drilling Program Expedition 306 is to place late Neogene-Quaternary climate proxies in the North Atlantic into a chronology based on a combination of geomagnetic paleointensity, stable isotope, and detrital layer stratigraphies, and in so doing generate integrated North Atlantic millennial-scale stratigraphies for the last few million years. To reach this aim, complete sedimentary sections were drilled by multiple advanced piston coring directly south of the central Atlantic "ice-rafted debris belt" and on the southern Gardar Drift. In addition to the North Atlantic paleoceanography study, a borehole observatory was successfully installed in a new ~180 m deep hole close to Ocean Drilling Program Site 642, consisting of a circulation obviation retrofit kit to seal the borehole from the overlying ocean, a thermistor string, and a data logger to document and monitor bottom water temperature variations through time. Preface Integrated Ocean Drilling Program (IODP) Expedition 306 is based on two separate proposals entitled (A) "Ice sheet-ocean atmosphere interactions on millennial timescales during the late Neogene-Quaternary using a paleointensity-assisted chronology (PAC) for the North Atlantic" and (B) "Installation of a circulation obviation retrofit kit (CORK) near Hole 642E to document and monitor bottom water temperature variations through time." Since both parts are independent of each other, the introductory sections of the Expedition 306 report (introduction, background, geological setting, and objectives) have been divided into Part A and Part B. Sites U1312, U1313, and U1314 are related to the North Atlantic paleoceanography study (Part A), whereas Site U1315 and the revisited Ocean Drilling Program (ODP) Hole 642E are related to the CORK program (Part B).
New dinoflagellate cyst (dinocyst) analyses were carried out at high-resolution in core MD99-2339... more New dinoflagellate cyst (dinocyst) analyses were carried out at high-resolution in core MD99-2339, retrieved from a contouritic field in the central part of the Gulf of Cadiz, for the Marine Isotope Stage (MIS) 3 interval, allowing to discuss paleohydrological changes over the last 50 ky in the subtropical NE Atlantic Ocean. Some index dinocyst taxa, according to their (paleo) ecological significance, shed light on significant sea-surface changes. Superimposed on the general decreasing pattern of dinocyst export to the seafloor over the last 50 ky, paralleling the general context of decreasing aeolian dust fertilization, a complex variability in dinocyst assemblages was detected at millennial timescale. Enhanced fluvial discharges occurred during Greenland Interstadials (GI) and especially GI 1, 8 and 12, while enhanced upwelling cell dynamics were suggested during the Last Glacial Maximum and Heinrich Stadials. Finally, during the early Holocene, and more specifically during the Sa...
ABSTRACT Marine Isotope Stage (MIS) 11 and the Holocene are often seen as equal as the orbital pa... more ABSTRACT Marine Isotope Stage (MIS) 11 and the Holocene are often seen as equal as the orbital parameters and greenhouse gas concentrations during the prominent interglacial 400 ky ago were similar to the ones of the present interglacial period. To study these analogues on the Portuguese Margin we use core MD03-2699 retrieved from the Estremadura Spur north of Lisbon for the MIS 11 record, while the Holocene variations are based on two sediment sequences: 1) MD03-2699 in conjunction with box core PO287-44B from the same location; and 2) core D13882, D13902 and a box cores from the Tagus prodelta off Lisbon, collected at 90m water. Sea Surface Temperatures (SST) reconstructed by the Uk&#39;37 alkenone index suggest that climate conditions during MIS 11 were not significantly different from those registed during the elapsed portion of the Holocene. The SST change at the glacial to interglacial transition (at the transition preceding the interglacials) is characterized by an abrupt rise of 6 degrees C in 1750 yr during for the MIS 11 and 8 degrees C in 280 yr for the transition to the Holocene. The maximum values of Holocene SST, close to 19ºC occur between 10,5 and 9,7 ky BP, while the MIS 11 record reveals two warmer phases: the first with maximum SST close to 18ºC (427 to 412 ky BP) and a second with temperatures close to 19ºC (407 to 395 ky BP). Both periods display a SST decrease following the maximum, which in the case of MIS 11, however, is interrupted by the second SST increase that marks the peak interglacial. Amplitude of SST variation of 1.5 degrees C is similar during both interglacial periods. The results obtained prove that the first part of MIS 11 (427 to 412 ky) and the last 10,5 ky have a similar SST record implying that hydrographic conditions must have been similar during both periods. The maximal SSTs are always associated with the insolation maximum during the respective interglacial, i.e. at the beginning of the Holocene and during MIS 11.3.1.
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Papers by Antje Voelker