North Atlantic Sediments Research Articles

Surface climate signals transmitted rapidly to deep North Atlantic throughout last millennium.

Instrumental observations of subsurface ocean warming imply that ocean heat uptake has slowed 20th-century surface warming. We present high-resolution records from subpolar North Atlantic sediments that are consistent with instrumental observations of surface and deep warming/freshening and in addition reconstruct the surface-deep relation of the last 1200 years. Sites from ~1300 meters and deeper suggest an ~0.5 degrees celsius cooling across the Medieval Climate Anomaly to Little Ice Age transition that began ~1350 ± 50 common era (CE), whereas surface records suggest asynchronous cooling onset spanning ~600 years. These data suggest that ocean circulation integrates surface variability that is transmitted rapidly to depth by the Atlantic Meridional Ocean Circulation, implying that the ocean moderated Earth's surface temperature throughout the last millennium as it does today.

A mid-Holocene stalagmite multiproxy record from southern Siberia (Krasnoyarsk, Russia) linked to the Siberian High patterns

A multiproxy record from a stalagmite collected from Torgashinskaya Cave (Southern Siberia, Russia) and growing between ca. 6 and 3.8 ka shows evidence for regional climatic changes occurring at ca. 5 ka. Interpretation of stable isotope ratios (δ18O and δ13C) and fluorescence data (intensity and wavelength of the emitted fluorescence) suggests that the interval between ca. 5 and 4.2 ka was generally warmer and drier than the interval between ca. 6 and 5 ka. The observed bipartitioning of the climate, attributable to the so-called ‘middle-late Holocene transition’, has a striking similarity to changes in K+ and Na+ concentration of Greenland ice cores (taken as indicators of the strength of the Siberian High and Icelandic Low, respectively), in the abundance of hematite-stained grains in subpolar North Atlantic sediments and, to lesser extent, in the summer Asian monsoon intensity deduced by δ18O from Chinese speleothems. In particular, the δ18O record at Torgashinskaya Cave can be interpreted as mostly driven by temperature changes. Besides several episodes of drift towards higher temperatures, it also strongly suggests the presence of short cooling events centered at 4.1+0.08/-0.07, 4.85+0.05/-0.06, 5.1+0.09/-0.09, 5.3+0.08/-0.07 and 5.8+0.12/-0.13 ka. Notably, the last three such events are in very good correspondence with spikes in the K+ and Na+ concentration of Greenland ice cores. Instead, the cooling around 4.1 ka could be the local response to the 4.2 event, a cold/dry episode identified in several records in the Northern Hemisphere. This suggests that δ18O of speleothem calcite from this area could be a useful proxy for defining the evolution of the Siberian High and its effect on the wider regional climate.

A 1.5-million-year record of orbital and millennial climate variability in the North Atlantic

Abstract. Climate during the last glacial period was marked by abrupt instability on millennial timescales that included large swings of temperature in and around Greenland (Daansgard–Oeschger events) and smaller, more gradual changes in Antarctica (AIM events). Less is known about the existence and nature of similar variability during older glacial periods, especially during the early Pleistocene when glacial cycles were dominantly occurring at 41 kyr intervals compared to the much longer and deeper glaciations of the more recent period. Here, we report a continuous millennially resolved record of stable isotopes of planktic and benthic foraminifera at IODP Site U1385 (the “Shackleton Site”) from the southwestern Iberian margin for the last 1.5 million years, which includes the Middle Pleistocene Transition (MPT). Our results demonstrate that millennial climate variability (MCV) was a persistent feature of glacial climate, both before and after the MPT. Prior to 1.2 Ma in the early Pleistocene, the amplitude of MCV was modulated by the 41 kyr obliquity cycle and increased when axial tilt dropped below 23.5∘ and benthic δ18O exceeded ∼3.8 ‰ (corrected to Uvigerina), indicating a threshold response to orbital forcing. Afterwards, MCV became focused mainly on the transitions into and out of glacial states (i.e. inceptions and terminations) and during times of intermediate ice volume. After 1.2 Ma, obliquity continued to play a role in modulating the amplitude of MCV, especially during times of glacial inceptions, which are always associated with declining obliquity. A non-linear role for obliquity is also indicated by the appearance of multiples (82, 123 kyr) and combination tones (28 kyr) of the 41 kyr cycle. Near the end of the MPT (∼0.65 Ma), obliquity modulation of MCV amplitude wanes as quasi-periodic 100 kyr and precession power increase, coinciding with the growth of oversized ice sheets on North America and the appearance of Heinrich layers in North Atlantic sediments. Whereas the planktic δ18O of Site U1385 shows a strong resemblance to Greenland temperature and atmospheric methane (i.e. Northern Hemisphere climate), millennial changes in benthic δ18O closely follow the temperature history of Antarctica for the past 800 kyr. The phasing of millennial planktic and benthic δ18O variation is similar to that observed for MIS 3 throughout much of the record, which has been suggested to mimic the signature of the bipolar seesaw – i.e. an interhemispheric asymmetry between the timing of cooling in Antarctica and warming in Greenland. The Iberian margin isotopic record suggests that bipolar asymmetry was a robust feature of interhemispheric glacial climate variations for at least the past 1.5 Ma despite changing glacial boundary conditions. A strong correlation exists between millennial increases in planktic δ18O (cooling) and decreases in benthic δ13C, indicating that millennial variations in North Atlantic surface temperature are mirrored by changes in deep-water circulation and remineralization of carbon in the abyssal ocean. We find strong evidence that climate variability on millennial and orbital scales is coupled across different timescales and interacts in both directions, which may be important for linking internal climate dynamics and external astronomical forcing.

Late Pleistocene–Holocene climatic implications of high‐resolution stable isotope profiles of a speleothem from south‐central Anatolia, Turkey

ABSTRACTHigh‐resolution 230Th ages and stable isotope (δ18O and δ13C) records from a stalagmite that grew between 39 and 2 ka in Incesu Cave located in south‐central Anatolia allow us to evaluate paleoclimate conditions for growth periods during Marine Isotope Stage 3 (MIS3) and the Holocene. High δ18O values and Heinrich events H3 and H4 are observed during the MIS3 interval. After a dry period in the Younger Dryas, low values between ca. 10 and 5.3 ka suggest a transition to wet mid‐ to early Holocene conditions. In the early Holocene, there are drier periods at 9.4 and 10.3 ka, coincident with cooling events recorded in the North Atlantic sediments and, after 5.3 ka a relatively dry late Holocene is seen. The IN‐01 isotope record is in phase with general trends of speleothem records in Turkey and the Eastern Mediterranean but differs in detail at the millennial scale. The more depleted δ18O values of IN‐01 compared to those of Eastern Mediterranean speleothems during the Holocene indicate that central Anatolian winter rainfall was isotopically influenced by the same air mass trajectories derived from the North Atlantic and Mediterranean with an isotopic rain shadow effect.

A framework for the timing of the final meltwater outbursts from glacial Lake Agassiz-Ojibway

The climate variability of the last deglaciation is often linked to meltwater discharges from the melting of large ice sheets. One of the best examples comes from the drainage of glacial Lake Agassiz-Ojibway (LAO) and its attendant perturbation of the Atlantic Meridional Overturning Circulation (AMOC), which has long been held responsible for a rapid cooling at ∼8.2 ka. However, recent modeling studies have argued that a large and sustained freshwater flux linked to increased surface melt and ensuing collapse of the Laurentide Ice Sheet (LIS) dam may have formed an efficient forcing for this cooling event. Yet, empirical (geological) evidence for a long-lasting meltwater flux is still equivocal while paleoceanographic data show that the freshening of the North Atlantic around the 8.2-ka cold event is characterized by multiple freshwater pulses. Part of this uncertainty arises from the lack of constraints on the structure (number) and timing of meltwater discharges involved in the drainage of LAO, which prevents a detailed assessment of the freshwater forcing mechanisms at work and their potential impact on AMOC—an important issue given the present-day increase in the melting of the cryosphere around the North Atlantic. Here, we review 597 14C ages from marine and continental sediment archives and use 296 of these 14C ages along with LAO geomorphological and varve records to present an integrated framework constraining the timing of LAO meltwater outbursts across the final deglaciation interval. Results show that LAO drained through two distinct events: first subglacially at ∼8.22 cal ka BP and then after the breakup of the ice dam at ∼8.16 cal ka BP. These LAO meltwater discharges are coeval with two important freshwater pulses in North Atlantic sediment cores, with the largest meltwater outburst matching the onset of the 8.2 ka event in Greenland ice cores. These results suggest that, in a fast-changing ocean-climate system influenced by melting ice sheets like that of the late deglaciation, massive and short-lived freshwater injections can potentially have an impact on AMOC.

Exceptional 20th Century Shifts in Deep-Sea Ecosystems Are Spatially Heterogeneous and Associated With Local Surface Ocean Variability

Traditionally, deep-sea ecosystems have been considered to be insulated from the effects of modern climate change, but with the recognition of the importance of food supply from the surface ocean and deep-sea currents to sustaining these systems, the potential for rapid response of benthic systems to climate change is gaining increasing attention. However, very few ecological time-series exist for the deep ocean covering the twentieth century. Benthic responses to past climate change have been well-documented using marine sediment cores on glacial-interglacial timescales, and ocean sediments have also begun to reveal that planktic species assemblages are already being influenced by global warming. Here, we use benthic foraminifera found in mid-latitude and subpolar North Atlantic sediment cores to show that, in locations beneath areas of major surface water change, benthic ecosystems have also changed significantly over the last ∼150 years. The maximum benthic response occurs in areas which have seen large changes in surface circulation, temperature, and/or productivity. We infer that the observed surface-deep ocean coupling is due to changes in the supply of organic matter exported from the surface ocean and delivered to the seafloor. The local-to-regional scale nature of these changes highlights that accurate projections of changes in deep-sea ecosystems will require (1) increased spatial coverage of deep-sea proxy records, and (2) models capable of adequately resolving these relatively small-scale oceanographic features.

Holocene Indian Summer Monsoon variations inferred from end-member modeling of sediment grain size in the Andaman Sea

Whether the Indian Summer Monsoon (ISM) Maximum started in the early Holocene or middle Holocene remains a controversial issue. Grain-size measurements were conducted on 157-cm-long core ADM-C1 from the Andaman Sea; the core spanned the last 11.2 kyr BP. Four end-members (EMs) were unmixed using lognormal parametric end-member modeling. Genetic analyses of the grain-size EMs suggested the EM1 and EM2 may correlate with suspension in the upper layers and transportation in the benthic nepheloid layer, respectively, while the EM3 and EM4 were tied with deposition affected by monsoonal currents and sedimentation under extreme events, respectively. The EM3 was dominated by ISM intensity and transported by summer monsoon currents, with increase in the proportion of EM3 reflecting increasing ISM intensity, and vice versa. Variations in EM3 were used to define three stages of ISM evolution, as follows: (1) During 11.2–9.1 kyr BP, EM3 was relatively low, indicating a weak ISM; (2) during 9.1–4.5 kyr BP, EM3 increased substantially, indicating a strengthened ISM; (3) during 4.5-0 kyr BP, EM3 decreased gradually, suggesting a gradual weakening of the ISM. The Holocene ISM Maximum started at ~9.1 kyr BP rather than in the early Holocene, which may have been partly due to the slowdown of the Atlantic meridional overturning circulation (AMOC) during the early Holocene that resulted in a decreased land-sea thermal contrast between the landmass and the Indian Ocean. An additional cause may have been the remnant Northern Hemisphere ice sheets that impeded the northward shift of the Intertropical Convergence Zone (ITCZ). Seven relatively brief decreases in the ISM intensity (events 7, 6, 5, 4, 3, 2, 1) occurred at ca. 10.3, 9.7, 7.3, 5.7, 4.2, 2.4, and 0.8 kyr BP, respectively. They corresponded, within the age uncertainties, to the increased supplies of ice-rafted detritus to North Atlantic sediments, implying that changes in the ISM intensity on millennial scale were dominated by climatic processes in northern high latitudes.

High-resolution varve sequences record one major late-glacial ice readvance and two drainage events in the eastern Lake Agassiz-Ojibway basin

Lake Ojibway developed at the southern Laurentide Ice Sheet margin during the last deglaciation and covered vast expanses of northeastern Ontario and northwestern Quebec (Canada). The late-stage history of the lake is complex, presumably marked by a coalescence with Lake Agassiz and by late-glacial ice readvances into the basin shortly before its final drainage ∼8200 years ago. However, these events are poorly defined in the Lake Ojibway varve record. Here we present evidence for the occurrence of one major ice readvance and two distinct drainage events from two varve sequences in northwestern Quebec. The Matagami section contains 231 varves that comprise an interval recording a late-glacial (Cochrane) ice readvance. The La Reine section spans 100 varves and the base of the sequence is characterized by a set of chaotic and coarse-grained (silty) rhythmites that marks the first lake drainage. These bottom rhythmites are overlain by a set of 65 thick varves, which are in turn capped by a thick silt bed associated with the final drainage of Lake Ojibway. Varve thickness measurements allow the correlation of these sections with the main Ojibway varve record that spans ∼2129 years in the region (where varve 1 represents the start of sedimentation). The Matagami sequence covers varve years 1644–1874 and the onset of the Cochrane readvance occurred in varve year 1817. The lower part of the La Reine sequence below the bottom chaotic rhythmites was deposited between varve years 1843–1877, while the overlying set of thick varves is correlated to varve years 2065–2129, thus indicating a hiatus of 188 varves. The distinct thick varves show strong compositional and stratigraphic similarities with the Connaught sequence reported elsewhere in the basin and point to a connection with the late-glacial (Cochrane) ice dynamics, which appear to have played an important role on the lake evolution. These results indicate that the late-stage history of Lake Ojibway was marked by the already known Cochrane major ice readvance which was followed by two drainage events separated by at least 65 years, consistent with North Atlantic sediment records that document a two-step drainage of Lake Agassiz-Ojibway.

Lithological and biogeochemical investigations of the North Atlantic sediment system (data from the 49th cruise of the R/V Akademik Ioffe)

Marine sediments and samples of suspended particulate matter from the North Atlantic were studied. It is shown that modern sedimentation in this area is controlled mainly by the efficiency of autochthonous marine phytoplankton and sediment supply by the system of near-bottom currents, as well as by means of the ice-rafting. The studied sediment sections were formed in the North Atlantic during the Middle Pleistocene to Holocene. The highest sedimentation rate, up to 58.3 cm/kyr, is established for AI-3359 core, collected in the area of the Gardar and Björn Drifts. This makes the core very interesting for detailed paleoreconstructions. AI-3378 and AI-3415 cores are characterized by low sedimentation rates. Nevertheless, they are of interest for spatial paleoreconstructions on a continuous timeline. The general pattern is established for the first time for the sediments from the central North Atlantic: biogeochemically significant processes are detected only in the thin surface layer (0–2 cm) and almost completely cease at depths of 90–180 cm. The processes are probably controlled by the position of the sampling stations and the composition of organic matter (OM) but not related to the age of the sedimentary layer. The low microbial activity in the sediments is most likely the result of a lack of OM.

Lithological and Biogeochemical Investigations of the North Atlantic Sediment System (Data from the 49th Cruise of the R/V Akademik Ioffe)

Marine sediments and samples of suspended particulate matter from the North Atlantic were studied. It is shown that modern sedimentation in this area is controlled mainly by the efficiency of autochthonous marine phytoplankton and sediment supply by the system of near-bottom currents, as well as by means of the ice-rafting. The studied sediment sections were formed in the North Atlantic during the Middle Pleistocene to Holocene. The highest sedimentation rate, up to 58.3 cm/kyr, is established for AI-3359 core, collected in the area of the Gardar and Bjorn Drifts. This makes the core very interesting for detailed paleoreconstructions. AI-3378 and AI-3415 cores are characterized by low sedimentation rates. Nevertheless, they are of interest for spatial paleoreconstructions on a continuous timeline. The general pattern is established for the first time for the sediments from the central North Atlantic: biogeochemically significant processes are detected only in the thin surface layer (0–2 cm) and almost completely cease at depths of 90–180 cm. The processes are probably controlled by the position of the sampling stations and the composition of organic matter (OM) but not related to the age of the sedimentary layer. The low microbial activity in the sediments is most likely the result of a lack of OM.

Last Glacial Maximum surface water properties and circulation over Laurentian Fan, western North Atlantic

Millennial scale events marked by the contribution of detrital sand are recorded in North Atlantic sediments during the Last Glacial Maximum (LGM), between Heinrich events (HE) 1 and 2, and left their imprint on Laurentian Fan (LF – 43°N) sediments off eastern Canada. The LF counterpart of the well-known detrital events consist of glacial red-brick sediments resulting from subglacial flows separated by olive-grey sediments appears at ∼21.4–19.9 and ∼19.5–18.65 cal kyr BP. High-resolution analyses of diatom assemblages and lithic grains coupled with planktonic oxygen isotopic records reveal that while the red sediment is almost barren of diatoms, foraminifera and lithics (>150 μm), they are abundant in the olive-grey sediment. Diatom assemblages reveal three phases during these events: (1) initial relatively warm/temperate conditions followed by (2) very cold surface water and drifting ice, and (3) a final phase characterized by relatively warmer waters and the appearance of detrital carbonate. Although these events possibly reflect the variability specific to the slope water region, they are likely the response to Atlantic Meridional Overturning Circulation perturbations and ice-sheet instability. Through a chain of mechanisms, meltwater inputs into the North Atlantic led ultimately to an increased volume of tropical waters and part of the heat stored in the subsurface was flushed by a brief convective episode that was not sustained, accounting for the return of cold conditions after the events. The sequence of mechanisms deduced from the paleo data here and elsewhere is consistent with previous modeling results. These data suggest that the detrital events between Heinrich event 1 and 2 may be synchronous across the North Atlantic, and that the LGM was probably not a time of prolonged steady state in the climate system.

Export of nutrient rich Northern Component Water preceded early Oligocene Antarctic glaciation

The onset of the North Atlantic Deep Water formation is thought to have coincided with Antarctic ice-sheet growth about 34 million years ago (Ma). However, this timing is debated, in part due to questions over the geochemical signature of the ancient Northern Component Water (NCW) formed in the deep North Atlantic. Here we present detailed geochemical records from North Atlantic sediment cores located close to sites of deep-water formation. We find that prior to 36 Ma, the northwestern Atlantic was stratified, with nutrient-rich, low-salinity bottom waters. This restricted basin transitioned into a conduit for NCW that began flowing southwards approximately one million years before the initial Antarctic glaciation. The probable trigger was tectonic adjustments in subarctic seas that enabled an increased exchange across the Greenland–Scotland Ridge. The increasing surface salinity and density strengthened the production of NCW. The late Eocene deep-water mass differed in its carbon isotopic signature from modern values as a result of the leakage of fossil carbon from the Arctic Ocean. Export of this nutrient-laden water provided a transient pulse of CO2 to the Earth system, which perhaps caused short-term warming, whereas the long-term effect of enhanced NCW formation was a greater northward heat transport that cooled Antarctica.

Grain size dependent magnetic discrimination of Iceland and South Greenland terrestrial sediments in the northern North Atlantic sediment record

We use isothermal and temperature dependent in-field and magnetic remanence methods together with electron microscopy to characterize different sieved size fractions from terrestrial sediments collected in Iceland and southern Greenland. The magnetic fraction of Greenland silts (3–63 μm) and sands (>63 μm) is primarily composed of near-stoichiometric magnetite that may be oxidized in the finer clay (<3 μm) fractions. In contrast, all Icelandic fractions dominantly contain titanomagnetite of a range of compositions. Ferrimagnetic minerals preferentially reside in the silt-size fraction and exist as fine single-domain (SD) and pseudo-single-domain (PSD) size inclusions in Iceland samples, in contrast to coarser PSD and multi-domain (MD) discrete magnetites from southern Greenland. We demonstrate the potential of using magnetic properties of the silt fraction for source unmixing by creating known endmember mixtures and by using naturally mixed marine sediments from the Eirik Ridge south of Greenland. We develop a novel approach to ferrimagnetic source unmixing by using low temperature magnetic susceptibility curves that are sensitive to the different crystallinity and cation substitution characteristics of the different source regions. Covariation of these properties with hysteresis parameters suggests sediment source changes have driven the magnetic mineral variations observed in Eirik Ridge sediments since the last glacial maximum. These observations assist the development of a routine method and interpretative framework to quantitatively determine provenance in a geologically realistic and meaningful way and assess how different processes combine to drive magnetic variation in the North Atlantic sediment record.

The 8.2 ka cooling event caused by Laurentide ice saddle collapse

The 8.2 ka event was a period of abrupt cooling of 1–3 °C across large parts of the Northern Hemisphere, which lasted for about 160 yr. The original hypothesis for the cause of this event has been the outburst of the proglacial Lakes Agassiz and Ojibway. These drained into the Labrador Sea in ∼0.5–5 yr and slowed the Atlantic Meridional Overturning Circulation, thus cooling the North Atlantic region. However, climate models have not been able to reproduce the duration and magnitude of the cooling with this forcing without including additional centennial-length freshwater forcings, such as rerouting of continental runoff and ice sheet melt in combination with the lake release. Here, we show that instead of being caused by the lake outburst, the event could have been caused by accelerated melt from the collapsing ice saddle that linked domes over Hudson Bay in North America. We forced a General Circulation Model with time varying meltwater pulses (100–300 yr) that match observed sea level change, designed to represent the Hudson Bay ice saddle collapse. A 100 yr long pulse with a peak of 0.6 Sv produces a cooling in central Greenland that matches the 160 yr duration and 3 °C amplitude of the event recorded in ice cores. The simulation also reproduces the cooling pattern, amplitude and duration recorded in European Lake and North Atlantic sediment records. Such abrupt acceleration in ice melt would have been caused by surface melt feedbacks and marine ice sheet instability. These new realistic forcing scenarios provide a means to reconcile longstanding mismatches between proxy data and models, allowing for a better understanding of both the sensitivity of the climate models and processes and feedbacks in motion during the disintegration of continental ice sheets.

Reconstruction of mid- to late-Holocene winter temperatures in the Skagerrak region using benthic foraminiferal Mg/Ca and δ18O

Reconstruction of Skagerrak deep-water renewal is used to assess regional changes in winter thermal conditions over the past 6800 years. Changes in winter climate conditions from the Skagerrak region are in turn linked to shifts in Holocene large-scale atmospheric circulation patterns prevailing over northern Europe. We use Melonis barleeanus Mg/Ca from two sediment cores in the central Skagerrak to reconstruct temperature of Skagerrak intermediate water, representing the warm season temperature variability, and deep water, for monitoring Skagerrak deep-water renewal, reflecting the winter temperature variability. In addition, M. barleeanus δ18O is used from the deeper core to reconstruct salinity, also monitoring the deep-water renewal. Our results show that the Skagerrak deep-water experienced phases of particularly enhanced renewal during the mid-Holocene reflecting severe winter conditions, followed by a general shift to reduced renewal as a consequence of milder winter conditions over the North Sea around 3500 cal. yr BP. The late-Holocene shift was most likely related to the onset of a regime with intensified winter westerly winds directed toward northern Europe and an increased inflow of North Atlantic water into the Skagerrak–North Sea reflecting more maritime climate conditions. On millennial scale, cold phases in our deep-water records match with low winter precipitation phases in western Norway. They are associated with distinct increases in ice rafted debris (IRD) in North Atlantic sediments, suggesting that phases of iceberg discharge in the Atlantic were associated with cold and dry winter conditions over northern Europe. Interestingly, the cold event centered around 5900 cal. yr BP appears to be only associated with winter variability, while the following one at 4200 cal. yr BP is documented in our winter record, as well as in records related to warmer seasons.

Age assessment and implications of late Quaternary periglacial and paraglacial landforms on Muckish Mountain, northwest Ireland, based on Schmidt-hammer exposure-age dating (SHD)

Schmidt-hammer exposure-age dating (SHD) was applied to a variety of late Quaternary periglacial and paraglacial landforms composed of coarse rock debris on Muckish Mountain, northwest Ireland. Landform ages were determined using a linear high-precision age-calibration curve, derived from young and old control surfaces of known age on the same rock type. The SHD ages represent maximum estimates of the time elapsed since the boulders stabilised and the landforms became inactive. Most ages are also minimum estimates for the start of landform development because older boulders are buried beneath the sampled surface boulders. Ages and 95% confidence intervals obtained for blockfield, boulder lobes and talus indicate these features were likely active during several of the early Holocene cold events evidenced in Greenland ice cores and North Atlantic sediment records. Activity ceased at different times ~9–7ka BP. These landforms are the first indication of a geomorphological response to early Holocene cooling in the oceanic mountains of Ireland. Late Holocene ages, obtained for rock-slope failure run-out debris and debris cone boulders, overlap with shifts to cooler and/or wetter conditions, including the Little Ice Age. Geomorphological impacts associated with these changes in climate have not previously been recorded in the Irish uplands. The SHD results indicate that previously implied timings for the stabilisation of some accumulations of coarse rock debris on mountain slopes are in need of revision.

Magnetic unmixing of first‐order reversal curve diagrams using principal component analysis

AbstractWe describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first‐order reversal curve (FORC) diagrams. For PCA, we resample FORC distributions on grids that capture diagnostic signatures of single‐domain (SD), pseudosingle‐domain (PSD), and multidomain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end‐member (EM) FORC diagrams, located at user‐defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt‐sized MD component, a fine silt‐sized PSD component, and a mixed clay‐sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA‐based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined data set of Rockall Trough and NADW sediments, which can be recast as a four‐EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multicomponent magnetic mixtures, a fundamental task of rock magnetic studies.

A combined paleomagnetic/dating investigation of the upper Jaramillo transition from a volcanic section at Tenerife (Canary Islands)

A coupled paleomagnetic/dating investigation has been conducted on a sequence of 25 successive lava flows, emplaced during the upper transition of the Jaramillo subchron in Tenerife, Canary Islands. This sequence is located along the western wall of the Güímar collapse scar, in the south central part of the island. Nine flows distributed throughout this sequence were dated using unspiked K/Ar and 40Ar/39Ar methods. They bracket the section between 1009±22 ka and 971±21 ka (2σ). A first group of 8 flows at the bottom of the sequence is characterized by normal polarity with paleointensity values of the order of present-day field intensity in the Canary Islands. The virtual geomagnetic poles (VGP) of these 8 flows describe a short loop at high latitudes. Seven overlying flows are transitional in directions and dated between 991±14 ka and 1002±11 ka consistently with published ages of the upper Jaramillo reversal. This second group of flows is characterized by low paleointensity values (around 8–12 μT) that are less than 30% of the present dipole value in Tenerife. The VGPs of the first two transitional flows lie over northeastern Pacific whereas the five following transitional flows have all negative inclinations and their VGPs lie initially over East Antarctica, then describe a northward loop almost reaching New Zealand. The final group of ten flows yield intensities varying between 20 and 35 μT and VGPs close to the southern pole with two of them describing a small amplitude second loop to southeastern Pacific. Assuming a constant extrusion rate as a very first approximation, the distribution of the obtained ages suggests a duration of 7.6±5.6 ka for the transitional interval. The obtained transitional positions of VGPs are consistent with the path reported for the same reversal from North Atlantic sediments but are different from the only other volcanic record from Tahiti. The intensity low characterizing the transitional interval remains the best tie point, centered at 996±7 ka (2σ) relative to 28.02 Ma FC sanidine.

A Holocene palynological record from the northeastern Laptev Sea and its implications for palaeoenvironmental research

A 844 cm long core PS51/118-3 (77°53.54′ N; 132°11.92′ E) recovered from the upper slope (122 m water depth) of the Laptev Sea (Russian Arctic) has been studied for pollen, spores and aquatic palynomorphs, including freshwater green algae and cysts of marine dinoflagellates. The age model was established on the basis of radiocarbon dates obtained on marine bivalve mollusk shells. The available dates suggest that the analyzed sediment was accumulated during the last ca. 10.8 cal. ka and reveal two intervals with markedly different sedimentation rates, reflecting the sedimentary regime changes of the Laptev Sea shelf during postglacial sea-level rise. Very high sedimentation rates (ca. 4.7 mm per year) in the lower part of the core (120–866 cm) between ca. 9.2 and 10.8 cal. ka BP reflect lower-than-present sea levels, high erosion activity and much closer position of the palaeo-shoreline with the Lena and Yana river mouths to the core site. Dramatic decrease in sedimentation rates (ca. 0.1 mm per year) during the middle and late Holocene interval reflects high sea-level and decreased amount of suspended material transported to the outer shelf by rivers. Despite the location of the core site at the continental slope and far away from the modern coastline pollen, spores and fresh-water algae constitute a major part of the microfossils throughout the whole record, indicating great impact of the Lena and Yana rivers and possibly prevalent wind regime on the pollen and non-pollen-palynomorph (NPP) assemblages. Although a number of short-term (decadal to multi-century) oscillations deviate from the mean Holocene values, pollen taxa percentages and pollen-based numerical biome reconstructions do not show very clear trends. The latter is likely a result of the mixed environmental signal and complex pollen contribution of several large environmental regions and vegetation zones of Siberia drained by the Lena and Yana rivers. The greater pollen contribution of the forested regions to the PS51/118-3 record reflects higher pollen production of the boreal trees and shrubs over the low-productive Arctic vegetation. The intervals of the relative increase in the tundra biome scores in the PS51/118-3 record reflect decreased arboreal pollen production or/and increased landscape openness within the pollen source area and can be correlated (within the uncertainty of the age models) with the cold episodes observed in the Greenland ice and North Atlantic sediment records.

The response of the Okhotsk Sea environment to the orbital-millennium global climate changes during the Last Glacial Maximum, deglaciation and Holocene

Reconstruction of regional climate and the Okhotsk Sea (OS) environment for the Last Glacial Maximum (LGM), deglaciation and Holocene was performed on the basis of high-resolution records of ice rafted debris (IRD), СаСО3, opal, total organic carbon (TOС), biogenic Ba (Ba_bio) and redox sensitive element (Mn, Mo) content, and diatom and pollen results of four cores that form a north-southern transect. Age models of the studied cores were earlier established by AMS 14C data, oxygen–isotope chronostratigraphy and tephrochronology. According to received results, since 25ka the regional climate and OS environmental conditions have changed synchronously with LGM condition, cold Heinrich event 1, Bølling–Allerød (BA) warming, Younger Dryas (YD) cooling and Pre-Boreal (PB) warming recorded in the Greenland ice core, North Atlantic sediment, and China cave stalagmites. Calculation of IRD MAR in sediment of north–south transect cores indicates an increase of sea ice formation several times in the glacial OS as compared to the Late Holocene. Accompanying ice formation, increased brine rejection and the larger potential density of surface water at the north shelf due to a drop of glacial East Asia summer monsoon precipitation and Amur River run off, led to strong enhancement of the role of the OS in glacial North Pacific Intermediate Water (NPIW) formation. The remarkable increase in OS productivity during BA and PB warming was probably related with significant reorganisation of the North Pacific deep water ventilation and nutrient input into the NPIW and OS Intermediate Water (OSIW). Seven Holocene OS millennial cold events based on the elevated values of the detrended IRD stack record over the IRD broad trend in the sediments of the studied cores have occurred synchronously with cold events recorded in the North Atlantic, Greenland ice cores and China cave stalagmites after 9ka. Diatom production in the OS was mostly controlled by sea ice cover changes and surface water stratification induced by sea-ice melting; therefore significant opal accumulation in sediments of this basin begin from 4 to 6ka ago simultaneously with a remarkable decrease of sea ice cover.