A multiproxy-based reconstruction of the mid- to late Holocene paleoenvironment in the Laptev Sea off the Lena River Delta (Siberian Arctic)

Highlights • Land-shelf interactions in vicinity to the Lena River delta since 6 cal. ka. • Arctic tundra and taiga zones drained by the Lena River form shelf pollen assemblages. • Warmer-than-present climatic conditions 6–4.5 cal. ka and subsequent gradual cooling. • Estuarine-like reversed (N to S) bottom current activity along submarine paleovalleys. • Principal diversion of the major Lena River outflows to the east since 0.5 cal. ka. Abstract Land-shelf interactions and related environmental changes were reconstructed for the past 6 cal. kyrs interpreting multiproxy records from the two AMS14C-dated sediment cores from the sites located to the north and northeast of the Lena River delta. Proxies used include terrestrial and aquatic palynomorphs, benthic/planktic foraminifers and ostracods paired with benthic δ18O and δ13C records of Haynesina orbiculare. The study focused on unravelling the relation between river runoff and the regional climate changes on the one hand, and its imprint on the shelf sea environment on timescales beyond the instrumental records on the other. The palynomorph records show that the Lena River outflow largely determined the composition of species associations and the magnitude of terrestrial matter influx from land. Pollen assemblages of the inner Laptev Sea shelf reflect complex pollen contribution of the Arctic tundra and remote taiga zones drained by the Lena River and indicate a vegetation response to warmer-than-present climatic conditions between 6.0 and 4.5 cal. ka and a subsequent gradual cooling. Fluvial influence in the records is manifested by (1) increases in sedimentation rates; (2) high influxes of pollen/spores and freshwater chlorophycean algae, wood and plant remains; (3) negative δ13C excursions due to the introduction of dissolved inorganic carbon (DIC) from river water. Episodes of enhanced freshwater influence in the surface water layers (fluvial events) correlate with positive benthic δ18O excursions and increasing representation of river-distal species among benthics. Altogether, this points to an estuarine-like reversed (north to south) bottom current activity along the submarine paleovalleys on the shelf. The most pronounced fluvial events are recognized at 5.3–5.9, 1.5 and <0.5 cal. ka. The oldest fluvial event coincides with the final stage of mid-Holocene climate warming. A fluvial event at 1.5 cal. ka is specifically strong at the northeastern site thus marking a direction change of the Lena River outflow at this time and the progressive protrusion of the delta. During the past 500 years distinct negative δ13C values at the northeastern site reflect enhanced riverine influence. It is therefore concluded that the unprecedented change in the δ13C trend corroborates the other evidence for a principal diversion of the major Lena River outflows into its present-day, easterly direction

Boreal (Eemian) Transgression in the Northeastern White Sea Region: Multiproxy Evidence from Bychye-2 Section

Reconstructing interglacial marine environments helps us understand the climate change mechanisms of the past. To contribute to this body of knowledge, we studied a high-resolution 455 cm-thick sediment sequence of the Boreal (Eemian) marine beds directly overlying Moscovian (Saalian) moraine in the Bychye-2 section on the Pyoza River. We analyzed lithological and microfossil (foraminifers, ostracods, pollen, aquatic palynomorphs) variations at the studied site. Stratigraphical zonation is based on the local and well-established regional pollen zones, correlated with the western European pollen zones. The studied marine beds accumulated from the end of the Moscovian glacial (&gt;131 ka) until ca. 119.5 ka. We distinguished three successive phases: a seasonally sea-ice-covered, relatively deep, freshened basin in the initial rapid flooding stage (&gt;131–130.5 ka); a deep basin in the maximum flooding phase with less extensive sea ice cover (130.5–130.25 ka); and a shallow basin with reduced sea ice cover (130.25–119.5 ka). According to a pollen zone comparison with other sites, the regional glacioisostatic rebound started ca. 130 ka. The diverse warm-water assemblages of benthic foraminifers and ostracods containing typical Baltic Sea species occurred during the regression, mainly 128–124 ka, thus giving evidence for a relatively long-lasting connection between the White and Baltic Seas.</jats:p

Postglacial environmental changes at the eastern Laptev Sea continental margin: evidence from benthic and planktic foraminiferal assemblages

In order to reconstruct the late glacial to Holocene history of Atlantic-derived water (ADW) inflow into the Arctic Ocean along the northern Eurasian margin, past freshwater inputs and variability in sea-ice extent we carry out investigation of the distribution and abundance of benthic and planktic foraminifers in the AMS14C-dated sediment core PS2458-4 from the eastern Laptev Sea continental slope (78°10.0’N, 133°23.9’E) located at 983 m water depth in front of the position of the former Lena and Yana river mouths on the shelf break (Spielhagen et al., 2005). The 822 cm long sediment sequence was sampled continuously in 2 cm thick slices. The core is well-dated from 201 cm down to 625 cm, this sediment interval corresponds in time to 8.9-14.6 cal.ka (Spielhagen et al., 2005), whereas the basal and uppermost parts of the core lack sufficient amount of datable material. Based on preliminary data on taxonomic composition and abundance of benthic and planktic foraminifers several intervals were distinguished corresponding to certain changes in past environments. The oldest lateglacial period (822-660 cm) is characterized by very low abundance of microfossils suggesting harsh environmental conditions with heavy sea-ice cover. However, high percentages of ADW-indicative benthic foraminifer Cassidulina neoteretis and the presence of diverse planktic subpolar species give evidence for the strong subsurface inflow of ADW along the Laptev Sea continental slope. The increasing representation of Elphidium clavatum, higher foraminiferal abundance and species diversity imply growing fluvial influence during early deglaciation (660-600 cm, until 14.2 cal.ka). The abundance of subpolar planktic foraminifers is the highest although their diversity considerably decreases compared with the earlier lateglacial times. Enhanced ADW inflow to the core site is manifested by the sharp dominance of C. neoteretis in the record during the Bølling-Allerød period (600-520 cm, 14.2-12.9 cal.ka). Transition to the Younger Dryas period (520-504 cm, ~12.9-12.6 cal.ka) is marked by a drastic decrease in the total abundance of all microfossils coincident with the previously recorded sharp negative excursion in the oxygen isotope composition of planktic foraminifers implying surface water freshening (Spielhagen et al., 2005). This layer appeared to be also enriched in authigenic vivianite concretions indicating anaerobic conditions at the sea floor. Previously, vivianite accumulations were recorded in lateglacial sediments from the western Laptev Sea and were provisionally related to the meltwater-induced stratification of the water column (Taldenkova et al., 2010). Generally, the Younger Dryas period in the studied core (520-380 cm, 12.9-10.8 cal.ka) is characterized by the dominance of typical arctic species (Cassidulina reniforme, E.clavatum, Islandiella helenae/norcrossi) and decreasing foraminiferal abundance. The core site was probably located close to the summer sea-ice marginal zone with increased seasonal productivity as evidenced by the appearance of Nonion labradoricum. The Early Holocene (380-215 cm, 10.8-9 cal. ka) stands out as a period of sharply increasing abundance and diversity of benthic foraminifers together with other benthic fossil groups (ostracods, bivalves, gastropods). However, the abundance of planktic foraminifers and representation of C. neoteretis decrease. Altogether this might be a result of surface water warming, diminished sea-ice cover, strong mixing of open-sea waters with freshened water masses formed on the flooded outer shelf. The latter conditions were unfavorable for planktic foraminifers and C. neoteretis (Lubinski et al., 2001). Mid-Late Holocene time interval (~215-35 cm, <9 cal.ka) is distinguished by decreasing total abundance and diversity of microfossils likely due to climate cooling. Growing representation of C. neoteretis indicates strong subsurface ADW inflow and reduced interaction of the open-sea and shelf water masses. References Lubinski, D.J., Polyak, L., Forman, S.L., 2001. Freshwater and Atlantic water inflows to the deep northern Barents and Kara seas since ca 13 14C ka: foraminifera and stable isotope. Quat. Sci. Rev., 20, 1851-1879. Spielhagen, R.F., Erlenkeuser, H., Siegert, C., 2005. History of freshwater runoff across the Laptev Sea (Arctic) during the last deglaciation. Glob. Planet. Change, 48 (1-3), 187-207. Taldenkova E., Bauch H.A., Gottschalk J., Nikolaev S., Rostovtseva Yu., Pogodina I., Ovsepyan Ya., Kandiano E., 2010. History of ice-rafting and water mass evolution at the northern Siberian continental margin (Laptev Sea) during Late Glacial and Holocene times. Quat. Sci. Rev., 29, 27-28, 3919-3935

Community changes at the North Siberian margin in response to Atlantic water mass variability since last deglacial times

The eastward penetration of Atlantic-derived water (ADW) into the Eurasian Basin of the Arctic Ocean was investigated at the western Laptev Sea continental margin for the time since c. 17.6 ka. Using a high-resolution investigation of the lithology, geochemistry, planktic and benthic foraminifers, and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized. In general, ADW was continuously present in the study area. Between 17.6 and 15.4 ka ADW manifested itself through open-water polynyas and associated upwelling events. Comparison between the Laptev Sea and northern Svalbard shelf using Cassidulina neoteretis allows assuming an unmodified subsurface inflow of ADW within its northern branch between 15.4-13.2, which was strongest after 14.7 ka and in line with the overall climate amelioration. A local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off-shelf displacement of ADW from the studied site as suggested by the disappearance of C. neoteretis between 12 and 7 ka. As evidenced by an abundance peak in Nonion labradoricum, the sea-ice marginal zone was located at the site around 12-11 ka but then shifted northward during the early Holocene warming. Enhanced ADW inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift-ice margin. The inflow of ADW during mid-late Holocene differed from deglacial times because of the combined influence of northern and eastern ADW branches

High-resolution record of the Late Saalian–Eemian environmental changes in the northeastern White Sea Region (Bychye section) inferred from benthic foraminifers

Paleoenvironmental history of the White Sea region during the late Saalian–Eemian transition is reconstructed on the basis of the high-resolution record of benthic foraminifers from a 4.5 m thick marine sediment sequence directly overlying Saalian till in the Bychye section on the Pyoza River, NE White Sea Region. Besides benthic foraminifers which are the most abundant microfossil group in the studied section, also ostracods, pollen, aquatic palynomorphs as well as lithology and benthic foraminiferal stable isotope composition (δ18O, δ13C) were investigated. Palynological correlation with the previously studied sections confirms the age estimation from c. 133 to 120 ka (Devyatova, 1982; Grøsfjeld et al., 2006). Paleoecological analysis of foraminiferal assemblages is based on ecological preferences of different benthic species distinguished in modern Barents and Kara seas (Polyak et al., 2002, Korsun et al., 1994) supported by factor analysis. Five Ecozones were established which reflect rapid postglacial flooding of the territory after the disappearance of the Saalian ice sheet and consequent shallowing of the sea basin likely due to glacial rebound. Ecozone 1 corresponds to the earliest stage of inundation. Composition of microfossils suggests with the predominance of the arctic opportunistic species Elphidium clavatum among foraminifers suggests it was a cold Arctic coastal environment probably with turbid waters, high sedimentation rates and heavy seasonal sea-ice cover. Relative deepening of the shelf basin continued in Ecozone 2 as suggested by the high percentage of river-distal foraminifers indicating bottom water salinities above 30 and less heavy seasonal sea-ice cover. Foraminiferal assemblages are very similar to those occurring in the mid-shelf regions of the Laptev and Kara seas. The maximum flooding and deepening of the basin registered in Ecozone 3 as indicated by the highest percentage of the relatively deep-water species Melonis barleeanus. According to palynological composition, Ecozone 3 correlates with the early Eemian times (c. 128-130 ka). Maximum relative abundances of foraminiferal species Nonion labradoricum and Cassidulina reniforme indicate the warmest surface water conditions and high seasonal productivity. Marine environments resemble the Arctic outer shelf conditions with normal marine bottom water salinity and restricted sea ice cover. However these alternations are hardly a result of Atlantic water penetration since planktic foraminifers are absent and no indicative species are recorded among benthic foraminifers. Ecozone 4 represents the onset of regressive stage likely caused by isostatic crustal rebound as is witnessed by accumulation of sands, gradually decreasing taxonomic diversity and abundance of all microfossils, and introduction of subarctic ostracods and bivalves. Climatic conditions characterized as the most humid and warm gradually deteriorate towards the end of the record. Shallow-water nearshore environment in Ecozone 5 is supported by the abundance of river-proximal species and epifaunal foraminifers indicative of hydrodynamically active bottom waters. References Devyatova, E.I., 1982. Late Pleistocene environments as related to human migrations in the Dvina River basin and in Karelia. Petrozavodsk, 156 pp. (in Russian). Grøsfjeld, K., Funder S., Seidenkrantz M.-S., Glaister C., 2006. Last Interglacial marine environments in the White Sea region, northwestern Russia. Boreas, 35, 493-520. Korsun,S.A., Pogodina I.A., Tarasov G.A., Matishov G.G., 1994. Foraminifers from the Barents Sea (gidrobiology and quaternary paleoecology). Apatity, 136 pp. (in Russian). Polyak, L., Korsun, S., Febo, L., Stanovoy, V., Khusid, T., Hald, M., Paulsen, B.E., Lubinski, D.A., 2002. Benthic foraminiferal assemblages from the southern Kara Sea, a river-influenced arctic marine environment. J. of Foraminiferal Research, 32, 3, 252-273