Climatic cyclicity at Site 806; the GRAPE record

We used the continuous saturated bulk density records collected by the gamma-ray attenuation porosity evaluator (GRAPE) at Ocean Drilling Program Site 806 on the top of the Ontong Java Plateau to evaluate the continuity of the recovered cores and to splice together a complete section from the multiple holes drilled at the site (for the upper 165 m, this is equivalent to approximately 0-5 Ma). The lack of offset in core breaks (between the 9.5-m-long, successive cores) from hole to hole made splicing difficult, and the results are not unambiguous. The composite section was converted to a time series by using biostratigraphy and supplementing this with oxygen-isotope datums for the interval between 2 and 5 Ma. Evolutionary spectra generated from the composite section clearly indicate the presence of Milankovitch frequencies throughout the record. We chose a final age model that was most consistent with a Milankovitch model but have not, as yet, spectrally tuned the data. The GRAPE (saturated bulk density) changes at Site 806 are the result of changes in grain size, with density decreasing as grain size increases. We attribute this to the removal of fine particles by winnowing, leaving a greater percentage of large hollow foraminifers behind— the winnowing effect. This is in contrast to the dissolution effect, which breaks up large hollow foraminifers into fragments but merely transfers intraparticle porosity to interparticle porosity and thus shows significant changes in grain size without significant changes in density. A 300-k.y. piston core record reveals that during this time interval increased winnowing has been associated with glacials and 100-k.y. cyclicity. For the time interval from 5 to 2 Ma, enhanced winnowing continues to be associated with isotopically heavy intervals dominated by 41-k.y. (obliquity) variance. In this band, the winnowing record is highly correlated with the ice-volume record, particularly since the onset of Northern Hemisphere glaciations. Before that time, the grain-size record continues to show variance in the obliquity band whereas the oxygen isotope record shows a shift to the dominance of precessional frequencies. We suggest that the winnowing signal is a response to increased thermohaline circulation and benthic storm activity associated with enhanced north-south thermal gradients during times of climatic degradation

Evolution of Pliocene climate cyclicity at Hole 806B (5-2 Ma); oxygen isotope record

A detailed Pliocene oxygen isotope record from the Ontong Java Plateau, based on measurements of the surface-dwelling planktonic foraminifer Globigerinoides sacculifer, was produced for the period from 5 to 2 Ma. The record documents major long and short-term climate changes. The results show periods of enhanced ice volume at 4.6 to 4.3 Ma and after 2.85 Ma, a long-term warming trend from 4.1 to 3.7 Ma, and a distinct cooling trend that was initiated at 3.5 Ma and progressed through the initiation of large-scale Northern Hemisphere glaciation after 2.85 Ma (according to the time scale of Shackleton and others proposed in 1990). Periods of high average ice volumes also show the highest δ 1 8 amplitudes. The pattern of climate cyclicity changed markedly at about 2.85 Ma. Earlier times were marked by high-frequency variability at the precessional frequencies or even higher frequencies, pointing to low-latitude processes as a main controlling factor driving planktonic δ 1 8 variability in this period. The high-frequency variability is not coherent with insolation and points to strong nonlinearity in the way the climate system responded to orbital forcing before the onset of large scale Northern Hemisphere glaciation. After 3 Ma, stronger 41-k.y. cyclicity appears in the record. The shift in pattern is clearest around 2.85 Ma (according to the time scale proposed by Shackleton and others in 1990), 100-200 k.y. before the most dramatic spread of Northern Hemisphere ice sheets. This indicates that high-latitude processes from this point on began to take over and influence most strongly the δ 1 8 record, which now reflects ice-volume fluctuations related to the climatic effects of obliquity forcing on the seasonality of high-latitude areas, most probably in the Northern Hemisphere. The general Pliocene trend is that high-latitude climate sensitivity and instability was increasing, and the causal factors producing the intensified glacial cyclicity during the Pliocene must be factors that enhance cooling and climate sensitivity in the subarctic areas

Planktonic Foraminifer Biostratigraphy of Norwegian Sea Sediments: ODP Leg 104

Sites 642, 643, and 644 were investigated for planktonic foraminifers. Frequent occurrences of barren intervals caused by dissolution as well as low- diversity and long-ranging assemblages reduce the stratigraphic resolution of these records. Based on Neogloboquadrina species a local zonation, correlated with high-latitude North Atlantic schemes, is proposed for the middle Miocene to Quaternary and correlated with paleomagnetic records. The middle to late Miocene boundary is defined by the last appearance datum of Neogloboquadrina mayeri and the first appearance datum of Neogloboquadrina acostaensis. Late Miocene age is defined by a N. acostaensis zone, whereas latest Miocene to Pliocene zones are defined by dextral and sinistrally coiling Neogloboquadrina atlantica. The top of the Pliocene is characterized by dextral and sinistrally coiling Neogloboquadrina pachyderma. The Pleistocene is defined by a nearly monospecific assemblage of Neogloquadrina pachy derma in its sinistrally coiled and encrusted variety

Insolation and gacial meltwater influence on sea‐ice and circulation variability in the Northeastern Labrador Sea during the last glacial period

The variable amounts of ice rafted debris (IRD) and foraminifers in North Atlantic sediments are related to the abrupt, millennial-scale alteration from Greenland stadials to interstadials during the last glacial period and indicate past ice sheet instabilities, changes in sea-ice cover and productivity. In the Norwegian Sea, Greenland stadials were likely characterized by an extensive, near-perennial sea-ice cover whereas Greenland interstadials were seasonally ice-free. The variability in other areas, such as the Labrador Sea, remains, however, obscure. We therefore investigated deep-sea sediment core GS16-204-22CC retrieved south of Greenland. Using a multiproxy approach, we distinguish two sediment regimes and hence different environmental conditions between ca. 65 and 25 ka b2k. Regime 1 (similar to 65-49 ka b2k) is characterized by the dominance of planktic foraminifers in the sediments. During late MIS4 and early MIS3, the site was covered by near-perennial sea-ice with occasional periods of iceberg discharge. During the younger part of regime 1 the northeastern Labrador Sea was seasonally ice-free with hardly any icebergs melting near the site and long-term environmental conditions were less variable. Regime 2 (similar to 49-25 ka b2k) is characterized by pronounced stadial-interstadial variability of foraminifer and IRD fluxes, suggesting an extensive sea-ice cover during most Greenland stadials and seasonally ice-free conditions during most Greenland interstadials. During MIS2 environmental conditions were very similar to those of the younger part of regime 1. While all Heinrich (H) related Greenland stadials are marked by depleted oxygen isotope values at our core site, only H4 and H3 are associated with pronounced IRD peaks. Plain Language Summary North Atlantic sediments contain variable amounts of sand-sized mineral grains and microorganism shells. Mineral grains indicate iceberg transport from continental ice sheets, like the Greenland ice sheet (more icebergs/melting sea-ice, more grains). If the sea-ice cover is too thick, no light can penetrate and fewer microorganisms live in the water beneath the ice. Using these indicators, we investigated ocean sediments from south of Greenland covering the time period between ca. 65 and 25 thousand years ago. This time period was characterized by several abrupt changes between cold and warm climates on millennial timescales. We find that the ocean south of Greenland was sea-ice covered for most of the year with occasional time periods of iceberg discharge between 65 to 56 thousand years ago. From 56 to 49 thousand years ago the ice-free season was extended and hardly any icebergs melted near the site. From 49 thousand years ago our study area was covered by sea-ice year-round during cold time intervals whereas warm time intervals were only seasonally sea-ice covered. Continental ice sheets were growing during this time interval and we observed two major calving events related to two of the four very cold climate intervals recorded in the analyzed sediment.Fundacao de a Ciencia e a Tecnologia IF/01500/2014 UID/Multi/04326/2019info:eu-repo/semantics/publishedVersio

Subarctic Front migration at the Reykjanes Ridge during the mid- to late Holocene:Evidence from planktic foraminifera

Expansion of fresh and sea-ice loaded surface waters from the Arctic Ocean into the sub-polar North Atlantic is suggested to modulate the northward heat transport within the North Atlantic Current (NAC). The Reykjanes Ridge south of Iceland is a suitable area to reconstruct changes in the mid- to late Holocene fresh and sea-ice loaded surface water expansion, which is marked by the Subarctic Front (SAF). Here, shifts in the location of the SAF result from the interaction of freshwater expansion and inflow of warmer and saline (NAC) waters to the Ridge. Using planktic foraminiferal assemblage and concentration data from a marine sediment core on the eastern Reykjanes Ridge elucidates SAF location changes and thus, changes in the water-mass composition (upper ˜200 m) during the last c. 5.8 ka BP. Our foraminifer data highlight a late Holocene shift (at c. 3.0 ka BP) in water-mass composition at the Reykjanes Ridge, which reflects the occurrence of cooler and fresher surface waters when compared to the mid-Holocene. We document two phases of SAF presence at the study site: from (i) c. 5.5 to 5.0 ka BP and (ii) c. 2.7 to 1.5 ka BP. Both phases are characterized by marked increases in the planktic foraminiferal concentration, which coincides with freshwater expansions and warm subsurface water conditions within the sub-polar North Atlantic. We link the SAF changes, from c. 2.7 to 1.5 ka BP, to a strengthening of the East Greenland Current and a warming in the NAC, as identified by various studies underlying these two currents. From c. 1.5 ka BP onwards, we record a prominent subsurface cooling and continued occurrence of fresh and sea-ice loaded surface waters at the study site. This implies that the SAF migrated to the southeast of our core site during the last millennium

High resolution benthic Mg/Ca temperature record of the intermediate water in the Denmark Strait across D-O stadial-interstadial cycles

Dansgaard‐Oeschger (D‐O) climate instabilities that took place during Marine Isotope Stage 3 are connected to changes in ocean circulation patterns and sea ice cover. Here we explore in detail the configuration of the water column of the Denmark Strait during D‐O events 8–5. How the ocean currents and water masses within the Denmark Strait region responded and were connected to the North Atlantic are discussed. We investigate sediment core GS15‐198‐36CC, from the northern side of the Greenland‐Iceland Ridge, at 30‐year temporal resolution. Stable carbon and oxygen isotope reconstructions based on benthic foraminifera, together with a high‐resolution benthic foraminiferal record of Mg/Ca paleothermometry, is presented. The site was bathed by warm intermediate waters during stadials and cool but gradually warming intermediate water during interstadials. We suggest that stadial conditions in the Denmark Strait are characterized by a well‐stratified water column with a warm intermediate water mass that lies beneath a cold fresh body of water where sea ice and brine rejection work in consort to uphold the halocline conditions. Interstadial periods are not a pure replicate of modern times, but rather have two modes of operation, one similar to today, and the other incorporating a brief period of warm intermediate water and increased ventilation.publishedVersio

Atlantic Ocean thermohaline circulation changes on orbital to suborbital timescales during the mid-Pleistocene

Mid-Pleistocene benthic ∂18O and ∂13C time series from the North Atlantic site 983 and Ceara Rise site 928 are compared to an array of existing isotopic records spanning the Atlantic basin and the geographic extremes of the North Atlantic Deep Water/Southern Ocean Water interface during both glacial and interglacial periods. This comparison allows the persistent millennial-scale intermediate depth North Atlantic ventilation changes recorded at site 983 to be placed within the context of the longer period water mass reorganizations taking place throughout the mid-Pleistocene. Our benthic ∂13C results suggest that the intermediate depth North Atlantic experienced millennial-scale changes in ventilation throughout the mid-Pleistocene climate shift. The times of poorest ventilation (low benthic ∂13C) persisted for only a few millennia and were associated with rapid decreases in benthic ∂18O, suggesting that ice sheet decay and melt water induced salinity changes were effective at throttling deep water production in the North Atlantic throughout the mid-Pleistocene. Similar but less pronounced decreases in the ∂13C of the middepth waters also punctuated interglacials, suggesting that large ice sheet fluctuations do not explain all of the observed thermohaline circulation mode shifts in the North Atlantic. Meanwhile, on orbital timescales, glacial deep to intermediate water ∂13C gradients evolved after ~0.95 Ma. Taken together, these observations provide a number of new constraints for understanding the timing and evolution of deep water circulation changes across the mid-Pleistocene

Independent tephrochronological evidence for rapid and synchronous oceanic and atmospheric temperature rises over the Greenland stadial-interstadial transitions between ca. 32 and 40 ka b2k

Understanding the dynamics that drove past abrupt climate changes, such as the Dansgaard-Oeschger (DO) events, depends on combined proxy evidence from disparate archives. To identify leads, lags and synchronicity between different climate system components, independent and robust chronologies are required. Cryptotephrochronology is a key geochronological tool as cryptotephra horizons can act as isochrons linking disparate and/or distant records. Here, we investigated marine sediment core MD99-2284 from the Norwegian Sea to look for previously identified Greenland ice core cryptotephra horizons and define time-parallel markers between the archives. We explored potential secondary transport and depositional mechanisms that could hamper the isochronous integrity of such horizons. We identified six cryptotephra layers of which four correlate to previously known Greenland ice core horizons. None of those were identified in other marine cores and thus, this study contributes greatly to the North Atlantic tephra framework tripling the original amount of existing isochrons between ca. 25 and 60 ka b2k. The latter allow a synchronization between MD99-2284 and the Greenland ice cores between ca. 32 e40 ka b2k, which is, in the North Atlantic, the shortest time-interval during the Last Glacial Period to be constrained by four independent tephra isochrons. These findings provide essential tephra-based evidence for synchronous and rapid oceanic and atmospheric temperature rises during the Greenland Stadial-Interstadial transitions. Furthermore, it enables us to estimate the average peak-duration of interstadial temperature overshoots at approximately 136 years. As such, this well-targeted high-resolution investigation successfully demonstrates the use of cryptotephra for geochronological purposes in the marine realm.publishedVersio

paleoclimatic and paleoceanographic changes in the Nordic Seas

[1] High-resolution records from IMAGES core MD95-2011 in the eastern Norwegian Sea provide evidence for relatively large-and small-scale high-latitude climate variability throughout the Holocene. During the early and mid-Holocene a situation possibly driven by consistent stronger westerlies increased the eastward influence of Arctic intermediate and near-surface waters. For the late Holocene a relaxation of the atmospheric forcing resulted in increased influence of Atlantic water. The main changes in Holocene climate show no obvious connection to changing solar irradiance, and spectral analysis reveals no consistent signature for any periodic behavior of Holocene climate at millennial or centennial timescales. There are, however, indications of consistent multidecadal variability