Paleo Sea Ice Research Articles

Holocene variability in sea ice and primary productivity in the northeastern Baffin Bay

Arctic sea ice is a critical component of the climate system, known to influence ocean circulation, earth’s albedo, and ocean–atmosphere heat and gas exchange. Current developments in the use of IP25 (a sea ice proxy with 25 carbon atoms only synthesized by Arctic sea ice diatoms) have proven it to be a suitable proxy for paleo-sea ice reconstructions over hundreds of thousands to even millions of years. In the NE Baffin Bay, off NW Greenland, Melville Bugt is a climate-sensitive region characterized by strong seasonal sea ice variability and strong melt-water discharge from the Greenland Ice Sheet (GIS). Here, we present a centennial-scale resolution Holocene sea ice record, based on IP25 and open-water phytoplankton biomarkers (brassicasterol, dinosterol and HBI III) using core GeoB19927-3 (73° 35.26′ N, 58° 05.66′ W). Seasonal to ice-edge conditions near the core site are documented for most of the Holocene period with some significant variability. In the lower-most part, a cold interval characterized by extensive sea ice cover and very low local productivity is succeeded by an interval (~ 9.4–8.5 ka BP) with reduced sea ice cover, enhanced GIS spring melting, and strong influence of the West Greenland Current (WGC). From ~ 8.5 until ~ 7.8 ka BP, a cooling event is recorded by ice algae and phytoplankton biomarkers. They indicate an extended sea ice cover, possibly related to the opening of Nares Strait, which may have led to an increased influx of Polar Water into NE-Baffin Bay. The interval between ~ 7.8 and ~ 3.0 ka BP is characterized by generally reduced sea ice cover with millennial-scale variability of the (late winter/early spring) ice-edge limit, increased open-water conditions (polynya type), and a dominant WGC carrying warm waters at least as far as the Melville Bugt area. During the last ~ 3.0 ka BP, our biomarker records do not reflect the late Holocene ‘Neoglacial cooling’ observed elsewhere in the Northern Hemisphere, possibly due to the persistent influence of the WGC and interactions with the adjacent fjords. Peaks in HBI III at about ~ 2.1 and ~ 1.3 ka BP, interpreted as persistent ice-edge situations, might correlate with the Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA), respectively, in-phase with the North Atlantic Oscillation (NAO) mode. When integrated with marine and terrestrial records from other circum-Baffin Bay areas (Disko Bay, the Canadian Arctic, the Labrador Sea), the Melville Bugt biomarker records point to close ties with high Arctic and Northern Hemispheric climate conditions, driven by solar and oceanic circulation forcings.

Autoxidation of the sea ice biomarker proxy IPSO25 in the near-surface oxic layers of Arctic and Antarctic sediments

Over the last decade or so, the mono- and di-unsaturated highly branched isoprenoid (HBI) lipids IP25 (Ice Proxy with 25 carbon atoms) and IPSO25 (Ice Proxy for the Southern Ocean with 25 carbon atoms) have emerged as useful proxies for sea ice in the Arctic and Antarctic, respectively. A more complete understanding of their respective proxy signatures, however, requires more detailed knowledge of their stability in the water column and in sediments. In the current study, we focused on the autoxidation of IPSO25, first by performing laboratory-based oxidation reactions on a purified sample and characterizing products based on detailed mass spectral analysis. We then analysed for the same oxidation products in near-surface sediments retrieved from the Arctic and the Antarctic, and some suspended organic matter from the Antarctic. Our data show that IPSO25 is susceptible to partial autoxidation within the oxic layers of Arctic and Antarctic sediments, while the same processes appear not to be so important in the water column. Although the number of primary autoxidation reactions identified in sediments was not as large as in laboratory experiments, there was evidence for their subsequent modification by biotic degradation. Quantifying the extent of degradation of IPSO25 and IP25 in sediments, and thus the impact of such process on the use of these biomarkers as paleo sea ice proxies, remains challenging at this stage, since most of the primary oxidation products do not accumulate, likely due to secondary biodegradation reactions. Some interesting differences in reactivity were also observed between IPSO25 and IP25 present in the same Arctic sediments. This suggests that factors other than environmental control may influence the IPSO25/IP25 ratio (i.e. DIP25) in Arctic sediments.

Paleo-sea ice distribution and polynya variability on the Kara Sea shelf during the last 12 ka

The Kara Sea is an important area for paleo-climatic research since sea ice and brine formation take place on its shelf—two processes inducing supra-regional climatic implications and thereby connecting regional environmental variability with global climatic conditions. To gain information about past sea ice coverage and variations, three sediment cores distributed in the southern and central parts of the marginal Sea were investigated. By applying the sea ice biomarker IP25 and the PIP25 index [phytoplankton biomarker (dinosterol)-IP25 index] post-glacial sea ice variability could be detected in the central Kara Sea (Core BP00-36/4), with most intense sea ice cover between 12.4 and 11.8 ka coinciding with the Younger Dryas (12.9–11.6 ka), and reduced sea ice cover between 10 and 8 ka during the Holocene Thermal Maximum. During the last ~ 7 ka, increasing sea ice indicators might indicate a Holocene cooling trend, probably induced by declining summer insolation. Furthermore, temporal changes in the fast ice—polynya distribution in the southern Kara Sea were detected: expanding fast ice during the late Holocene and a cyclic short-term Holocene climate variability documented by abrupt changes in the sea ice coverage at the BP00-07/7 core site. Core BP99-04/7 from the Yenisei estuary recorded consistently seasonal sea ice cover since ~ 9.3 ka, apart from five short phases of fast ice expansion to the core site. The strong influence of river run-off as well as estuary processes might prevent the detection of (short-term) climatic signals at this study site.

Linking the Modern Distribution of Biogenic Proxies in High Arctic Greenland Shelf Sediments to Sea Ice, Primary Production, and Arctic‐Atlantic Inflow

AbstractThe eastern north coast of Greenland is considered to be highly sensitive to the ongoing Arctic warming, but there is a general lack of data on modern conditions and in particular on the modern distribution of climate and environmental proxies to provide a baseline and context for studies on past variability. Here we present a detailed investigation of 11 biogenic proxies preserved in surface sediments from the remote High Arctic Wandel Sea shelf, the entrance to the Independence, Hagen, and Danmark fjords. The composition of organic matter (organic carbon, C:N ratios, δ13C, δ15N, biogenic silica, and IP25) and microfossil assemblages revealed an overall low primary production dominated by benthic diatoms, especially at the shallow sites. While the benthic and planktic foraminiferal assemblages underline the intrusion of chilled Atlantic waters into the deeper parts of the study area, the distribution of organic‐walled dinoflagellate cysts is controlled by the local bathymetry and sea ice conditions. The distribution of the dinoflagellate cyst Polarella glacialis matches that of seasonal sea ice and the specific biomarker IP25, highlighting the potential of this species for paleo sea ice studies. The information inferred from our multiproxy study has important implications for the interpretation of the biogenic‐proxy signal preserved in sediments from circum‐Arctic fjords and shelf regions and can serve as a baseline for future studies. This is the first study of its kind in this area.

First records of winter sea ice concentration in the southwest Pacific sector of the Southern Ocean

AbstractWe use a Generalized Additive Model (GAM) to provide the first winter sea ice concentration record from two cores located within the southwest Pacific sector of the Southern Ocean. To compliment the application of GAM, a time series analysis on satellite records of sea ice concentration data was used to extend the standard 13.25 year time series used for paleoceanography. After comparing GAM sea ice estimates with previously published paleo sea ice data we then focus on a new paleo winter sea ice record for marine sediment core E27‐23 (59°37.1'S, 155°14.3'E), allowing us to provide a more comprehensive view of winter sea ice dynamics for the southwest Pacific Ocean. The paleo winter sea ice concentration estimates provide the first suggestion that winter sea ice within the southwestern Pacific might have expanded during the Antarctic Cold Reversal. Throughout the Holocene, core E27‐23 documents millennial scale variability in paleo winter sea ice coverage within the southwest Pacific. Holocene winter sea ice expansion may have resulted from the Laurentide Ice Sheet deglaciation, increased intensity of the westerly winds, as well as a northern migration of the Subtropical and/or Sub‐Antarctic Fronts. Brief consideration is given to the development of a paleo summer sea ice proxy. We conclude that there is no evidence that summer sea ice ever existed at core sites SO136‐111 and E27‐23 over the last 220 and 52,000 years, respectively.

Characterisation and isomerisation of mono- and di-unsaturated highly branched isoprenoid (HBI) alkenes: Considerations for palaeoenvironment studies

We report the structures and analytical data for several highly branched isoprenoid (HBI) mono- and di-unsaturated alkenes (monoenes and dienes). This has been achieved by isolating several well characterised HBI dienes from diatom cultures and sediments, and transforming these to structurally related monoenes and other dienes using partial hydrogenation and clay-catalysed, or diphenyl disulphide induced, isomerisation reactions. HBI double bond (DB) positions have been determined through analysis of a combination of GC–MS and NMR data, along with RuO4 oxidation of HBI monoenes that produce characteristic ketones. Our structural identifications have also enabled the assignments of a number of previously reported HBI monoenes and dienes to be confirmed or corrected. Monoenes and dienes undergo clay-catalysed isomerisation in the laboratory, which likely has implications for the sedimentary occurrences of such biomarkers. Under our conditions, alkenes with C6/17 DBs are particularly sensitive to isomerisation, yielding isomers with C5/6, C6/7, C7/8 and C7/20 DBs. Isomers with C5/6 and C7/20 DBs also isomerise with each other and to isomers with C6/7 and C7/8 DBs, but not to ones with a C6/17 DB. In contrast, we see no evidence for isomerisation of the Arctic sea ice biomarker IP25 which has a DB at C23/24. These observations may have implications for the use of certain monoenes (e.g. IP25) and dienes for paleo sea ice reconstruction. A scheme to summarise the (monoene) isomerisation reactions is proposed. On the basis of the usual DB positions in HBIs obtained from diatom cultures, the outcomes from our isomerisation reactions, and of previous sedimentary reports, we discuss the potential origins (biogenic versus sedimentary transformation) of HBIs with different DB positions. However, confirmation of such assignments will require further investigation.

Identification and characterisation of a novel mono-unsaturated highly branched isoprenoid (HBI) alkene in ancient Arctic sediments

The occurrence of a novel C25 highly branched isoprenoid (HBI) mono-unsaturated hydrocarbon in Arctic marine sediments from the Fram Strait is described. The structure was determined following synthesis from a closely related diene and NMR spectroscopic analysis. The position of the double bond was confirmed by oxidation (RuO4) and comparison of the mass spectrum of the resulting C19 ketone with that of an authentic sample synthesised previously. The same C19 ketone was also present following oxidation of the Fram Strait sediment extracts. The source of the novel HBI monoene biomarker is not known, but is believed to be certain marine diatoms on the basis of structural similarities with other HBIs from such sources. It does not, however, appear to be biosynthesised by Arctic sea ice diatoms, unlike its regio isomer, IP25, an established proxy for Arctic sea ice. Alternatively, it may represent a diagenetic product of the sea ice diatom biomarker IP25; this seems unlikely, however, on the basis of their respective structures. Since the new HBI monoene and IP25 possess similar gas chromatographic and mass spectral properties, but probably have contrasting sources, we suggest that future paleo sea ice reconstruction studies based on the occurrence of IP25 should pay further attention to the rigorous assignment of this biomarker in marine sediment extracts. Some suggestions for achieving this, based on analytical purification and mass spectrometric measurements, are provided.

Reconstructing past sea ice cover of the Northern Hemisphere from dinocyst assemblages: status of the approach

Dinocysts occur in a wide range of environmental conditions, including polar areas. We review here their use for the reconstruction of paleo sea ice cover in such environments. In the Arctic Ocean and subarctic seas characterized by dense sea ice cover, Islandinium minutum, Islandinium? cezare, Echinidinium karaense, Polykrikos sp. var. Arctic, Spiniferites elongatus–frigidus and Impagidinium pallidum are common and often occur with more cosmopolitan taxa such as Operculodinium centrocarpum sensu Wall & Dale, cyst of Pentapharsodinium dalei and Brigantedinium spp. Canonical correspondence analyses conducted on dinocyst assemblages illustrate relationships with sea surface parameters such as salinity, temperature, and sea ice cover. The application of the modern analogue technique permits quantitative reconstruction of past sea ice cover, which is expressed in terms of seasonal extent of sea ice cover (months per year with more than 50% of sea ice concentration) or mean annual sea ice concentration (in tenths). The accuracy of reconstructions or root mean square error of prediction (RMSEP) is ±1.1 over 10, which corresponds to perennial sea ice. Such an error is close to the interannual variability (standard deviation) of observed sea ice cover. Mismatch between the time interval of instrumental data used as reference (1953–2000) and the time interval represented by dinocyst populations in surface sediment samples, which may cover decades if not centuries, is another source of error. Despite uncertainties, dinocyst assemblages are useful for making quantitative reconstruction of seasonal sea ice cover.

Biomarker proxy shows potential for studying the entire Quaternary Arctic sea ice history

A novel and promising biomarker proxy for reconstruction of Arctic sea ice conditions was developed and is based on the determination of a highly branched isoprenoid with 25 carbons (IP25). IP25 records have been restricted to the last 150kyr BP. We present a biomarker record from Ocean Drilling Program (ODP) Site 912, going back to the Pliocene–Pleistocene boundary and indicating that sea ice of variable extent occurred in the Fram Strait/southern Yermak Plateau area at least since about 2.2Ma. Furthermore, our data support the idea that a combination of IP25 and open water, phytoplankton biomarker data (“PIP25 index”) may give a more reliable and quantitative estimate of past sea ice cover (at least for the study area). The study reveals that the novel IP25/PIP25 biomarker approach has potential for semi-quantitative paleo-sea ice studies covering the entire Quaternary and could motivate further detailed high resolution research on ODP/IODP material using this proxy.

Insights into Circum-Arctic sea ice variability from molecular geochemistry

Geological records of past sea ice, such as those contained in Arctic marine sediments, offer an opportunity to strengthen our understanding of long-term sea ice variability, provided unambiguous paleo-sea ice proxies can be developed. One such recently proposed proxy is IP25, a highly branched isoprenoid alkene biosynthesized exclusively by sea-ice dwelling diatoms (Haslea spp.), which is well preserved in marine sediments and could be used to reconstruct past changes in spring sea-ice extent. However, little is known about regional-scale controls on IP25 production in sea ice, limiting its wider applicability as a paleo-sea-ice proxy. To address this issue we examined the distributions of IP25 and the marine productivity biomarkers dinosterol and brassicasterol in a suite of surface sediment samples distributed across the Arctic. We find a statistically significant, logarithmic relationship between IP25 and spring sea ice cover in sediment samples from arctic and subarctic sites in the Pacific (n = 96, R2 = 0.67, P < 0.0001) and the Atlantic n = 25, R2 = 0.50, P < 0.0001), though the absolute concentrations of IP25 are generally higher in the Atlantic (1.6–166.4 μg/g OC) than in the Pacific (0–38.5 μg/g OC) for equivalent sea-ice cover, and there are regional and basin-specific differences in the slope of the IP25 – sea ice relationship. Using the PDIP25 index, which also includes a biomarker for marine productivity (e.g., dinosterol) the relationship with sea ice cover is improved in all of the study areas (R2 = 0.86 and R2 = 0.75 for Atlantic and Pacific, respectively) and most basin specific differences in the proxy–sea ice relationships are significantly reduced. However, there remain differences in the absolute magnitudes of the PDIP25 index for equivalent amounts of sea ice cover between the Atlantic and the Pacific, and with previously published data from Fram Strait. Nevertheless, this work demonstrates that the PDIP25 index can provide a valuable tool for sea ice reconstructions at local and regional scales.

Late Quaternary paleoceanography and paleo-sea ice conditions in the Mackenzie Trough and Canyon, Beaufort SeaThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

The Mackenzie Trough provides a high resolution signal for paleoceanography as a result of high sedimentation rates at the mouth of the Mackenzie River. Three cores were collected along a transect covering a depth range of 58–671 m and the time period of the last 11 500 cal BP. Prior to the last ∼10 000 cal BP, the distal core is characterized by laminated sediment and a foraminiferal fauna of Arctic Bottom Water calcareous species and abundant planktic foraminifera suggesting little freshwater runoff and (or) perennial sea-ice cover. This occurs at a similar time as laminated sediments from the west of this site, which have been suggested to be part of the Lake Agassiz flood outburst and (or) cold period. If this outburst occurred, the very positive oxygen isotope values from our core (PC3; &gt;+3.0 ppm) indicate that it did not flow through the Mackenzie Trough. After 9000 cal BP, the faunas in the three cores differ because of timing and different water depths. However, it is possible to see a progression of cold saline water prior to 10 000 cal BP, with a freshening of surface water after 10 000 cal BP where tintinnids (brackish water ciliates) occur with incursions of deep water Arctic calcareous species to ∼3000 years BP. A sequence of mixed faunas appears as sea ice returns, at least periodically in the last 3000 cal BP; but (in core PC2 only) a return to more sea ice is recorded by both foraminifera and dinocysts in the last few hundred years.

Paleo–sea ice conditions of the Amundsen Gulf, Canadian Arctic Archipelago: Implications from the foraminiferal record of the last 200 years

Four boxcores were collected as part of the Canadian Arctic Exchange Shelf Study (CASES) in the Amundsen Gulf at water depths of 59 m to 600 m. Data from these cores help to develop a record of changes in the oceanographic history of the area over the last 200 years, with particular reference to the indication of paleo–sea ice formation, a key element of the Arctic ecosystem. The four sites cover a range of water depths and environments to provide a basis for comparison. The benthic foraminifera of sites CA‐06 (253 m water depth) and CA‐18 (600 m water depth) show an increase in Arctic Surface Water associated agglutinated foraminifera over the last ∼100 years (uppermost 8 to 16 cm). These are indicating a decrease in sea ice cover and in cold saline Arctic Bottom Water influence; these are similar to Canadian Arctic Archipelago postglacial faunas. This contrasts with abundant planktic foraminifera at the same stations, suggesting strong, oceanic Arctic surface influence (little freshwater) in the central Gulf. The foraminifera of sites 403B (59 m water depth) and 415B (56 m water depth), at the outermost edges of Amundsen Gulf, indicate that the present‐day location of the winter flaw lead has been in place for at least the last 100 years, with foraminiferal faunas similar to those of the Beaufort Shelf. Additionally, station 415 is on an earlier Holocene shoreline that is covered with cobbles.