Warm Intervals Research Articles

North Atlantic versus Southern Ocean contributions to a deglacial surge in deep ocean ventilation

Research Article| June 01, 2013 North Atlantic versus Southern Ocean contributions to a deglacial surge in deep ocean ventilation L.C. Skinner; L.C. Skinner 1Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar A.E. Scrivner; A.E. Scrivner 1Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar D. Vance; D. Vance 2School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar S. Barker; S. Barker 3School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar S. Fallon; S. Fallon 4Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar C. Waelbroeck C. Waelbroeck 5LCSE/IPSL, Laboratoire CNRS-CEA-UVSQ, Bat. 12, Avenue de la Terrasse, F-91198, Gif-sur-Yvette CEDEX, France Search for other works by this author on: GSW Google Scholar Author and Article Information L.C. Skinner 1Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK A.E. Scrivner 1Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK D. Vance 2School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK S. Barker 3School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK S. Fallon 4Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia C. Waelbroeck 5LCSE/IPSL, Laboratoire CNRS-CEA-UVSQ, Bat. 12, Avenue de la Terrasse, F-91198, Gif-sur-Yvette CEDEX, France Publisher: Geological Society of America Received: 15 Oct 2012 Revision Received: 12 Jan 2013 Accepted: 15 Jan 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (6): 667–670. https://doi.org/10.1130/G34133.1 Article history Received: 15 Oct 2012 Revision Received: 12 Jan 2013 Accepted: 15 Jan 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation L.C. Skinner, A.E. Scrivner, D. Vance, S. Barker, S. Fallon, C. Waelbroeck; North Atlantic versus Southern Ocean contributions to a deglacial surge in deep ocean ventilation. Geology 2013;; 41 (6): 667–670. doi: https://doi.org/10.1130/G34133.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Past glacial-interglacial climate transitions were accompanied by millennial-scale pulses in atmospheric CO2 that are widely thought to have resulted from the release of CO2 via the Southern Ocean. However, direct proxy evidence for a Southern Ocean role in regulating past ocean-atmosphere CO2 exchange is scarce. Here we use combined radiocarbon and neodymium isotope measurements from the last deglaciation to confirm greatly enhanced overturning and/or air-sea exchange rates relative to today, in particular during the Bølling-Allerød warm interval. We show that this deglacial pulse in ocean ventilation was not driven by the North Atlantic overturning alone, and must have involved an increase in the ventilation of southern-sourced deep waters. Our results thus confirm the removal of a physical and/or dynamical barrier to effective air-sea (CO2) exchange in the Southern Ocean during deglaciation, and highlight the Antarctic region as a key locus for global climate/carbon-cycle feedbacks. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Abrupt changes in deep Atlantic circulation during the transition to full glacial conditions

Six Ocean Drilling Program (ODP) sites, in the Northwest Atlantic have been used to investigate kinematic and chemical changes in the “Western Boundary Undercurrent” (WBUC) during the development of full glacial conditions across the Marine Isotope Stage 5a/4 boundary (~70,000 years ago). Sortable silt mean grain size measurements are employed to examine changes in near bottom flow speeds, together with carbon isotopes measured in benthic foraminifera and % planktic foraminiferal fragmentation as proxies for changes in water‐mass chemistry. A depth transect of cores, spanning 1.8–4.6 km depth, allows changes in both the strength and depth of the WBUC to be constrained across millennial scale events. measurements reveal that the flow speed structure of the WBUC during warm intervals (“interstadials”) was comparable to modern (Holocene) conditions. However, significant differences are observed during cold intervals, with higher relative flow speeds inferred for the shallow component of the WBUC (~2 km depth) during all cold “stadial” intervals (including Heinrich Stadial 6), and a substantial weakening of the deep component (~3–4 km) during full glacial conditions. Our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation (involving Glacial North Atlantic Intermediate Water) that was quantitatively distinct from preceding cold stadial events. Furthermore, our chemical proxy data show that the physical response of the WBUC during the last glacial inception was probably coupled to basin‐wide changes in the water‐mass composition of the deep Northwest Atlantic.

Temperature changes over the past 2000 yr in China and comparison with the Northern Hemisphere

Abstract. We use principal component regression and partial least squares regression to separately reconstruct a composite series of temperature variations in China, and associated uncertainties, at a decadal resolution over the past 2000 yr. The reconstruction is developed using proxy temperature data with relatively high confidence levels from five regions across China, and using a temperature series from observations by the Chinese Meteorological Administration, covering the period from 1871 to 2000. Relative to the 1851–1950 climatology, our two reconstructions show four warm intervals during AD 1–AD 200, AD 551–AD 760, AD 951–AD 1320, and after AD 1921, and four cold intervals during AD 201–AD 350, AD 441–AD 530, AD 781–AD 950, and AD 1321–AD 1920. The temperatures during AD 981–AD 1100 and AD 1201–AD 1270 are comparable to those of the Present Warm Period, but have an uncertainty of ±0.28 °C to ±0.42 °C at the 95% confidence interval. Temperature variations over China are typically in phase with those of the Northern Hemisphere (NH) after 1000, a period which covers the Medieval Climate Anomaly, the Little Ice Age, and the Present Warm Period. In contrast, a warm period in China during AD 541–AD 740 is not obviously seen in the NH.

The amplification of Arctic terrestrial surface temperatures by reduced sea-ice extent during the Pliocene

Many past warm periods exhibited greatly reduced latitudinal temperature gradients as a result of amplified Arctic surface temperatures as well as more seasonably equable temperatures. The Pliocene is a period of particular interest because CO2 forcing was comparable to today and yet Arctic temperatures were significantly warmer than today. Here we describe an atmospheric general circulation model experiment assessing the response of terrestrial temperatures in the mid-Pliocene (3.02 to 3.26Ma) to an ice-free Arctic, and we compare the simulation with a compilation of proxy-based Pliocene paleotemperature reconstructions. Our experiments indicate that the amplification of Arctic surface temperatures is much more sensitive to the extent of sea ice than continental ice. The removal of Arctic sea ice results in simulated mean annual surface temperatures that better match terrestrial proxy data (RMSE=2.9°C) than experimental conditions that included seasonal sea ice (RMSE=4.5°C). Our simulations also show a decrease in the seasonal amplitude of temperatures in the absence of sea-ice, which is consistent with theory predicting more equable climates in the Arctic during warmer intervals in Earth's history. Our results demonstrate that once sea-ice is removed, latent heat is lost from the ocean to the atmosphere as water vapor that can be circulated by the atmosphere, which results in warming of continental interiors. Although our sensitivity experiment does not help to identify the full array of feedback mechanisms responsible for the amplification of Arctic surface temperatures during the Pliocene, it does demonstrate that Arctic terrestrial surface temperatures are extremely sensitive to the spatial and seasonal extent of sea-ice.

The role of foraminifera as indicators of the Late Pleistocene–Holocene palaeoclimatic fluctuations on the deltaic environment: The example of Tiber delta succession (Tyrrhenian margin, Italy)

The palaeoclimatic and palaeoenvironmental reconstruction of the last 39.0 kyr of the central Tyrrhenian shelf are presented in this paper. Micropalaeontological, sedimentological and stable isotopic data integrated by radiocarbon dates were determined from three cores drilled in the Tiber delta sequence (TDS), which constitutes one of the most complete Late Quaternary high-frequency depositional sequences in the Mediterranean area. Short-period climate variations that induced short-period events caused changes in the Tiber run-off and increased the sediment supply and grain size in the delta system. The three cores, with different thicknesses, record all the system tracts forming the TDS. Three intervals corresponding to the Early and Late Lowstand, Transgressive and Highstand System Tracts (TST and HST) were recognised. The first is characterised by shoreface/inner shelf deposits and corresponds to a general cooling period characterised by short warmer periods (approximately 1.0 kyr), confirming the climatic variability of MIS 3/MIS 2. A very condensed deposition affected the second interval, which is characterised by a very low sedimentation rate corresponding to the TST of TDS. The third interval, corresponding to Holocene shelf deposits, shows small climatic oscillations that mark an enhanced seasonality typical of the modern Tyrrhenian regime. At the millennial scale, two warm intervals (Copper Age and Late Bronze Age/II century AC) alternating with two cold humid intervals (Bond cycle B3/RCC: 4.2–3.8 kyr BP of Mayewski events and the Little Ice Age) have been recognised.

Tree-ring based summer minimum temperature reconstruction for the southern edge of the Qinghai-Tibetan Plateau, China

Information about historical summer temperature variability is rare on the Qinghai- Tibetan Plateau (QTP). Here we report a 290 yr summer (June to August) mean minimum tem - perature (SMMT) record reconstructed from tree-ring widths of alpine juniper Sabina squamata in Yadong County at the southern edge of QTP, China. The reconstruction accounts for 51.2% of the SMMT variance in the instrumental period from 1956 to 2002. Five cold intervals in 1725�1734, 1745�1789, 1817�1825, 1860�1869 and 1967�1978, and 4 warm intervals in 1735� 1744, 1790�1816, 1870�1879 and 1990�2002 were identified. Wavelet analysis of the recon- structed temperatures suggested the existence of 2 to 7 yr cycles in discontinuous periods and 20 to 25 yr cycles in the 1850s to 1880s. The reconstructed summer temperature variations were consistent with other temperature records from tree rings, ice cores and glacier activities on the QTP, which demonstrate the fidelity of the reconstruction and suggested that the climate in the study region was part of a large-scale climate system. The 1980s to 2000s were the warmest period in the last 3 centuries in our tree-ring records, but the amplitude of the warming was still within the natural variation.

Late Holocene climate and environmental changes in Kamchatka inferred from the subfossil chironomid record

This study presents a reconstruction of the Late Holocene climate in Kamchatka based on chironomid remains from a 332 cm long composite sediment core recovered from Dvuyurtochnoe Lake (Two-Yurts Lake, TYL) in central Kamchatka. The oldest recovered sediments date to about 4500 cal years BP. Chironomid head capsules from TYL reflect a rich and diverse fauna. An unknown morphotype of Tanytarsini, Tanytarsus type klein, was found in the lake sediments. Our analysis reveals four chironomid assemblage zones reflecting four different climatic periods in the Late Holocene. Between 4500 and 4000 cal years BP, the chironomid composition indicates a high lake level, well-oxygenated lake water conditions and close to modern temperatures (∼13 °C). From 4000 to 1000 cal years BP, two consecutive warm intervals were recorded, with the highest reconstructed temperature reaching 16.8 °C between 3700 and 2800 cal years BP. Cooling trend, started around 1100 cal years BP led to low temperatures during the last stage of the Holocene. Comparison with other regional studies has shown that termination of cooling at the beginning of late Holocene is relatively synchronous in central Kamchatka, South Kurile, Bering and Japanese Islands and take place around 3700 cal years BP. From ca 3700 cal years BP to the last millennium, a newly strengthened climate continentality accompanied by general warming trend with minor cool excursions led to apparent spatial heterogeneity of climatic patterns in the region. Some timing differences in climatic changes reconstructed from chironomid record of TYL sediments and late Holocene events reconstructed from other sites and other proxies might be linked to differences in local forcing mechanisms or caused by the different degree of dating precision, the different temporal resolution, and the different sensitive responses of climate proxies to the climate variations. Further high-resolution stratigraphic studies in this region are needed to understand the spatially complex pattern of climate change in Holocene in Kamchatka and the surrounding region.

Synchronous Change of Atmospheric CO 2 and Antarctic Temperature During the Last Deglacial Warming

Understanding the role of atmospheric CO2 during past climate changes requires clear knowledge of how it varies in time relative to temperature. Antarctic ice cores preserve highly resolved records of atmospheric CO2 and Antarctic temperature for the past 800,000 years. Here we propose a revised relative age scale for the concentration of atmospheric CO2 and Antarctic temperature for the last deglacial warming, using data from five Antarctic ice cores. We infer the phasing between CO2 concentration and Antarctic temperature at four times when their trends change abruptly. We find no significant asynchrony between them, indicating that Antarctic temperature did not begin to rise hundreds of years before the concentration of atmospheric CO2, as has been suggested by earlier studies.

Changes in Pacific Ocean circulation following the Miocene onset of permanent Antarctic ice cover

We integrate micropaleontological and geochemical records (benthic stable isotopes, neodymium isotopes, benthic foraminiferal abundances and XRF-scanner derived elemental data) from well-dated Pacific Ocean successions (15–12.7Ma) to monitor circulation changes during the middle Miocene transition into a colder climate mode with permanent Antarctic ice cover. Together with previously published records, our results show improvement in deep water ventilation and strengthening of the meridional overturning circulation following major ice expansion at ∼13.9Ma. Neodymium isotope data reveal, however, that the provenance of intermediate and deep water masses did not change markedly between 15 and 12.7Ma. We attribute the increased δ13C gradient between Pacific deep and intermediate water masses between ∼13.6 and 12.7Ma to more vigorous entrainment of Pacific Central Water into the wind-driven ocean circulation due to enhanced production of intermediate and deep waters in the Southern Ocean. Prominent 100kyr ventilation cycles after 13.9Ma reveal that the deep Pacific remained poorly ventilated during warmer intervals at high eccentricity, whereas colder periods (low eccentricity) were characterized by a more vigorous meridional overturning circulation with enhanced carbonate preservation. The long-term δ13C decline in Pacific intermediate and deep water sites between 13.5 and 12.7Ma reflects a global trend, probably related to a re-adjustment response of the global carbon cycle following the last 400kyr carbon maximum (CM6) of the “Monterey Excursion”.

Estimated strength of the Atlantic overturning circulation during the last deglaciation

The Atlantic meridional overturning circulation affects the latitudinal distribution of heat, and is a key component of the climate system. Proxy reconstructions, based on sedimentary 231 Pa= 230 Th ratios and the difference between surface- and deep-water radiocarbon ages, indicate that during the last glacial period, the overturning circulation was reduced during millennial-scale periods of cooling 1‐5 . However, much debate exists over the robustness of these proxies 6‐8 . Here we combine proxy reconstructions of sea surface and air temperatures and a global climate model to quantitatively estimate changes in the strength of the Atlantic meridional overturning circulation during the last glacial period. We find that, relative to the Last Glacial Maximum, the overturning circulation was reduced by approximately 14 Sv during the cold Heinrich event 1. During the Younger Dryas cold event, the overturning circulation was reduced by approximately 12 Sv, relative to the preceding warm interval. These changes are consistent with qualitative estimates of the overturning circulation from sedimentary 231 Pa= 230 Th ratios. In addition, we find that the strength of the overturning circulation during the Last Glacial Maximum and the Holocene epoch are indistinguishable within the uncertainty of the reconstruction. The Atlantic meridional overturning circulation (AMOC) is an important feature of the Earth’s climate system. It is characterized by northward-flowing water masses at the surface ocean, deepwater formation in the North Atlantic and southward-flowing water masses in the intermediate to deep ocean. This overturning cell is complemented by an underlying, generally weaker, reversed overturning cell that originates in the Southern Ocean. In the following, we use AMOC to refer only to the upper overturning cell. Owing to the large heat capacity of water, the northwardflowing near-surface waters contribute effectively to the heat transport from the tropics to the mid- and high latitudes and therefore affect the regional climate around the North Atlantic 9,10 . Hence, changes of the AMOC strength can substantially influence climate in the north, especially in Europe. The depth of the southward-flowing waters and the strength of the circulation have changed in the past. During the last glacial period, abrupt warming events of several degrees within a few decades followed by slower cooling were detected in Greenland ice cores 11 . As effects of the so-called DansgaardOeschger events were found far beyond the Greenland ice sheet 1214 with patterns reminiscent of varying inter-hemispheric heat distribution 15,16 , they are believed to be the result of rapid changes of the AMOC strength 17 . Other examples of a reduction or even a complete shutdown of the

Lipid biomarkers record fundamental changes in the microbial community structure of tropical seas during the Late Ordovician Hirnantian glaciation

The Late Ordovician mass extinction was linked to climate cooling and glaciation of Gondwana during the terminal Ordovician Hirnantian Age (444.7–443.4 Ma). Extinction patterns have been well described for many marine taxa, but much less is known about marine microbial communities through this interval. To elucidate the structure of microbial communities in tropical marine basins through the Late Ordovician, we analyzed lipid biomarkers in thermally well preserved strata from the Taconic foreland (Anticosti Island, Canada), the Cincinnati Arch (midwestern United States), and the western continental margin (Vinini Formation, Nevada, United States). Despite clear oceanographic differences, lipid biomarker profiles show similarities between these three localities. Major shifts in biomarker distributions of Anticosti Island and the Vinini Formation, mainly hopane/sterane ratios, record changes in the balance of bacterial versus algal primary production. Bacterial contributions to sedimentary organic matter were highest during warm intervals, both before and after Hirnantian cooling. In particular, 3β-methylhopanes, likely sourced from aerobic methanotrophic bacteria, occur in high relative abundance (many times the Phanerozoic average) across Laurentia throughout most of the interval studied. 3β-methylhopane abundances also reveal an overall positive relationship with paleotemperature proxies, implying increased methane cycling during warm intervals. These results suggest that enhanced methane cycling could have provided an important positive feedback on climate during extended intervals of early Paleozoic time.

Exceptional preservation of microbial lipids in Paleozoic to Mesoproterozoic sediments

Exceptional preservation of microbial lipids in Paleozoic to Mesoproterozoic sediments

Tracing the Nd isotope evolution of North Pacific Intermediate and Deep Waters through the last deglaciation from South China Sea sediments

The intermediate and deep waters of the Pacific Ocean play a crucial role for regulating global climate changes on the millennium timescale. However, due to poor preservation of carbonate sediments in the deep Pacific, little attention has been received to better understand deep ocean circulation and its relationship with abrupt climate events. Here we present the first authigenic Nd isotope (εNd) record extracted from Fe–Mn oxyhydroxides of the South China Sea (SCS) sediments (SO17940-2, 1727m water depth) in order to investigate changes of intermediate and deep-water circulations in the Pacific Ocean since the Last Glacial Maximum (LGM).In general, the seawater εNd record extracted from the SCS sediments paralleled the existing Nd isotopes of fish teeth/debris from the eastern North Pacific core during the last deglaciation, demonstrating that our authigenic εNd can faithfully record the seawater εNd evolution in the deep Pacific Ocean. At the onset of the deglacial period, our data exhibit a clear negative shift in εNd towards a gradually increased component of Antarctica Intermediate Water (AAIW) and reached a maximal influx during the mid Heinrich Stadial 1 (HS1, ∼16kyr BP). Between the HS1 and Younger Dryas, deep-water circulation gradually shifted back to the modern condition during the Bølling-Allerød warm interval. A pronounced negative εNd excursion had occurred during the Pre-Boreal (PB), suggesting a rapid reorganization of deep-water ventilation in the Pacific Ocean and was predominated by AAIW. This is most likely due to poor ventilation in the western North Pacific associated with an intensified summer monsoon during the PB-early Holocene warm period. After the PB period, the seawater εNd values were once again dominated by Pacific Deep Water at the study site.

Global warming in an independent record of the past 130 years

AbstractThe thermometer‐based global surface temperature time series (GST) commands a prominent role in the evidence for global warming, yet this record has considerable uncertainty. An independent record with better geographic coverage would be valuable in understanding recent change in the context of natural variability. We compiled the Paleo Index (PI) from 173 temperature‐sensitive proxy time series (corals, ice cores, speleothems, lake and ocean sediments, historical documents). Each series was normalized to produce index values of change relative to a 1901–2000 base period; the index values were then averaged. From 1880 to 1995, the index trends significantly upward, similar to the GST. Smaller‐scale aspects of the GST including two warming trends and a warm interval during the 1940s are also observed in thePI. ThePIextends to 1730 with 67 records. The upward trend appears to begin in the early 19th century but the year‐to‐year variability is large and the 1730–1929 trend is small.

The June-September maximum mean temperature reconstruction from Masson pine (<italic>Pinus massoniana</italic> Lamb.) tree rings in Macheng, southeast China since 1879 AD

High-resolution and accurately dated tree-ring material is one of the major tools for studying past climate change during the last millennia. However,to date,dendroclimatological studies in southeast China are scarce because of the complicated relationship between tree rings and climatic factors,as well as the paucity of old trees and difficulties in dating. In the present paper,two Masson pine tree-ring chronologies(HTG0 and HTG2) were developed in Macheng,a junction area of three provinces(Hubei,Anhui and Henan) in southeast China. Strongly negative relationships were detected between the ring width and the seasonal mean and maximum temperatures from September to October,current March to April and June to September. The strongest correlation was identified between HTG0 and the maximum mean temperature from June to September(Tmax6–9,r=-0.61,P0.001). A simple regression model was designed to reconstruct Tmax6–9 from 1879 to 2011. The reconstruction accounts for 37.2% of the explained variance of instrumentally observed Tmax6–9 over the period 1959–2008. The reconstructed temperature displayed an asymmetric "W" type. A long,gradually decreasing trend from 1879 to 1951,and an increasing trend since 1983 were observed. Moreover,two cold(1940–1957; 1971–1998) and two warm(1959–1970; 1999–2011) intervals were identified during the past 133 years. In addition,spatial correlation analysis revealed that the Tmax6–9 reconstruction was regionally representative for a large area of southeast China,especially extending to the west of the sample site. This was also verified by other tree-ring based temperature series around this region. This study is expected to clarify the climatic conditions in southeast China beyond the instrumental record,as well as the possible response of Masson pine to future global warming for forest management purposes.

Occurrences of the amphi-Atlantic brown mussel Perna perna (Linné, 1758) (Mollusca, Bivalvia) in South Portugal since the Atlantic “climatic optimum”

The large, edible, and near-cosmopolitan brown mussel Perna perna (Linné, 1758) has a history of few and infrequent occurrences in Portugal since the Atlantic “climatic optimum” of the Holocene. Moreover, it is likely that some previous citations of Mytilus spp. from other Holocene deposits, including kitchen middens, may represent true Perna specimens. The species was recently found by Lourenço et al. (2012) in the south Portuguese coastal localities of Ilha do Farol and Vila Moura. Here, we summarize these and other previously published occurrences of P. perna in the areas of Armação de Pêra, Lagos and Aljezur, discussing morphologic, ecologic and biogeographic aspects of this species in the Portuguese fauna. The brown mussel is also known from archaeological records from Padrão I (Sagres – Vila do Bispo, Ancient Neolithic) and Arrifana (Aljezur, XII century). These chronologies are contemporaneous with the climatic warming intervals of the Atlantic Period (circa 8000-5000 BP) and “Medieval Warm Period” (circa 1000- 1200 AD). During these and other related intervals of warmer coastal surface waters, it is very likely that this subtropical “warm guest” colonized open marine rocky areas of the South and Southwest Portuguese coast, with settlement of stable and permanent populations.
 Ocorrências do mexilhão castanho anfiatlântico Perna perna (Linné, 1758) (Mollusca, Bivalvia) no sul de Portugal desde o “ótimo climático” Atlântico - O mitilídeo Perna perna (Linné, 1758), espécie comestível de dimensão apreciável e repartição quase cosmopolita, apresenta um historial escasso de ocorrências em Portugal desde o “ótimo climático” Atlântico do Holocénico. É provável, também, que algumas das ocorrências publicadas de Mytilus spp. noutros depósitos holocénicos, incluindo concheiros, possam corresponder a espécimes de Perna. Recolhas recentes na costa sul de Portugal, mencionadas por Lourenço et al. (2012) ampliam as ocorrências conhecidas às localidades de Ilha do Farol e de Vila Moura, em paralelo com citações mais antigas para Armação de Pêra, Lagos e Aljezur. Estas ocorrências são sumariadas no presente estudo, assim como discutidos aspetos de ordem morfológica, ecológica e biogeográfica relativos à presença desta espécie na fauna portuguesa. No registo arqueológico, o mexilhão castanho é conhecido nos arqueossítios de Padrão I (Sagres – Vila do Bispo, Neolítico Antigo) e Arrifana (Aljezur, século XII). Estas cronologias são coevas com os intervalos de melhoria climática do período Atlântico (circa 8000-5000 BP) e de finais da IdadeMédia (circa 1000-1200 DC). Durante estes e outros intervalos comparativos de melhoria das condições hidroclimáticas das águas superficiais costeiras, considera-se bastante provável que este “warm guest” subtropical tenha atingido, por diversas vezes, áreas não restritas do litoral rochoso do sul e sudoeste dePortugal, com o estabelecimento de populações estáveis e permanentes.

Interglacial Pluvial Periods in Southern Arabia

The role of mid-latitude precipitation in the hydrological forcing leading to the deposition of sapropels in the Mediterranean Sea remains unclear. The new GDEC-4-2 borehole, East Corsica margin (northern Tyrrhenian Sea), provides the first precisely dated evidence for enhanced rainfall in the Western Mediterranean during warm intervals of interglacial periods over the last 547 kyr. Comparison of GDEC-4-2 proxy records with pollen sequences and speleothems from the central and eastern Mediterranean reveals that these pluvial events were regional in character and occurred probably in response to the intensification of the Mediterranean storm track along the northern Mediterranean borderlands in autumn/winter. Our dataset suggests that the timing of maxima of the Mediterranean autumn/winter storm track precipitation coincide with that of the North African summer monsoon and sapropel deposition. Besides highlighting a close coupling between mid- and low-latitude hydrological changes, our findings suggest that during warm intervals of interglacial periods the reduced sea-surface water salinities, together with the high flux of nutrient and organic matter, produced by the monsoonal Nile (and wadi-systems) floods, were maintained throughout the winter by the Mediterranean rainfall. This provides an important additional constraint on the hydrological perturbation causing sapropel formation.

Community replacement of neritic carbonate organisms during the late Valanginian platform demise: A new record from the Provence Platform

The Valanginian is marked by a major platform demise inducing a hiatus in the northern Tethyan neritic carbonate record from the top of the lower Valanginian to the lower Hauterivian. New biostratigraphic and chemostratigraphic data from the Ollioules section (Provence Platform, southern France) are presented here, demonstrating that a large part of the upper Valanginian is preserved in an inner platform environment. The thick, upper Valanginian, aggrading carbonate succession is observed in an aborted rift domain, implying relatively low subsidence. In this context, a relatively long-term sea-level rise was required to sustain a keep-up style of carbonate production. Like the Apulian Platform, the remarkable preservation of the Provence Platform may have been favored by its remoteness from terrigenous source areas, as suggested by the low clastic inputs and low P-accumulation rates. Two main biotic community replacements are observed in Ollioules. The first saw the development of abundant microbialites and algae at the onset of the late Valanginian. A Tubiphytes concentration occurred during the coolest climatic conditions and the transition towards arid conditions, whereas the subsequent Lithocodium–Bacinella and orbitolinids assemblages developed under low nutrient conditions during a warmer interval. Both assemblages may have been triggered by increased alkalinity. The second community replacement saw the installation of coral- and rudist-dominated communities during the latest Valanginian to early Hauterivian. They indicate a change to oligotrophic, open marine conditions. Six medium-scale sequences have been defined in Ollioules, indicating short-term transgressive–regressive trends superimposed on a long-term transgression. Low nutrient inputs and relatively low subsidence in an aggradational context may explain the survival of the isolated Provence Carbonate Platform during a time of widespread drowning episodes and platform demise in the northern Tethyan domain.

Large-Amplitude Variations in Carbon Cycling and Terrestrial Weathering during the Latest Paleocene and Earliest Eocene: The Record at Mead Stream, New Zealand

AbstractThe late Paleocene to early Eocene was marked by major changes in Earth surface temperature and carbon cycling. This included at least two, and probably more, geologically brief (<200-k.yr.) intervals of extreme warming, the Paleocene-Eocene thermal maximum (PETM) and the Eocene thermal maximum-2 (ETM-2). The long-term rise in warmth and short-term “hyperthermal” events have been linked to massive injections of 13C-depleted carbon into the ocean-atmosphere system and intense global climate change. However, the causes, environmental impact, and relationships remain uncertain because detailed and coupled proxy records do not extend across the entire interval of interest; we are still recognizing the exact character of the hyperthermals and developing models to explain their occurrence. Here we present lithologic and carbon isotope records for a 200-m-thick sequence of latest Paleocene–earliest Eocene upper slope limestone exposed along Mead Stream, New Zealand. New carbon isotope and lithologic anal...

Modern seasonal variability and deglacial/Holocene change of central Arctic Ocean sea-ice cover: New insights from biomarker proxy records

For the reconstruction of sea-ice variability, a biomarker approach which is based on (1) the determination of sea-ice diatom-specific highly-branched isoprenoid (IP25) and (2) the coupling of phytoplankton biomarkers and IP25 has been used. For the first time, such a data set was obtained from an array of two sediment traps deployed at the southern Lomonosov Ridge in the central Arctic Ocean at water depth of 150m and 1550m and recording the seasonal variability of sea ice cover in 1995/1996. These data indicate a predominantly permanent sea ice cover at the trap location between November 1995 and June 1996, an ice-edge situation with increased phytoplankton productivity and sea-ice algae input in July/August 1996, and the start of new-ice formation in late September. The record of modern sea-ice variability is then used to better interpret data from sediment core PS2458-4 recovered at the Laptev Sea continental slope close to the interception with Lomonosov Ridge and recording the post-glacial to Holocene change in sea-ice cover.Based on IP25 and phytoplankton biomarker data from Core PS2458-4, minimum sea-ice cover was reconstructed for the Bølling/Allerød warm interval between about 14.5 and 13 calendar kyr BP, followed by a rapid and distinct increase in sea-ice cover at about 12.8 calendar kyr BP. This sea-ice event was directly preceded by a dramatic freshwater event and a collapse of phytoplankton productivity, having started about 100 years earlier. These data are the first direct evidence that enhanced freshwater flux caused enhanced sea-ice formation in the Arctic at the beginning of the Younger Dryas. In combination with a contemporaneous, abrupt and very prominent freshwater/meltwater pulse in the Yermak Plateau/Fram Strait area these data may furthermore support the hypothesis that strongly enhanced freshwater (and ice) export from the Arctic into the North Atlantic could have played an important trigger role for the onset of the Younger Dryas cold reversal. During the Early Holocene, sea-ice cover steadily increased again (ice-edge situation), reaching modern sea-ice conditions (more or less permanent sea-ice cover) probably at about 7–8 calendar kyr BP.