Giuseppe Cortese

Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning

During the last glacial termination, the upwelling strength of the southern polar limb of the Atlantic Meridional Overturning Circulation varied, changing the ventilation and stratification of the high-latitude Southern Ocean. During the same period, at least two phases of abrupt global sea-level rise--meltwater pulses--took place. Although the timing and magnitude of these events have become better constrained, a causal link between ocean stratification, the meltwater pulses and accelerated ice loss from Antarctica has not been proven. Here we simulate Antarctic ice sheet evolution over the last 25 kyr using a data-constrained ice-sheet model forced by changes in Southern Ocean temperature from an Earth system model. Results reveal several episodes of accelerated ice-sheet recession, the largest being coincident with meltwater pulse 1A. This resulted from reduced Southern Ocean overturning following Heinrich Event 1, when warmer subsurface water thermally eroded grounded marine-based ice and instigated a positive feedback that further accelerated ice-sheet retreat.

Radiolarian-based paleotemperatures during the last 160 kyr at ODP Site 1089 (Southern Ocean, Atlantic Sector)

Two cores,Site 1089 (ODP Leg 177) and PS2821-1,recovered from the same location (40‡56 PS; 9‡54PE) at the Subtropical Front (STF) in the Atlantic Sector of the Southern Ocean,provide a high-resolution climatic record,with an average temporal resolution of less than 600 yr. A multi-proxy approach was used to produce an age model for Core PS2821-1,and to correlate the two cores. Both cores document the last climatic cycle,from Marine Isotopic Stage 6 (MIS 6,ca. 160 kyr BP,ka) to present. Summer sea-surface temperatures (SSSTs) have been estimated,with a standard error of ca. 8 1.16‡C,for the down core record by using Q-mode factor analysis (Imbrie and Kipp method). The paleotemperatures show a 7‡C warming at Termination II (last interglacial,transition from MIS 6 to MIS 5). This transition from glacial to interglacial paleotemperatures (with maximum temperatures ca. 3‡C warmer than present at the core location) occurs earlier than the corresponding shift in N 18 O values for benthic foraminifera from the same core; this suggests a lead of Southern Ocean paleotemperature changes compared to the global ice-volume changes,as indicated by the benthic isotopic record. The climatic evolution of the record continues with a progressive temperature deterioration towards MIS 2. High-frequency,millennial-scale climatic instability has been documented for MIS 3 and part of MIS 4,with sudden temperature variations of almost the same magnitude as those observed at the transitions between glacial and interglacial times. These changes occur during the same time interval as the Dansgaard^Oeschger cycles recognized in the N 18 Oice record of the GRIP and GISP ice cores from Greenland,and seem to be connected to rapid changes in the STF position in relation to the core location. Sudden cooling episodes (‘Younger Dryas (YD)-type’ and ‘Antarctic Cold Reversal (ACR)-type’ of events) have been recognized for both Termination I (ACR-I and YD-I events) and II (ACR-II and YD-II events),and imply that our core is located in an optimal position in order to record events triggered by phenomena occurring in both hemispheres. Spectral analysis of our SSST record displays strong analogies,particularly for high,sub-orbital frequencies,to equivalent records from Vostok (Antarctica) and from the Subtropical North Atlantic ocean. This implies that the climatic variability of widely separated areas (the Antarctic continent,the Subtropical North Atlantic,and the Subantarctic South Atlantic) can be strongly coupled and co-varying at millennial time scales (a few to 10-ka periods),and eventually induced by the same triggering mechanisms. Climatic variability has also been documented for supposedly warm and stable interglacial intervals (MIS 1 and 5),with several cold events which can be correlated to other Southern Ocean and North Atlantic sediment records. D 2002 Elsevier Science B.V. All rights reserved.

Radiolarian faunal provinces in surface sediments of the Greenland, Iceland and Norwegian (GIN) Seas

The overall abundance and species composition of the polycystine and phaeodarian radiolaria have been determined in 63 surface sediment samples from the GIN Seas. These results are compared to chemical and physical properties of the overlying water masses. There are three abundance maxima in the distribution of radiolarian skeletal debris preserved in these surface sediments, centered on the Iceland Plateau, southern Norwegian Basin and northern Norwegian Basin. The most commonly encountered species were: (a) Spumellarida — Actinomma boreale, A. leptoderma, Larcospira minor, and Phorticium clevei, (b) Nassellarida — Amphimelissa setosa, Artobotrys boreale, Lithomelissa setosa, Lithocampe platycephala, Pseudodictyophimus gracilipes , Cycladophora davisiana ,a ndLithomitra lineata. Based on factor analysis of the core-top assemblages, the radiolarian species were grouped into three associations: Factor 1 —a polar- and arctic water association dominated by Amphimelissa setosa (varimax factor score 5.269), high factor component values are found in the Iceland Plateau area and in a wedge just north of the Iceland‐Faeroe Ridge; Factor 2 —an Atlantic water association dominated by Pseudodictyophimus gracilipes(3.247), Lithomelissa setosa (2.731), and Actinomma boreale (1.851), high factor component values are found in the eastern part of the Norwegian Sea, particularly in the area under the influence of the Norwegian Current; and Factor 3 —an Atlantic and arctic water mixed association dominated by Lithocampe platycephala (3.251), Lithomelissa setosa ( 2.176), Actinomma leptoderma(1.994) and Artobotrys boreale (1.504), high factor component values are found immediately to the east of the Iceland Plateau in correspondence to the deepest part of the Norwegian Basin. The correlation coefficients obtained between seasonal sea surface temperature and Factors 1 .R 2 D 0:835/ ,a nd 2.R 2 D 0:891/ show a good fit, whilst for Factor 3 there was a less marked.R 2 D 0:497/, but still significant at the 5% confidence level, correlation for fifth-degree polynomial regression functions. The factors correlated better with the summer than the winter sea surface temperatures. However, at 20, 50, 100 and 200 m depth, the correlation became increasingly better, particularly so for the winter situation. The highest species richness for polycystine radiolarians (>28 species) was found in the warm Atlantic domain, the lowest (<24 species) was found in the colder arctic and polar domains, whilst an area approximating the position of the Arctic front had between 24 and 28 species.

A review of bipolarity concepts: History and examples from Radiolaria and Medusozoa (Cnidaria)

Abstract Bipolarity, its history and general interpretation are investigated and discussed herein. Apart from the classical view, namely that a bipolar distribution is a peculiar biogeographical phenomenon, we propose that it is ecologically controlled too. This approach was used for bipolarity assessment within the following groups: Phaeodaria, Nassellaria, Spumellaria (Radiolaria) and Medusozoa (Cnidaria). We recognize 46 bipolar radiolarian species and three radiolarian genera. However, although species concepts in radiolarians are relatively stable and well known, the high-rank taxonomy of radiolarians is still not well defined. Caution should therefore be taken in the interpretation of distribution data at a taxonomic level higher than the species. In the Medusozoa, bipolarity is observed for 23 species and 32 genera. The different ways in which bipolarity can develop are discussed under the different groups, but preference has been given to the recent and most probable routes of migration. In our investigation of the bipolarity phenomenon, we reviewed more than 400 articles dealing with taxonomy, ecology and biogeography of the modern fauna in both groups.

Polycystine radiolarians in the Greenland-Iceland-Norwegian Seas: species and assemblage distribution

Cluster analysis and Q-mode factor analysis have been applied to polycystine radiolarian census data from 160 core-top samples. This allowed us to recognize four faunal assemblages in the Greenland-Iceland-Norwegian Seas, each related to different oceanographic conditions. A regression equation for deriving palaeotemperatures from these assemblages has also been developed. The standard error of estimate for this equation is - 1.2°C. The relative abundance of the species having the higher loadings in the core-top assemblages has been mapped, in order to identify and analyse water mass and environmental requirements for these species. Cluster analysis has also been performed on the same data set, providing results which are in good harmony with those derived by Q-mode factor analysis.

The taxonomy of boreal Atlantic Ocean Actinommida (Radiolaria)

The boreal Atlantic Ocean is characterized by its limited number of Polycystine radiolarian species, about 70 in total. The taxonomy and ontogeny of all the Actinommida species in these waters are discussed and the following species illustrated: Actinomma boreale, Actinomma leptoderma leptoderma (emended), A. leptoderma longispina n. subsp., A. trinacria (emended) and Drymyomma elegans. A key to the boreal Atlantic Ocean Actinommida is provided.

Sea surface temperature control on the distribution of far‐traveled Southern Ocean ice‐rafted detritus during the Pliocene

The flux and provenance of ice-rafted detritus (IRD) deposited in the Southern Ocean can reveal information about the past instability of Antarcticas ice sheets during different climatic conditions. Here we present a Pliocene IRD provenance record based on the 40Ar/39Ar ages of ice-rafted hornblende grains from Ocean Drilling Program Site 1165, located near Prydz Bay in the Indian Ocean sector of the Southern Ocean, along with the results of modeled sensitivity tests of iceberg trajectories and their spatial melting patterns under a range of sea surface temperatures (SSTs). Our provenance results reveal that IRD and hence icebergs in the Prydz Bay area were mainly sourced from (i) the local Prydz Bay region and (ii) the remote Wilkes Land margin located at the mouth of the low-lying Aurora Subglacial Basin. A series of IRD pulses, reaching up to 10 times background IRD flux levels, were previously identified at Site 1165 between 3.3 and 3.0 Ma. Our new results reveal that the average proportion of IRD sourced from distal Wilkes Land margin doubles after 3.3 Ma. Our iceberg trajectory-melting models show that slower iceberg melting under cooling SSTs over this middle Pliocene interval allowed Wilkes Land icebergs to travel farther before melting. Hence, declining SSTs can account for a large part of the observed IRD provenance record at Site 1165. In early Pliocene IRD layers, sampled at suborbital resolution around 4.6 Ma, we find evidence for significant increases in icebergs derived from Wilkes Land during very warm interglacials. This is suggestive of large-scale destabilization of the East Antarctic Ice Sheet in the Aurora Subglacial Basin, as far-traveled icebergs would have to overcome enhanced melting in warmer SSTs. Our results highlight the importance of considering SSTs when interpreting IRD flux and provenance records in distal locations.

The morphometric variation of Actinomma boreale (Radiolaria) in Atlantic boreal waters

Abstract A morphometric study on the polycystine radiolarian species Actinomma boreale (Cleve) from ten trigger weight core-tops from the Norwegian-Iceland Seas, three piston cores taken offshore western Norway and three surface sediment samples from Lygrepollen, Sogndalsfjord and Hoyangsfjord (western Norwegian fjords) shows a variation in morphology that groups A. boreale into three distinct clusters, interpreted to be related to different oceanographic settings. The largest specimens of A. boreale are found in the western Norwegian fjords, the smallest in the Iceland Sea, giving an apparent positive correlation with temperature. Down core studies in piston cores from the Norwegian Sea demonstrate a considerable size variation of the cortical shell of A. boreale. In the eastern Norwegian Sea, the climatically cold Younger Dryas had a population of A. boreale that was characterized by large cortical shells, while the climatically warm Holocene population was dominated by small sized cortical shells, showing a negative correlation with temperature. We suggest that the large sized conical shell population of A. boreale in the Younger Dryas is not reflecting precisely the sea-surface water temperature. Another factor must play the dominant role here, probably nutrients.

The late Pleistocene South Atlantic and Southern Ocean surface waters - A summary of time slice and time series studies

Central to global climate evolution is the paleoceanographic development of the South Atlantic as it represents the passageway for inter-hemispheric heat exchange within global thermohaline circulation (THC). Processes in the adjacent Southern Ocean regulate the heat import into the South Atlantic via the Agulhas “warm water route” (WWR) and the Drake Passage “cold water route” (CWR), and amplify climate change through various feedback mechanisms and teleconnections. For paleoceanographic reconstruction an inventory of new data sets and methods is now available, allowing for the estimation of Pleistocene sea-surface water temperatures and sea-ice distribution on time-slices and time-series based on the calcareous and siliceous microfossil record. Reconstruction of the Last Glacial Maximum (LGM) reveals distinct cooling in the Southern Ocean (up to 4 – 6 °C) accompanied by an expansion of winter and summer sea ice, cooling in the African upwelling regimes (up to 10°C) and in the Equatorial Atlantic ( 4 – 5 °C), but the Subtropical Gyre region remains relatively warm and unchanged compared with the present. While the WWR was not strongly altered during the LGM, heat transport via the CWR was most probably much weaker. The reconstruction of time-slices representing a warm climate end-member at the onset of the last climate cycle documents a distinct lead of southern high-latitudes in global climate development that also affects the south-west African upwelling regions. It is at the Marine Isotope Stage (MIS) 6/MIS 5 transition when Southern Ocean surface temperatures reach maximum values and sea ice is at a minimum, marking a period of South Atlantic heat piracy. During the isotopic minimum of MIS 5.5, the tropical South Atlantic was slightly colder than at present, likely the result of an enhanced poleward heat export. Time-series studies from key areas document that climate variability related to orbital forcing is overprinted by THC changes driven by meltwater injections into the North Atlantic and the Southern Ocean, changes in atmospheric circulation and greenhouse gas concentration, as well as sea ice that amplify climate change at global, hemispheric and regional scales. The study of centennial-scale variability during interglacial optima, such as MIS 5.5 and MIS 11, suggests that the presence of large ice sheets, meltwater events, changes in greenhouse gas concentration and sea-ice distribution are not the only prerequisite to trigger millennial-centennial-scale variability, but that another external agent, changes in solar irradiance, must be considered as an important factor in climate development.

Glacial‐interglacial size variability in the diatom Fragilariopsis kerguelensis: Possible iron/dust controls?

[1] The valve area of Fragilariopsis kerguelensis, the most abundant diatom species in the Southern Ocean, strongly changes in size in response to varying conditions in the surface ocean. We examined the link, both in two iron fertilization experiments and in sediment samples covering several glacial Terminations, between size variability in this species and environmental conditions across the Antarctic Polar Front, including sea ice extent, sea surface temperature, and the input of eolian dust. The iron fertilization experiments show valve area to be positively correlated with iron concentrations in ambient waters, which suggests the possibility of a causal relation between valve size of Fragilariopsis kerguelensis and ambient surface water iron concentration. Larger valves are usually found during glacial times and thus seem to be related to lower sea surface temperature and wider sea ice coverage. Moreover, our results indicate that there usually is a strong correlation between larger valve size and increased input of eolian dust to the Southern Ocean. However, this correlation, obvious for the fertilization experiments and for glacial Terminations I, II, III, and V, does not seem to be valid for Termination VI, where size appears to be inversely correlated to dust input. Citation: Cortese, G., R. Gersonde, K. Maschner, and P. Medley (2012), Glacial-interglacial size variability in the diatom Fragilariopsis kerguelensis: Possible iron/dust controls?, Paleoceanography, 27, PA1208, doi:10.1029/2011PA002187.