Nalan Koc

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene.

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (361N–751N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from � 0.27 to � 0.151C kyr � 1 . Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.91C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition fromthe Hypsithermal interval ( B9–5.7 ka B.P.) to the Neoglaciation (B5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T fromearly to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. r 2002 Elsevier Science Ltd. All rights reserved.

High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger

Early Holocene lacustrine, tree-ring, ice-core, and marine records reveal that the Northern Hemisphere underwent a short cooling event at 10 300 calendar yr B.P. (9100 14 C yr B.P.). The records were compared on a common high-resolution time scale and show that the event lasted less than 200 yr, with a cooling peak of 50 yr, and the event coincides with a distinct Holocene thermohaline disturbance recognized in the North Atlantic Ocean. In spite of wellknown freshwater forcings at the time of the event, the negligible difference between the modeled D 14 C record, based on the GISP2 (Greenland Ice Sheet Project 2) 10 Be data, and the measured values, does not allow for detectable D 14 C changes related to global ocean ventilation. We can, however, show that the onset of the cooling coincides with the onset of one of the largest Holocene 10 Be flux peaks. This finding may imply that the climate system is more

Mid-Cenozoic tectonic and paleoenvironmental setting of the central Arctic Ocean

Drilling results from the Integrated Ocean Drilling Program’s Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown.

Holocene climate dynamics in Fennoscandia and the North Atlantic

Set against the high amplitude climatic variations that characterised the Pleistocene, the majority of the Holocene epoch was once considered to be comparatively stable. Indeed, observed post-19th century “global” warming is frequently thought of as an anomaly. However, the Holocene in the high latitude terrestrial environment of the PAGES PEP III transect has not been climatically stable, as was indicated by 19th century studies of Scandinavian and Scottish peat bogs by Blytt (1876) and Sernander (1908). Taken from Andersson’s early 20th century review of Late-Quaternary Swedish climate research, the

Cyclicity in the middle Eocene central Arctic Ocean sediment record: Orbital forcing and environmental response

[1] Continuous X-ray fluorescence scanning of middle Eocene (� 46 Ma) core M0002A-55X (� 236–241 m composite depth), recovered during Integrated Ocean Drilling Program Expedition 302, revealed a strong cyclical signal in some major and trace geochemical elements. We performed a multiproxy study of the same core, which included organic geochemical, sedimentological, and biological parameters, and integrated our results with available geochemical and physical properties data. The target was to look for cyclicity in the several proxies, investigate their frequency, and understand the environmental response to the potential forcing. Results indicate that a higher terrigenous component corresponds to lower organic carbon concentration, smaller contributions by angiosperm pollen and spores, organic-walled dinoflagellate cysts, and chrysophyte cysts (lower productivity, shorter growing season for flowering plants, and lower stratification) but higher contributions by bisaccate pollen and diatoms (drier conditions on land, more marine conditions) and higher terrigenous sand (ice-rafted debris (IRD)). Our investigation shows that physical proxy parameters hold cyclicity with periods of about 50 and 100 cm and that these frequency components are compatible with a Milankovitchtype orbital forcing, representing precession and obliquity, respectively. The longer 100 cm cyclicity is also present in the biological (pollen, dinoflagellate cysts, and siliceous microfossils) and in the sedimentological (IRD) proxies. The environmental signal derived from the integrated multiproxy analysis suggests that in an enclosed Arctic Ocean at time of ice (sea ice and glacial ice) initiation the biological proxies responded more strongly to growing season length/darkness, whereas the terrigenous components, directly driven by sea ice and/ or glacial ice formation and extent, responded more directly to seasonal insolation.

Holocene climate variations at the entrance to a warm Arctic fjord: evidence from Kongsfjorden trough, Svalbard

Abstract The North Atlantic Current transports warm and salty water into the Nordic Seas and continues northwards into the Arctic Ocean as the West Spitsbergen Current. This current flows along the west coast of the Svalbard archipelago and into the fjords and troughs on the Svalbard shelf. We have investigated a core (NP05-11-21GC) from the Kongsfjorden trough, which spans the last 12 ka. The core site presently experiences seasonal inflow of Atlantic Water masses, and may therefore provide a record of past variations in Atlantic Water inflow to the Arctic. Lithological analysis and benthic foraminifera have been used to reconstruct the palaeoceanographic development in the area. The results show that cold and harsh conditions prevailed during the late part of the Younger Dryas and that the site was in proximity to glaciers. After 11.8 ka BP the first influence of Atlantic Water is seen in the fauna. This was followed by changes in glacial activity at 11.5 ka BP. During the period 11.5–10.6 ka BP the fauna indicate increased influence of Atlantic Water and the final deglaciation of the fjord after the Younger Dryas period. These initial ameliorated conditions were interrupted by a 250-year long cooling starting at 11.3 ka BP, corresponding to the Preboreal Oscillation. After 10.6 ka BP a marked change from an ice proximal to ice distal environment occurred accompanied by the strong influence of Atlantic Water masses. In the Mid-Holocene at 7 ka BP, the influence of Atlantic Water diminished but no sign of an immediate response of the glaciers to this are found in the core. The evidence suggests that intensification of glacial activity started as late as c. 3.5 ka BP. Comparing the core from Kongsfjorden trough with two shelf records from Svalbard generally shows similar faunal development although some differences exist. These are assumed to be related to the distance of each record to the position of the Arctic front.

Multicentennial Variability of the Sea Surface Temperature Gradient across the Subpolar North Atlantic over the Last 2.8 kyr

A 2800-year-long August Sea Surface Temperature (aSST) record based on fossil diatom assemblages is generated from a marine sediment core from the northern subpolar North Atlantic. The record is compared with the aSST record from the Norwegian Sea to explore the variability of the aSST gradient between these areas during the late Holocene. The aSST records demonstrate the opposite climate tendencies towards a persistent warming in the core site in the subpolar North Atlantic and cooling in the Norwegian Sea. At the multicentennial scale of aSST variability of 600–900 years, the records are nearly in anti-phase with warmer (colder) periods in the subpolar North Atlantic corresponding to the colder (warmer) periods in the Norwegian Sea. At the shorter time scale of 200–450 years, the records display a phase-locked behaviour with a tendency for the positive aSST anomalies in the Norwegian Sea to lead by ~30 years the negative aSST anomalies in the subpolar North Atlantic. This apparent aSST seesaw might have an effect on two major anomalies of the European climate of the past Millennium: Medieval Warm Period (MWP) and the Little Ice Age (LIA). During the MWP warming of the sea surface in the Norwegian Sea occurred in parallel with cooling in the northern subpolar North Atlantic whereas the opposite pattern emerged during the LIA. The results suggest that the observed aSST seesaw between the subpolar North Atlantic and the Norwegian Sea could be a surface expression of the variability of the eastern and western branches of the Atlantic Meridional Circulation (AMOC) with a possible amplification through atmospheric feedback.

High frequency climate variability of the Norwegian Atlantic Current during the early Holocene period and a possible connection to the Gleissberg cycle.

A high-resolution sediment core (MD95-2011) from the Vøring Plateau has been studied to document the variability of the surface water conditions during the Younger Dryas—Preboreal (13—9 kyr BP) in the eastern Norwegian Sea, in order to assess the climate variability in a period considered to be highly dynamic. Quantitative summer sea surface temperatures (SSSTs) with a time resolution of 20—40 years are reconstructed using three different diatom transfer function methods. The Younger Dryas—Preboreal transition at site MD95-2011 is documented with a temperature increase of 3.5°C/3°C within 100 years. Following, the record resolves for the first time PBO as two prominent cooling events centred at 11.2 and 11.3 kyr BP. In addition, a broad cooling event documented from 10.3—9.9 kyr BP coincides with previously documented climatic events as the 10.3 event and Erdalen Event 1 (10.1—9.9 kyr BP) and 2 (~9.7 kyr BP), respectively. Constrained by the observed SSST changes and diatom assemblages, short-term SSST changes with a periodicity of 80—120 years are observed, and the length of this period might indicate a possible connection to the solar Gleissberg cycle. The North Atlantic heat transport is highly sensitive to freshwater inputs, and the interplay of freshwater forcing with insolation forcing is considered as the controlling mechanism of climate changes at site MD05-2011 during the early Holocene. However, the century-scale variability of 80—120 years cannot explain the large-scale variability during the early Holocene, but is of importance for understanding the underlying small-scale oscillations.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.