Elena V. Ivanova

Primary production in the Arabian Sea during the last 135 000 years

Abstract Variations in primary productivity (PP) have been reconstructed in eutrophic, mesotrophic and oligotrophic parts of the Arabian Sea over the past 135 000 years applying principal component analysis and transfer function to planktic foraminiferal assemblages. Temporal variation in paleoproductivity is most pronounced in the mesotrophic northern (NAST site) and oligotrophic eastern (EAST site) Arabian Sea, and comparatively weak in the western eutrophic GeoB 3011-1 site in the upwelling area off Oman. Higher PP during interglacials (250–320 g C m−2 year−1) than during cold stages (210–270 g C m−2 year−1) at GeoB 3011-1 could have been caused by a strengthened upwelling during intensified summer monsoons and increased wind velocities. At NAST, during interglacials, PP is estimated to exceed 250 g C m−2 year−1, and during glacials to be as low as 140–180 g C m−2 year−1. These fluctuations may result from a (1) varying impact of filaments that are associated to the Oman coastal upwelling, and (2) from open-ocean upwelling associated to the Findlater Jet. At EAST, highest productivity of about 380 g C m−2 year−1 is documented for the transition from isotope stage 5 to 4. We suggest that during isotope stages 2, 4, 5.2, the transition 5/4, and the end of stage 6, deep mixing of surface waters was caused by moderate to strong winter monsoons, and induced an injection of nutrients into the euphotic layer leading to enhanced primary production. The deepening of the mixed layer during these intervals is confirmed by an increased concentration of deep-dwelling planktic foraminiferal species. A high-productivity event in stage 3, displayed by estimated PP values, and by planktic foraminifera and radiolaria flux and accumulation rate, likely resulted from a combination of intensified SW monsoons with moderate to strong NE monsoons. Differential response of Globigerina bulloides, Globigerinita glutinata and mixed layer species to the availability of food is suited to subdivide productivity regimes on a temporal and spatial scale.

Climate and oceanographic variability in the SW Barents Sea during the Holocene

The Holocene section of the marine sediment core PSh-5159N, located in the SW Barents Sea, has been studied at high resolution with a multiproxy approach. A well-stratified water column occurred at the site 11—9.8 ka BP. The stratification was probably a result of a winter sea ice cover and/or fresh, warm surface waters during summer. Stratification and resultant reduction in air—sea interaction allowed for warmer bottom water temperatures. The general situation 11—9.8 ka BP could have been associated with an anomalous high-pressure system over the Nordic Seas and the Arctic Ocean. During the 11—10.5 ka BP interval the polar front was located close to the Barents Sea margin. The polar front moved towards the site from 10.5 ka BP, and from 9.8 to 7.5 ka BP it was probably located close to the site. At 7.5 ka BP the polar front retreated eastwards as the present-day oceanographic pattern established. The mid Holocene was in general characterized by rather stable conditions. In contrast, highly variable conditions are recorded throughout the late Holocene. Episodic expansions of the coastal water influenced zone are typical for the last 2.5 ka BP. Predominantly cold conditions and reduced southwesterly wind strength are suggested during these episodes. The Holocene temperature variability seems in general to be of larger amplitude than instrumentally recorded temperature changes in the SW Barents Sea.

Early Holocene temperature variability in the Nordic Seas: The role of oceanic heat advection versus changes in orbital forcing

Received 7 January 2011; revised 15 July 2011; accepted 21 July 2011; published 22 October 2011. [1] The separate roles of oceanic heat advection and orbital forcing on influencing early Holocene temperature variability in the eastern Nordic Seas is investigated. The effect of changing orbital forcing on the ocean temperatures is tested using the 1DICE model, and the 1DICE results are compared with new and previously published temperature reconstructions from a transect of five cores located underneath the pathway of Atlantic water, from the Faroe‐Shetland Channel in the south to the Barents Sea in the north. The stronger early Holocene summer insolation at high northern latitudes increased the summer mixed layer temperatures, however, ocean temperatures underneath the summer mixed layer did not increase significantly. The absolute maximum in summer mixed layer temperatures occurred between 9 and 6 ka BP, representing the Holocene Thermal Maximum in the eastern Nordic Seas. In contrast, maximum in northward oceanic heat transport through the Norwegian Atlantic Current occurred approximately 10 ka BP. The maximum in oceanic heat transport at 10 ka BP occurred due to a major reorganization of the Atlantic Ocean circulation, entailing strong and deep rejuvenation of the Atlantic Meridional Overturning Circulation, combined with changes in the North Atlantic gyre dynamic causing enhanced transport of heat and salt into the Nordic Seas.

Late Weichselian to Holocene paleoenvironments in the Barents Sea

Paleoceanographic changes since the Late Weichselian have been studied in three sediment cores raised from shelf depressions along a north–south transect across the central Barents Sea. AMS radiocarbon dating offers a resolution of several hundred years for the Holocene. The results of lithological and micropaleontological study reveal the response of the Barents Sea to global climatic changes and Atlantic water inflow. Four evolutionary stages were distinguished. The older sediments are moraine deposits. The destruction of the Barents Sea ice sheet during the beginning of the deglaciation in response to climate warming and sea level rise resulted in proximal glaciomarine sedimentation. Then, the retreat of the glacier front to archipelagoes during the main phase of deglaciation caused meltwater discharge and restricted iceberg calving. Fine-grained distal glaciomarine sediments were deposited from periodic near-bottom nepheloid flows and the area was almost permanently covered with sea ice. The dramatic change in paleoenvironment occurred near the Pleistocene/Holocene boundary when normal marine conditions ultimately established resulting in a sharp increase of biological productivity. This event was diachronous and started prior to 10 14C ka BP in the southern and about 9.2 14C ka in the northern Barents Sea. Variations in sediment supply, paleoproductivity, sea-ice conditions, and Atlantic water inflow controlled paleoenvironmental changes during the Holocene.

The Role of Thermohaline Circulation in Global Teleconnections

Evidence for global teleconnections via the ocean and atmosphere is found throughout marine and terrestrial climate records at glacial–interglacial, millennial, centennial, and shorter time scales; however, their mechanisms are still enigmatic. The examples of linkages between the North Atlantic and several low and high latitude areas at millennial and shorter time scales are reviewed with special emphasis to oceanic teleconnections and feedbacks. Paleo-observations and modeling results suggest that changes in the THC mode exert significant control on the spatial and temporal variability of heat and moisture exchange between the ocean and atmosphere and hence on the global climate at orbital and sub-orbital timescales. The widely debatable “bipolar seesaw” and “eddy” hypotheses were proposed to explain the anti-phase millennial-scale temperature changes in high northern and southern latitudes. The assumed coupling between weakened upwelling in the North Pacific and reduced NADW formation during the stadials of the DO cycles still needs further investigation. The atmospheric teleconnections operate mainly on short-term scales via the major wind systems, ENSO, NAO, and other oscillations.

Methods and Proxies of Paleoceanographic Reconstructions

This chapter provides a brief overview of the existing traditional and upcoming micropaleontological, geochemical, and other proxies, methods, and techniques of paleoceanographic reconstructions referred to in the monograph. Special emphasis is given to the foraminiferal transfer functions and stable isotopes as the concert of these techniques enables to obtain the quantitative estimates of the basic parameters of the thermohaline circulation, notably temperature and salinity. The carbon isotope composition of planktic and epibenthic species and several conservative and non-conservative water mass tracers are mentioned as the tools to discriminate the advection and propagation of the waters from different sources.

Paleoceanography of the Northern Indian Ocean: Linkages to Monsoon and Global Thermohaline Paleocirculation

The foraminiferal assemblages and other marine and terrestrial indicators suggest the onset of the Indian monsoon in the middle or even early Miocene. A number of marine and terrestrial paleoarchives monitor the general pattern of the intensified SW (summer) Indian monsoon during warm stages and NE (winter) monsoon during cold stages at glacial–interglacial and millennial timescales. The numerous short-lasting events were superimposed on this pattern during the Pleistocene–Holocene. The primary forcing mechanisms of the monsoon variability are contrast in land–ocean sensible heating and tropospheric latent heating which in turn largely depend on the orbital parameters of the Earth. Remarkable monsoon-induced variations are ascertained in surface circulation, coastal and open-ocean upwellings, primary production, and intensity of the oxygen minimum zone. Besides, the hydrological parameters and microfossil assemblages in the northern Indian Ocean have been significantly affected by variations in the intensity of the return branch of the global thermohaline circulation as well as the rate of the NADW and Southern Ocean waters propagation into the tropical Indian Ocean during the high (interglacial) and low (glacial) sea level stands. The most pronounced environmental changes are associated with the reorganization of the THC over the terminations.

The Global Thermohaline Circulation and the Main Stages of Its Development During the Cenozoic

The concept of the global conveyor suggested in the 1980s evolved in the generally accepted theory of the global thermohaline circulation (THC). One of the major mechanisms driving the THC is the deep convection in the Subpolar North Atlantic due to the high density of the surface waters. However, significant amount of deep, bottom, and intermediate waters are formed in several other areas of the ocean, notably in the Antarctic, providing the large-scale overturning. The large oceanic heat transport, ocean–atmosphere interaction, heat and moisture release and uptake in particular energy-active areas exert a strong control on the global climate. Along with the millennial and longer-term variations linked to the THC, the atmospheric circulation modes are of crucial importance for the interannual and decadal-scale variability. The development of the modern THC pattern during the Neogene resulted from several dramatic tectonic and paleogeographic events which contributed to the reduction of interoceanic exchange in low latitudes and its strengthening in high southern latitudes.

Variability of the Meridional Overturning Circulation and Paleoceanographic Events in the North Atlantic During the Last Climatic Cycle

The THC is known to exhibit pronounced variability at glacial–interglacial, millennial, and shorter timescales. Along with the orbital forcing and global feedbacks controlling the glacial–interglacial variability of the THC, millennial-to-centennial changes in its modes were strongly influenced by the ice sheets dynamics and interaction with the atmospheric oscillations. During the Last Interglaciation, the THC mode resembled its modern pattern but the climate was characterized by rather prominent variability. Recent data disprove the shutdown of the THC during the Last Glacial Maximum and point to a shift into the convection cells. However, a shutdown or at least dramatic slowdown of the AMOC is suggested for some Heinrich events as a result of massive freshwater discharge into the North Atlantic. The abrupt climate changes during the Dansgaard–Oeschger cycles, Younger Dryas, and 8.2 cal. ka cooling event are also thought to be associated with the weakening of the AMOC due to the large freshwater input into the North Atlantic. However, the origin of the above-mentioned events is different and still debatable.

Influence of the Thermohaline Circulation on Paleoceanographic Events in the South China Sea

During the last climatic cycle, the monsoon-governed marginal South China Sea and surrounding lands experienced dramatic climate changes on millennial-to-decadal scales. At glacial intervals, the influence of the upper limb of the THC on the South China Sea decreased due to the sea level drop and emergence of the Sunda subcontinent that cut off the inflow of warm Indo-Pacific waters via the Borneo Strait. The sea became a semi-closed basin with the estuarine circulation, oxygen-minimum layer, and the only passageway to the open western Pacific via the Luzon Strait in the northeast. Unlike interglacials, the amplification of climatic signal in the South China Sea due to its sharp isolation from the THC during the glacials along with the strengthening of winter monsoon and weakening of summer monsoon was manifested by remarkable decrease in winter SST and surface water salinity, and increase in seasonality, mixed-layer, and thermocline depths over the major part of the basin, except for the upwelling area off Luzon tip. The short-term variability of hydrological parameters superimposed on the glacial–interglacial cyclicity suggests the global teleconnections as several climatic events in the South China Sea are coeval with DO cycles and changes in the Indian monsoon.