Nalân Koç

A siliceous microfossil view of middle Eocene Arctic paleoenvironments: A window of biosilica production and preservation

Integrated Ocean Drilling Program (IODP) Expedition 302, “The Arctic Coring Expedition” (ACEX), unearthed the most significant find of Paleogene siliceous microfossils in nearly 2 decades. 100 m of early middle Eocene, organic-rich, finely laminated sediments contain abundant marine and freshwater siliceous microfossils allowing intriguing insights into central Arctic paleoenvironments during the start of Cenozoic cooling. Largely endemic assemblages of marine diatoms and ebridians are preserved along with very high abundances of chrysophyte cysts, the endogenously formed resting stage of freshwater algae. An overall brackish environment is invoked, but variations in group dominance suggest episodic changes in salinity, stratification, and trophic status. With the backing of inorganic geochemistry we synthesize the sediment characteristics by hypothesizing an environmental model for the cooccurrence of these diverse siliceous microfossil groups. We also report on initial insights into the composition of some of the laminations, which may help explain the formation of this rich sediment archive.

Holocene climate in the Atlantic sector of the Southern Ocean: Controlled by insolation or oceanic circulation?

ABSTRACTThe Holocene climate of the Southern Ocean is not well un-derstood, mainly because of the lack of high-resolution reconstruc-tions of ocean surface properties. Here we present a 12,500-yr-long,decadal-scale record of Holocene sea-surface temperatures and sea-ice presence from the Polar Front of the East Atlantic SouthernOcean. The record shows gradual climate change, with no abruptNeoglacial cooling, and an unprecedented late Holocene warming.The dominant forcing factor appears to be precessional insolation;Northern Hemisphere summer insolation correlates to at least theearly to middle Holocene climate trend. Spectral analysis revealscentennial-scale cyclic climate changes with periods of 1220, 1070,400, and 150 yr. The record shows good correlation to East Ant-arctic ice cores and to climate records from South Georgia andBunger Oasis. However, the record shows out-of-phase behaviorwith regard to climate records from the western Antarctic Penin-sula and the Peru-Chile Current; such behavior hints at a climaticdivide through Patagonia, the Drake Passage, and between Westand East Antarctica.Keywords: Southern Ocean, paleoclimate, diatoms, modern analogtechnique, Holocene, insolation, Antarctic Polar Front.INTRODUCTIONThe Holocene climate variability has received less attention thanthe more dramatic changes during the last glacial period. The Holoceneis generally assumed to be a stable interglacial period when climatechanges in the North Atlantic are transmitted to the rest of the worldthrough a strong thermohaline circulation (Imbrie et al., 1992). Thelarge fluctuations of the last glacial period appear to continue into thisinterglacial, although in a subdued manner (Bond et al., 2001).The Southern Ocean has received even less attention than the highnorthern latitudes; oceanic records are few and coarse. They generallydisplay a warm early Holocene followed by cooling (Labracherie etal., 1989; Pichon et al., 1992; Hodell et al., 2001), similar to NorthAtlantic climate (e.g., Koc¸ and Jansen, 1994). High-resolution recordsare restricted to Antarctic ice cores, which mostly display little climatevariability during the Holocene (Jouzel et al., 2001). An exception isthe Taylor Dome deuterium record, in which the Holocene appears tobe split by an abrupt cooling event ca. 5.5–5 ka (Steig et al., 1998).This cooling appears to fit the late Holocene global cooling trend (Por-ter, 2000) as a nonlinear response from the climate system to insolationchanges (deMenocal et al., 2000; Hodell et al., 2001).High-resolution Holocene records available for Southern Oceanclimate reconstruction are rarely from the Antarctic Circumpolar Cur-rent proper (e.g., Domack et al., 2001; Stenni et al., 2001; Lamy et al.,2002), with the exception of a multiproxy approach to Holocene cli-mate that includes a high-resolution description of lithic fragments(Hodell et al., 2001).We present data from site TN057-17 near the Polar Front in theEast Atlantic Southern Ocean. It is based on quantitative estimates ofsummer sea-surface temperature (SSST) and sea-ice presence (SIP),utilizing the down-core variability of diatom assemblages. The recordshows good correlation to Antarctic ice cores, but a gradual change inphase relationships. During the early Holocene, the record shows anin-phase relationship with the Ross Sea region ice cores; during thelate Holocene, the record shows an in-phase relationship with the EastAntarctic region ice cores.Antarctic Polar FrontThe Antarctic Polar Front (APF, Fig. 1) is an important climateboundary in terms of air-sea fluxes and the heat and salt budgets ofthe oceans. At the surface, the APF is a steep sea-surface temperaturegradient, while the subsurface expression is the southern limit for sub-duction of cold, fresh Antarctic Surface Water below warmer and moresaline Subantarctic Surface Water (Orsi et al., 1995). Globally, the APFcould be the southern limit of synchrony with Northern Hemisphereclimate (Broecker, 1996). The APF is subject to large spring and sum-mer blooms of diatoms (Fischer et al., 2002), and achieves the largestbiogenic sedimentation rates of the diatom ooze belt of the SouthernOcean (Burckle and Cirilli, 1987). It is these high sedimentation ratesof diatomaceous oozes and the sensitivity of diatoms especially to lightand temperature that make sites in the APF region suitable for pro-ducing high-resolution records of climate change.

Consistently large marine reservoir ages in the Norwegian Sea during the Last Deglaciation

With the exception of the GS-1/Younger Dryas coldperiodmarine reservoir ages for the Last Deglaciation in the North Atlantic– Norwegian Sea are generally assumed to have been around 400–500 radiocarbon years in magnitude (Earth Planet. Sci. Lett. 126 (1994) 275; Radiocarbon 37 (1995) 53; Quat. Res. 52 (1999) 104; Nature 412 (2001) 724). By comparing the climate records obtained from the GRIP ice-core (Nature 359 (1992) 311; J. Quat. Sci. 13(4) (1998) 283) andfrom North Atlantic/Norwegian Sea cores (Quat. Res. 52 (1999) 104; Geology 23 (12) (1995) 1059; Nature 356 (1991) 757; Nature 356 (1992) 757; Paleoceanography 3(1) (1988) 1; Nature 343 (1990) 612; Earth Planet. Sci. Lett. 126 (1994) 275), with radiocarbon-dated European continental records, we show that marine reservoir ages in the Norwegian Sea were of the order of 1000 14 C yr, including large uncertainties. This approach rests on the reasonable assumption that climate changes throughout the NE Atlantic andEurope were more or less synchronous at the centennial scale. Fairly large variations in reservoir ages over time may have been causedby changing atmospheric 14 C content. The results indicate that detailed land-sea correlations for the North Atlantic during the Last Deglaciation are not feasible using radiocarbon dating alone. r 2003 Elsevier Science Ltd. All rights reserved.

Late glacial palaeoceanography of Hinlopen Strait, northern Svalbard

Timing and structure of the Late and post-glacial development of the northern Svalbard margin, together with the initial influx of Atlantic water into the Arctic Ocean are still very poorly constrained. We investigated a sediment core (NP94-51) from a high accumulation area on the continental shelf north of Hinlopen Strait with the purpose of resolving the timing and structure of the last deglaciation. Detailed analyses of ice-rafted detritus, benthic and planktonic foraminiferal fauna, diatom flora, grain size and radiocarbon dates are used to reconstruct the palaeoceanographic evolution of the area. Our results indicate that the disintegration of Hinlopen Strait ice and possibly the northern margin of the Svalbard Ice Sheet commenced between 13.7 and 13.9 14C Ky BP. Influx of subsurface Atlantic waters into the area (12.6 14C Ky BP) and the retreat of the sea ice cover, with the accompanying opening of the surface waters (10.8 14C Ky BP), happened at different times and both much later than the disintegration of the ice sheets. The transition into the Holocene shows a two-step warming.

Holocene Palaeoenvironmental Changes in North-West Europe: Climatic Implications and the Human Dimension

Holocene climate changes and variability in Europe are outlined on three time scales: long-term changes throughout the period as a whole; shorter-term fluctuations at centennial to millennial scales; and events with an annual to multi-decadal duration. Human population history in Europe during the Holocene is considered in relation to this history of climatic change and variability; in particular, evidence is reviewed that may indicate that human populations responded to, and perhaps also contributed to, climatic changes. The limitations of the data sources and chronological techniques available are briefly outlined.

Evolution of the East Greenland Current between 1150 and 1740 AD, revealed by diatom‐based sea surface temperature and sea‐ice concentration reconstructions

Sediment core MD99-2322 from the East Greenland shelf has been studied to assess the variability of the East Greenland Current between 1150 and 1740 AD. Only the top 100 cm of the high-resolution core—with a 4-year resolution through the upper 20 cm, and a 10-year resolution through the remaining section—was studied. Diatoms were utilized to reconstruct both the August sea-surface temperature (SST) and the May sea-ice concentration, using the weighted averages–partial least squares and maximum likelihood transfer function methods, respectively. The record can be divided into three periods: two periods of relatively stable August SSTs and May sea-ice concentrations, separated by a period of higher August SSTs and decreasing May sea-ice concentrations between 1500 and 1670 AD, and between 1450 and 1610 AD, respectively. Both trends are statistically significant, based on the SIZER (significant zero crossing of the derivatives) analysis of the records. These changes are explained by a decrease in the strength of the East Greenland Current between 1450 and 1670 AD, which was responsible for bringing cold polar water and sea ice to the core site. Simultaneous changes observed in both these parameters points to a strong coupling between them. Because of the high resolution of the record, the natural variability of the system over the period of almost 700 years can be assessed. This variability is about 1°C (0.9°C) for August SSTs and 12% (7.4%) for May sea-ice concentrations.