Late Quaternary paleoproductivity and deep water circulation in the eastern South Atlantic Ocean: Evidence from benthic foraminifera

Two late Quaternary sediment cores from the northern Cape Basin in the eastern South Atlantic Ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition. The locations of the cores were selected such that both of them presently are bathed by North Atlantic Deep Water (NADW) and past changes in deep water circulation should be recorded simultaneously at both locations. However, the areas are different in terms of primary production. One core was recovered from the nutrient-depleted Walvis Ridge area, whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high. Recent data served as the basis for the interpretation of the late Quaternary faunal fluctuations and the paleoceanographic reconstruction.

During the last 450,000 years, NADW flux into the eastern South Atlantic Ocean has been restricted to interglacial periods, with the strongest dominance of a NADW-driven deep water circulation during interglacial stages 1, 9 and 11. At the continental margin, high productivity faunas and very low epibenthic δ¹³C values indicate enhanced fluxes of organic matter during glacial periods. This can be attributed to a glacial increase and lateral extension of coastal upwelling. The long term glacial-interglacial paleoproductivity cycles are superimposed by high-frequency variations with a period of about 23,000 yr. Enhanced productivity in surface waters above the Walvis Ridge, far from the coast, is indicated during glacial stages 8, 10 and 12. During these periods, cold, nutrient-rich filaments from the mixing area were probably driven as far as to the southeastern flank of the Walvis Ridge.     

Late Quaternary development of the Java upwelling system (eastern Indian Ocean) as revealed by coccolithophores

The paleoceanographic potential of coccolithophores was used to decipher the paleoproductivity changes in the eastern Indian Ocean during the past 300,000 years. Core SO139-74KL was taken at the seaward limit of a fore-arc basin of the Indonesian continental shelf located beneath the Java upwelling system. Coccolithophores occur in all samples, and total coccolith concentration exhibit distinct variations over the entire section. Peak abundances occur every 20,000 to 25,000 years with the highest peak at isotope stage 7. Abundances increase during the glacials but peak abundances also occur during interglacials. The preservation of coccoliths is good to moderate in most of the samples. The most abundant species is Florisphaera profunda with a mean relative abundance of 41.5% followed by Gephyrocapsa ericsonii and Emiliania huxleyi (EhuxGeric) and Gephyrocapsa oceanica. These four taxa dominate the assemblage throughout the core, forming on average 90.5% of the total assemblage. The species composition suggests that warm tropical conditions prevailed throughout the investigated time period indicating that temperature was not the driving force for the assemblage variations at this site. The geologic record for present-day and Holocene oceanographic conditions seemed to be predominantly characterised by high productivities in combination with an unstable water column. Indications for oligotrophic open ocean conditions were sparse. However, during most of the year oligotrophic conditions prevail and upwelling recurs only for a short time period but upwelling indicating proxies dominate the geological record. A contrasting fully oligotrophic scenario characterised by peaks in the abundances of total coccolithophores, Umbellosphaera irregularis, and in the percentage ratio of EhuxGeric to G. oceanica can be seen with a periodical recurrence every 20,000 to 25,000 years. Synchronously the records of the high productivity indicators total organic carbon and G. oceanica are characterised by distinct minima. We believe that upwelling was totally cut off during these times and oligotrophic conditions with a pronounced water column stratification prevailed throughout all seasons. An obvious correspondence between the shut down times of upwelling and insolation minima suggests that surface water conditions were driven by orbital forcing.     

The surface ocean productivity response of deeper water benthic foraminifera in the Atlantic Ocean

We test the relationship of deep sea benthic foraminiferal assemblage composition to the surface ocean productivity gradient in the low latitude Atlantic Ocean using 81 surface sediment samples from a water depth range between 2800 and 3500 m. The samples are selected so that the surface ocean productivity gradient, controlling the flux of organic carbon to the seabed, will be the most important environmental variable. The first two principal components of the assemblage data account for 73% of data variance and are clearly linked to the productivity gradient across the Atlantic. These components show that under higher productivity the assemblages contain a higher abundance of Uvigerina peregrina, Melonis barleeanum, Globobulimina spp. and other taxa with probable infaunal microhabitats. Alabaminella weddellensis, a species linked to episodic phytoplankton debris falls, is also important in these assemblages. As productivity decreases there is a regular shift in assemblage composition so that low productivity assemblages are dominated by Globocassidulina subglobosa and several Cassidulina species along with Epistominella exigua. We hypothesize that these taxa are epifaunal to very shallow infaunal since nearly all organic carbon oxidation occurs near the sediment-water interface in low productivity settings. Discriminant function analysis of the foraminiferal assemblages, with groups selected on the basis of surface ocean productivity, shows clear separation among five productivity levels we used. This analysis demonstrates that productivity variations have a strong influence on assemblage composition. Finally, we used two groups of samples from the Rio-Grande Rise representing water depths from 2007 to 2340 m and 2739 to 3454 m to test for effects produced by changing water depth. All these samples are from a low productivity region and represent nearly identical environmental conditions. Although the low productivity nature of all the Rio-Grande Rise samples is obvious, there are assemblage differences between our depth groups. We cannot account for the assemblage differences with changes in organic carbon flux, dissolution effects or other physical/chemical properties of the ocean. Thus there are as yet unidentified factors related to water depth which cause some assemblage variation in the low productivity setting we investigated.     

Forest response to climate changes in Atlantic Equatorial Africa during the last 4000 years BP and inheritance on the modern landscapes

This review paper synthesizes the recent published palaeoecological results obtained in Atlantic Equatorial Africa (ECOFIT program) on the history of forest ecosystems and inferred climate changes during the past 4000 years. Evidence are mainly provided by pollen analysis carried out at nine sites from Congo, Cameroon and Ghana, locally supported by macroflora remains, phytoliths, diatoms, δ¹³C and mineralogical data. At all the sites, except Lake Bosumtwi (Ghana), following a large expansion of rain and mesophilous forests until 3000 years BP , a major change is registered, affecting floristic composition, structure and geographical distribution. According to the hydrological sensitivity of the different sites, local openings of the forests with development of heliophilous formations and/or isolated enclosed savannas are observed at the most humid sites; complete disappearance of forested formations at the driest. The agreement between pollen records, hydrological and hydrobiological data definitely demonstrates that an arid event has been the primary driving factor of this change and is responsable for the main features of the modern landscapes in Atlantic Equatorial Africa. Moreover, the most recent palaeoecological data obtained in Congo (Lake Sinnda), indicate that this Late Holocene increasing aridity was of longer duration, from 4000 to 1300 years BP , and more progressive than previously inferred. A new expansion of forests is locally detected c. 900–600 BP despite increased human impact.     

Late Quaternary millennial-scale variability in pelagic aragonite preservation off Somalia

In order to better understand Late Quaternary pelagic aragonite preservation in the western Arabian Sea we have investigated a high-resolution sediment core 905 off Somalia. Pteropod preservation is enhanced in times of reduced monsoon-driven productivity, indicated by low amounts of C_org and low barium to aluminium (Ba/Al) ratios. All periods corresponding to Heinrich events in the North Atlantic are represented by maxima in shell preservation of the common pteropod Limacina inflata (LDX values < 2, except for H5-equivalent with a poorer shell preservation, LDX > 2.66). Good shell preservation is also found during stadials at 52.1–53.2, 36, 33.2, and 31.9 ka. Relative abundance of pteropods and their fragments in the coarse fraction reaches maxima during Marine Isotope Stage (MIS) 5.2, during time-equivalents of Heinrich events 4–6 and in stadials at  53,  42.5, and 41.4 ka.On longer time scales, the pteropod abundance corresponds to the ‘Indo-Pacific carbonate preservation type’ with poor preservation during interglacials and better preservation during glacials. Late MIS 5 to early MIS 4 sections (84.1–64.8 ka) and the Late Holocene interval (6.5–0 ka) of core 905 contain only traces of pteropods. The early Holocene (9.2–6.5 ka) part is characterized by low pteropod amounts. Between 64.8 and 43.4 ka strong fluctuations occur and an intermediate average relative pteropod abundance is revealed. Between 43.4 and 9.2 ka the highest amounts in relative pteropod abundance in core 905 are observed. Besides the regional monsoonal influence on deepwater chemistry, changes in deepwater circulation occurring on glacial/interglacial and stadial/interstadial time scales might have affected pteropod preservation. However, it remains elusive whether 1) deep water formation in the Arabian Sea, 2) inflow of Glacial North Atlantic Intermediate Water or 3) change in water mass properties of the Circumpolar Deep Water (which is the water mass currently bathing this site) contributed to the observed pteropod preservation pattern.     

Late Quaternary human settlement patterning in the Jebel Gharbi

This paper presents the latest results of geoarchaeological research on the Upper Pleistocene sequence in the Jebel Gharbi (previously called Jebel Nafusah), a mountain range located in Tripolitania, northwestern Libya. Numerous archaeological sites are found adjacent to springs that formed as a consequence of tectonic activities. The springs originated when Upper Pleistocene earthquakes produced ground displacements that created water outlets, some of which are still active. Springs are spread all along the massif and at the foot of the mountains in Jebel Gharbi. We suggest that they offered attractive resources to populations coming from drier parts of North Africa or the near-by Sahara. The earliest sites associated with the springs include Aterian lithic techno-complexes that have been dated between 80,000 and 40,000 BP. Since then, these springs have attracted many populations, as documented here by settlements belonging to the Later Stone Age (Upper Palaeolithic), Iberomaurusian (Late Upper Palaeolithic or Epipalaeolithic), Neolithic, Roman period, and present time.     

Parallel adaptive evolution of Atlantic cod on both sides of the Atlantic Ocean in response to temperature

Despite the enormous economic and ecological importance of marine organisms, the spatial scales of adaptation and biocomplexity remain largely unknown. Yet, the preservation of local stocks that possess adaptive diversity is critical to the long-term maintenance of productive stable fisheries and ecosystems. Here, we document genomic evidence of range-wide adaptive differentiation in a broadcast spawning marine fish, Atlantic cod (Gadus morhua), using a genome survey of single nucleotide polymorphisms. Of 1641 gene-associated polymorphisms examined, 70 (4.2%) tested positive for signatures of selection using a Bayesian approach. We identify a subset of these loci (n = 40) for which allele frequencies show parallel temperature-associated clines (p < 0.001, r² = 0.89) in the eastern and western north Atlantic. Temperature associations were robust to the statistical removal of geographic distance or latitude effects, and contrasted ‘neutral’ loci, which displayed no temperature association. Allele frequencies at temperature-associated loci were significantly correlated, spanned three linkage groups and several were successfully annotated supporting the involvement of multiple independent genes. Our results are consistent with the evolution and/or selective sweep of multiple genes in response to ocean temperature, and support the possibility of a new conservation paradigm for non-model marine organisms based on genomic approaches to resolving functional and adaptive diversity.     

Reproductive biology of Vinciguerria nimbaria in the equatorial waters of the eastern Atlantic Ocean

Adult Vinciguerria nimbaria in an area of the Atlantic Ocean (0–5°N; 10–20°W), collected from November 1994 to February 1998, exhibited two different patterns in diel behaviour: 'typical' behaviour which consisted of large diel vertical migration and 'atypical', characterized by concentrations of schools that remained in the surface layers during the daytime. The total life span of V. nimbaria was estimated at 6–7 months. Females were mature when they reached 30·6 mm standard length ( L _S), which corresponded to an age of 85 days. Once the females were mature, spawning was continuous in the population as a whole, and V. nimbaria spawned continuously throughout the year. Spawning took place during two restricted times of the day: typical females spawned at dusk and atypical females spawned at dawn. Using the post-ovulatory follicles method, spawning frequency was estimated at 2 days. Batch fecundity was estimated at 1236 oocytes or 1230 oocytes g⁻¹ of total body mass, and egg size was 650 µm whatever the period of the year. The lifetime fecundity of V. nimbaria was estimated at 9000 eggs (109 000 eggs if mortality rate was not taken into account), and the maximum stock egg production of a theoretical cohort occurred at 37 mm L _S. Young adults thus contributed the most to the reproductive output for the survival of the population.     

Fluvial response to late Quaternary climate change in NE Queensland, Australia

Samples of alluvial and colluvial deposits from the coastal plain and coastal valleys north and south of Cairns (17°S) have been dated using optically stimulated luminescence (OSL) protocols, with additional thermoluminescence (TL) and Radiocarbon assays. Coarse fanglomerates from elevated coastal terraces date back to 81 ka, but most are clustered in oxygen isotope stage three (OIS3, 64-28 ka), indicating high-energy conditions during this period. Extensive fans and terraces of finer calibre sediments are widely represented grading from coarse gravels and cobbles in proximal zones fine sand and silt in distal areas. Dates show that vertical accumulation of 10-15 m of sediment occurred between ∼ 28 and 14 ka (OIS2), after which the fans were dissected and Holocene deposits become fragmentary. A number of deposits indicating hillslope instability were successfully dated and these fall mainly into OIS3 and post 12 ka. These results are interpreted as catchment responses to Late Pleistocene climate and vegetation changes, documented elsewhere from local pollen and ocean drilling sites. Correlation with these records and with evidence for regional climate change from the Austral-Asian region is good and indicates that these changes were sufficient to transform fluvial activity and slope processes.     

The significance of extant coccolithophores as indicators of ocean water masses, surface water temperature, and palaeoproductivity: a review

Coccolithophores are one of the main groups of marine phytoplankton playing key roles in the marine eco-system as primary producers and in marine biogeochemistry. These organisms have gained considerable attention as they play a unique role in the global carbon cycle because of their combined effects on both the organic carbon and the carbonate pump. In addition to steady advances in research on coccoliths as biogeochemical agents and palaeontological proxies were obtained knowledge of the biology of these organisms has progressed considerably in recent years. It is now clear that holococcolithophores are not autonomous but stages in the life cycle of heterococcolithophores. A general introduction to the taxonomy and biology of extant coccolithophores is followed by a brief overview of the environmental parameters affecting these phytoplanktonic organisms and their biogeography. Another chapter summarizes the investigations on coccolithophores in the Arabian Sea which were conducted during the past years to produce a synthesis of the production of living coccolithophores in the photic zone, their transformation to settling assemblages, their accumulation on the seafloor, and their final burial in the Sediments. In the following, applications of coccolithophores to palaeoenvironmental analyses are provided as case studies. In particular, the usage of both ecologically restricted species, such asFlorisphaera profunda, for palaeoproductivity studies and of a new coccolithophore-based palaeother-mometry for surface-water reconstructions are presented. In addition, haptophyte-specific biomarkers (long-chain alkenones) are reviewed and their applicability for palaeoceanographical reconstructions is demonstrated.      

Understanding Late Quaternary extinctions: the case of Myotragus balearicus (Bate, 1909)

Aim In this study we present a new view on the extinction of Myotragus balearicus, an extinct highly modified dwarf caprine from the Gymnesic Islands (or eastern Balearic Islands), as a methodological case study for interpretation of Late Quaternary extinctions (LQEs). Methods We analyse all available ¹⁴C ages obtained from M. balearicus bones from the uppermost part of the Pleistocene and the Holocene, together with the available chronological data of the putative causes of Myotragus extinction. Results It has been possible to define two critical dates that allow us to establish an ‘uncertainty period for the Myotragus extinction’ (UPME) in each analysed island (Mallorca, Menorca and Cabrera). For Mallorca, the UPME corresponds to the interval c. 3700 to 2030 calbc (i.e. c. 1670 years of uncertainty). In the case of Menorca, the UPME spans from 10,000 to 1930 calbc (8070 years of uncertainity). In Cabrera the UPME is placed between 3650 and 300 calbc (3350 years of uncertainty). These periods, together with a review of the available information on the chronology of human arrival and the chronology of Holocene climatic change, shed light on the possible causes of the extinction of this species. Main conclusions Extinction of Myotragus because of climatic change can be definitively rejected. The Myotragus extinction must be attributed to the rapid effects of the first human occupation. The use of uncertainity periods for the disappearance of species represents a useful tool for the analysis of LQEs.     

Climate‐driven ecological stability as a globally shared cause of Late Quaternary megafaunal extinctions: the Plaids and Stripes Hypothesis

Controversy persists about why so many large‐bodied mammal species went extinct around the end of the last ice age. Resolving this is important for understanding extinction processes in general, for assessing the ecological roles of humans, and for conserving remaining megafaunal species, many of which are endangered today. Here we explore an integrative hypothesis that asserts that an underlying cause of Late Quaternary megafaunal extinctions was a fundamental shift in the spatio‐temporal fabric of ecosystems worldwide. This shift was triggered by the loss of the millennial‐scale climate fluctuations that were characteristic of the ice age but ceased approximately 11700 years ago on most continents. Under ice‐age conditions, which prevailed for much of the preceding 2.6 Ma, these radical and rapid climate changes prevented many ecosystems from fully equilibrating with their contemporary climates. Instead of today's ‘striped’ world in which species' ranges have equilibrated with gradients of temperature, moisture, and seasonality, the ice‐age world was a disequilibrial ‘plaid’ in which species' ranges shifted rapidly and repeatedly over time and space, rarely catching up with contemporary climate. In the transient ecosystems that resulted, certain physiological, anatomical, and ecological attributes shared by megafaunal species pre‐adapted them for success. These traits included greater metabolic and locomotory efficiency, increased resistance to starvation, longer life spans, greater sensory ranges, and the ability to be nomadic or migratory. When the plaid world of the ice age ended, many of the advantages of being large were either lost or became disadvantages. For instance in a striped world, the low population densities and slow reproductive rates associated with large body size reduced the resiliency of megafaunal species to population bottlenecks. As the ice age ended, the downsides of being large in striped environments lowered the extinction thresholds of megafauna worldwide, which then increased the vulnerability of individual species to a variety of proximate threats they had previously tolerated, such as human predation, competition with other species, and habitat loss. For many megafaunal species, the plaid‐to‐stripes transition may have been near the base of a hierarchy of extinction causes whose relative importances varied geographically, temporally, and taxonomically.     

Diatom-inferred productivity changes in the eastern equatorial Pacific: The quaternary record of ODP leg 111, site 677

Recent marine planktonic diatom assemblages from 47 sediment surface samples retrieved from the eastern equatorial Pacific yielded four significant Q-mode factors that are related to surface water properties. They include a northern subtropical (Factor 1), a southern subtropical (Factor 2), a Peru Current (Factor 3) and an eastern Equatorial Pacific (Factor 4) factor.A transfer function (based on 42 surface sediment samples) with a productivity range of 109 to 210 gC m^–2 y^–1 was defined using the geographic distribution of the northern and southern subtropical assemblages (Factor 1 and 2) and their relation to mapped surface water primary productivity. This transfer function has a standard error of estimate of 11.9 gC m^–2 y^–1 and it is used to estimate late Pleistocene paleo-productivity changes at ODP Leg 111 Site 677 off Ecuador.The Site 677 samples (usually spaced 10 cm apart and with a linear sedimentation rate of 4.3 cm y^–3) were the same samples that were used for stable oxygen/carbon isotopic analysis from planktonic Foraminifera. The well constrained oxygen isotope stratigraphy permitted assignment of accurate ages to downcore samples.The interval between oxygen isotope stages 3 and 9 had variable productivity that is generally higher than present values of 175 gC m^–2 y^–1 A major decline in paleo-productivity below the present value occurred in stage 9, the early part of stage 8 and in the early and late stage 5.     

The last 100,000 years in the western Mediterranean: sea surface water and frontal dynamics as revealed by coccolithophores

A quantitative analysis was carried out on coccolith assemblages from two Pleistocene cores (K1 and K10) from the western Mediterranean. The distribution of selected coccolithophore species provides new paleoclimatic and paleoceano-graphic data. A continuous sequence from the top of Isotope Stage 5 to the Holocene was recorded. The reversal in dominance between Gephyrocapsa muellerae and Emiliania huxleyi was dated in both cores at ca. 73 ka. At about 47 ka, E. huxleyi shows a regular increase, whereas G. muellerae progressively decreases in abundance. During interglacial periods, high concentrations of coccoliths are observed, whereas in glacial times, coccoliths are more diluted and the percentage of reworked forms increases as a consequence of the higher terrigenous input. After taking careful account of the dilution factor, we conclude that the production of coccolithophores was higher during warm periods. Maxima in coccolith concentrations coincide with highstand episodes, probably as a result of the intensification of the Atlantic flux into the Mediterranean across the Gibraltar Strait. This intensification could have produced an increase in nutrient content in the surface Mediterranean waters. During cold periods, the western Mediterranean front underwent a reduction in activity, probably due to an increase in the saline and/or thermal gradients between the superficial waters, and intermediate waters in the Liguro Provençal basin.     

Late Quaternary variations in water mass in the Shatsky Rise area, northwest Pacific Ocean

Oxygen isotope analysis of planktonic and benthic foraminifera in piston core S-2 collected from the Shatsky Rise (33°21.75N, 159°07.70E; water depth 3107 m) provides a paleoceanographic record for the last 540 000 years in the northwestern Pacific Ocean. Although peaks in the abundance of sinistral Neogloboquadrina pachyderma occur during Marine Isotope Stage 2, and particularly 6 and 12, the southward shifting of the Subarctic front did not reach the core site during these glacial periods. However, mass accumulation rates of total organic carbon, biogenic opal, and terrigenous matter contents indicate that surface productivity increased during cold periods. In addition, the C/N ratio analyzed in organic matter reached values of up to 10 during glacial periods. These results imply that delivery of eolian dust to this site was enhanced by strengthened westerly winds during glacial periods. Down-core fluctuations in δ¹³C values of Globigerinoides ruber and Globorotalia inflata nearly overlap, particularly during the period from 540 to 260 ka. This latter trend suggest that the subtropical surface water mass prevailed at the core site throughout that period, based upon the very small vertical δ¹³C gradient through water column in modern Kuroshio Current water.     

Chronology of the major palaeohydrological events in NW Africa during the late Quaternary: PALHYDAF results

On the basis of African examples, the paper draws attention to some geochemical and sedimentological problems which commonly occur in the establishment of a reliable chronology from lacustrine sediments. New results are then provided on the ages of the successive wet climatic phases in NW Africa since 150 ka ago.     

Abyssal Megafaunal Necrophages: Latitudinal Differences in the Eastern North Atlantic Ocean

A pop-up time-lapse camera system (BATHYSNACK) was used to study megafaunal organisms attracted to bait at three abyssal localities in the eastern North Atlantic Ocean, a northern site on the Porcupine Abyssal Plain and two sub-tropical sites on the Madeira Abyssal Plain. Fish and decapod crustaceans appeared on 47% of photographs taken during deployments of up to seven days duration. The fishes Coryphaenoides (Nematonurus) armatus and Pachycara bulbiceps and the decapod Munidopsis spp. dominated appearances at the northern locality whereas over 95% of records at the two subtropical localities were of the decapod Plesiopenaeus armatus. Cyclical variations in necrophage abundance did not correlate with tidal cycles of near-bottom currents, although high P. armatus numbers were inversely related to current speed. Most of the frequently recorded entities showed contagious intraspecific distributions. In contrast, many of the interspecies relationships were non-random, with co-occurrences lower than expected, probably as a result of physical disturbance or potential predation. Population densities based on times of first arrival were calculated. Assemblages dominated by fish at higher latitudes and crustaceans at lower latitudes are discussed in terms of organic carbon supply to the deep sea and pelagic community structures.     

Late Quaternary Palaeoichnological Sites from the Southern Atlantic Coast of Buenos Aires Province,Argentina: Mammal,Bird and Hominid Evidence

Palaeontological and palaeoichnological studies were made along the southeastern coast of Buenos Aires province, Argentina. Continental Late Pleistocene and continental, transitional and marine Holocene stratigraphic units were identified. The former are represented by fossil mammal bones and ichnites dated from 16,000 to 12,000 y. B.P. Continental Early Holocene beds dated circa 8,100 years B.P. contain fish, rodent and bird remains. Middle Holocene sediments 6,930 to 6,570 years B.P. are recorded by deposits bioturbated by estuarine crabs. During the Late Holocene, between 5,000 and 4,800 B.P., a marine transgression is recorded by a very rich mollusc fauna. The palaeoenvironmental variety recorded along a few tens of kilometers of the coast is a distinctive feature of this part of the Argentine coastal region. At Monte Hermoso beach, in the eastern part of the studied zone, archaeological materials were dated 7,300 years B.P., and human footprints dated 7,125 years B.P. were also recorded. In January 2005 some isolated human footprints were found at Pehuen-Co palaeoichnological site, which are older than those found at Monte Hermoso beach and of the same age as the oldest human archaeological findings in Patagonia, that is about 12,000 yeas B.P.