Carin Andersson

Holocene glacier fluctuations of Flatebreen and winter-precipitation changes in the Jostedalsbreen region, western Norvay, based on glaciolacustrine sediment records:

The history of Holocene glacier variations of Flatebreen, an independent glacier close to the SW part of the Jostedalsbreen ice cap, has been reconstructed from lacustrine sediments in the proglacial lake Jarbuvatnet. The sedimentary succession shows evidence of three maini episodes of Holocene glacier expansion. The first is recorded in the basal part of the core up to 370 cm. According to the age/depth relationship in the sediment core (based on 12 AMS radiocarbon dates), this glacier expansion episode terminated about 10200 cal. yr BP. The second major glacier phase lasted from 8400 to 8100 cal. yr BP, while the third was initiated around 4000 cal. yr BP and has continued up to the present. At 43 cm in the core, the medium silt content increases significantly, accompanied by a minor increase in the sand content. This textural change is interpreted as the first time that the tenninus of Flatebreen extended inlto an] upstream lake at 1083 m a.s.l. The age model suggests that this event took place around 800 cal. yr BP (-AD 1150), as a response to the initial ‘Little Ice Age’ glacier expansion after the ‘Mediaeval Warm Period’. By using a Holocene-inferred summer-temperature curve from central southern Norway in the exponential relationship between annual winter precipitation (snow) and ablation-season temperature at the ELA, periods of higher winter precipitation than the 1961-90 nomial in the Jostedalsbreen region are inferred for 9700-9400, 9200-8300, 8200-6500, 5700-5100, 4700-4600, 4500-4300, 3800-3000, 2100-1800, 1600-1300 and 1200-1000 cal. yr BP, and from 900 cal. yr BP to the present. The intervening periods of lower than normal winter precipitation correlate with periods of enhanced ice-rafting in the North Atlantic.

Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene.

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (361N–751N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from � 0.27 to � 0.151C kyr � 1 . Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.91C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition fromthe Hypsithermal interval ( B9–5.7 ka B.P.) to the Neoglaciation (B5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T fromearly to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. r 2002 Elsevier Science Ltd. All rights reserved.

Late Holocene surface ocean conditions of the Norwegian Sea (Vøring Plateau)

[1] Late Holocene sea surface ocean conditions of the eastern Norwegian Sea (Voring Plateau) are inferred from planktic stable isotopes and planktic foraminiferal assemblage changes in cores JM97-948/2A and MD952011 (66.97� N, 7.64� E). Strong covariance between the planktic stable oxygen isotopic record and abundance changes of N. pachyderma (sin) show that major changes in surface ocean conditions are reflected both in the geochemical composition of the foraminiferal tests as well as in the composition of the foraminiferal fauna. Surface ocean conditions warmer than present were common during the past 3000 years. During the so-called Medieval Warm Period, surface conditions were highly variable with marked changes in sea surface temperature. The warmest sea surface temperatures during this period occurred between 800 and 550 years BP (0 BP = AD 2000). Climatic deterioration, recorded as decreases in sea surface temperature, occurred at about 2750, 1550, 400, and 100 years BP. The cooling events at about 2750 and 1550 years BP appear to correlate with increases in ice-rafted debris in the North Atlantic. Based on the results from JM97-948/2A and MD952011, the onset of the Little Ice Age cooling trend seems to have occurred around 700–600 years BP. Faunal changes indicate two cooling events during the Little Ice Age (at 400 and 100 years BP) that correspond to decreases in Fennoscandian summer temperatures and increases in ice-rafted debris in the eastern North Atlantic. INDEX TERMS: 3030 Marine Geology and Geophysics: Micropaleontology; 4267 Oceanography: General: Paleoceanography; 4870 Oceanography: Biological and Chemical: Stable isotopes; KEYWORDS: paleoceanography, stable isotope, Holocene, micropaleontology Citation: Andersson, C., B. Risebrobakken, E. Jansen, and S. O. Dahl, Late Holocene surface ocean conditions of the Norwegian Sea (Voring Plateau), Paleoceanography, 18(2), 1044, doi:10.1029/2001PA000654, 2003.

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.

Neogloboquadrina pachyderma (dex. and sin.) Mg/Ca and δ18O records from the Norwegian Sea

Magnesium/calcium (Mg/Ca) records based on N. pachyderma (dex.) and N. pachyderma (sin.) from the last 1200 years have been retrieved from high-resolution cores in the Norwegian Sea. Comparing temperatures inferred from Mg/Ca and the oxygen isotopic composition of calcite (δ18OC) with instrumental temperature records from Ocean Weather Ship Mike suggests that N. pachyderma calcifies during the summer season with an offset to oxygen isotope equilibrium of 0.6‰. In this region, summer temperatures below the thermocline are related to the winter season ventilation and breakdown of the seasonal thermocline; hence the deeper dwelling N. pachyderma (sin.) provides a winter season signal. Down-core N. pachyderma (dex.) Mg/Ca temperatures display larger variance than observed in δ18OC temperatures record derived from the same morphotype. The smaller variance in the δ18OCN. pachyderma (dex.) temperatures is probably linked to salinity changes in the upper 50 m of the water column. The Mg/Ca temperature and δ18OC records are used to reconstruct changes in the oxygen isotopic composition of water (δ18Ow) during the last 1200 years. The reconstructed δ18OW records show variation within a realistic range, allowing the influence of other water masses than those present at the site today, and suggest that the Mg/Ca signal in morphotypes of N. pachyderma in the Norwegian Sea responds to changes in climatic parameters.

Reconstructions of surface ocean conditions from the northeast Atlantic and Nordic seas during the last millennium

We undertake the first comprehensive effort to integrate North Atlantic marine climate records for the last millennium, highlighting some key components common within this system at a range of temporal and spatial scales. In such an approach, careful consideration needs to be given to the complexities inherent to the marine system. Composites therefore need to be hydrographically constrained and sensitive to both surface water mass variability and three-dimensional ocean dynamics. This study focuses on the northeast (NE) North Atlantic Ocean, particularly sites influenced by the North Atlantic Current. A composite plus regression approach is used to create an inter-regional NE North Atlantic reconstruction of sea surface temperature (SST) for the last 1000 years. We highlight the loss of spatial information associated with large-scale composite reconstructions of the marine environment. Regional reconstructions of SSTs off the Norwegian and Icelandic margins are presented, along with a larger-scale reconstruction spanning the NE North Atlantic. The latter indicates that the ‘Medieval Climate Anomaly’ warming was most pronounced before ad 1200, with a long-term cooling trend apparent after ad 1250. This trend persisted until the early 20th century, while in recent decades temperatures have been similar to those inferred for the ‘Medieval Climate Anomaly’. The reconstructions are consistent with other independent records of sea-surface and surface air temperatures from the region, indicating that they are adequately capturing the climate dynamics of the last millennium. Consequently, this method could potentially be used to develop large-scale reconstructions of SSTs for other hydrographically constrained regions.

Holocene Climate Variability in the Northern North Atlantic Region: A Review of Terrestrial and Marine Evidence

The Holocene epoch, which followed the last major pulse of glaciation (the Younger Dryas) at the end of the last glaciation, encompasses a period before there was any substantial anthropogenic forcing of climate. A synthesis of climatic development during the Holocene (ca. 11,500 cal. yr BP to the present) is based on pollen-based quantitative temperature reconstructions, tree-line variations, chironomids, tree-ring records, speleothem data, glacier variations, and marine records (stable isotopes, species abundance, lithological changes) from the Nordic Seas. The Holocene has been regarded as a period of relatively stable climate, but recent evidence suggests there have been several significant millennial-scale climate fluctuations (larger than the post mid-19th century warming trend) throughout the Holocene. A general climate warming in the first part of the Holocene was punctuated by a few, abrupt climate reversals, centred at 10,000, 9,700, and 8,200 cal. yr BP. The data suggest there was a period of relatively warm conditions in the first half of the Holocene, in many areas warmer than in the 20th century, after which temperatures generally declined. The temperature decline was punctuated by centennial-scale warmer and colder periods with the most recent cold episode (∼AD 1550-1925), including the Little Ice Age, being one of the coldest of the entire Holocene. The kind of data presented here can be used for detecting mechanisms and forcing factors behind the reconstructed climate variations and to study leads and lags in the Earths climate system.

History of ice rafting at South Atlantic ODP Site 177-1092 during the Gauss and Late Gilbert Chrons

Abstract We have carried out a multiphase analysis of samples from ODP Site 177-1092, Meteor Rise, subantarctic South Atlantic. Samples were analyzed for ice-rafted debris (IRD) and stable isotopes from benthic foraminifera. Both analyses were performed on the same samples. Additional work was performed to identify the paleomagnetic stratigraphy. The analyzed samples range in age from about 2.6(?) Ma to 4.6 Ma, a time span that saw considerable global warmth, but witnessed overall global refrigeration and the transition to truly bipolar glaciations. A tentative oxygen isotopic stratigraphy was established by comparison with Shackleton et al. [R. Soc. Edinburgh Trans. Earth Sci. 8 (1990) 251–261] and shackleton et al. [Proc. ODP Sci. Results 138 (1995) 337–355]. Paleomagnetic results show that the Gauss Normal Chron, including subchrons, is identified, although uncertainties plague the exact definitions of the reversals. The subchrons of the Gilbert Reversed Chron, unfortunately, could not be identified. IRD arrived frequently during the Early and early Late Pliocene, but only as ‘background rafting’ (occasional grains per sample). The first identifiable IRD above background rafting is associated with marine isotope stage (MIS) KM4 (∼3.18 Ma). Successive IRD peaks become larger, the same pattern as noted at nearby Site 114-704. A very large peak near the top of the record, approximately 2.8 Ma, is considered to represent a hiatus. Peaks below 51.3 meters composite depth (mcd) coincide with positive excursions of the oxygen isotopic record, and with negative excursions of the carbon isotopic curve, a pattern also noted at Site 114-704. However, the reasonably large IRD peak at 51 mcd (tentatively identified with MIS G11) coincides with a positive excursion on the carbon isotopic curve and negative excursion on the oxygen isotopic curve. This relationship suggests a northern hemisphere interglacial, rising sea level, destabilization of the Antarctic margin, and delivery of Antarctic icebergs to the Southern Ocean. Such a mechanism has recently been suggested by Kanfoush et al. [Science 288 (2000) 1815–1818] for latest Pleistocene stadial/interstadial oscillations. Here we suggest that such a mechanism may have been in place on glacial/interglacial time scales as early as the Late Pliocene. One interval in the lowermost Gauss Normal Chron and several short intervals in the upper Gilbert Reversed Chron have no IRD. However, oxygen isotopic values of benthic foraminifera are only about 0.62‰ lighter than modern, and must be ascribed to temperature effects in the area of water-mass formation [Hodell and Warnke, Quat. Sci. Rev. 10 (1991) 205–214; Hodell and Venz, Antarctic Research Series 56 (1992) 265–310; Warnke et al., Mar. Micropaleontol. 27 (1996) 237–251]. The East Antarctic Ice Sheet was therefore stable – but the stability of the West Antarctic Ice Sheet may have been compromised.

Exploratory Comparisons of Quantitative Temperature Estimates Over the Last Deglaciation in Norway and the Norwegian Sea

This paper presents an exploratory synthesis of quantitative temperature reconstructions during the last deglaciation in Norway and the Norwegian Sea. The variety of proxy data available permits an overview of climate development and comparisons between land and sea records. Temperature reconstructions from each available proxy were averaged for the time slices late-glacial interstadial (Greenland Interstadial 1; ca. 14,000-12,700 cal BP) and Younger Dryas (Greenland Stadial 1; 12,700-11,500 cal BP) and at the end of the early Holocene rapid warming phase. When mapped, the data reveal spatial and temporal differentiation. Interstadial terrestrial July temperatures were ca. 10°C in the south to ca. 6°C in the north (gradient of ca. 4°C). Summer sea-surface temperatures were ca. 8°C in the south and ca. 5°C in the north (gradient of 2-3°C). The Younger Dryas July gradient was reduced on land to ca. 2-3°C and to 0°C at sea because southern temperatures cooled more than northern ones. Interstadial and Younger Dryas winter temperatures were similar on land and in the sea. In the early Holocene, the terrestrial July gradient steepened slightly to about 3°C. This is less than today (14°C and 10°C at the south and north Norwegian coasts, respectively) because southern July temperatures then were still 1-2°C lower than at present, but northern temperatures had already reached present values. The marine gradient reestablished but was weaker than the Interstadial gradient at ca. 1-3°C. Winter temperatures rose by at least 5°C on land, but less in the sea. The early Holocene climate was more seasonal than today. These results are compared with other data syntheses and model results.

The mid-Pliocene (4.3^2.6 Ma) benthic stable isotope record of the Southern Ocean: ODP Sites 1092 and 704, Meteor Rise

Abstract We present mid-Pliocene (4.3–2.6 Ma) benthic stable oxygen and carbon isotope data from Ocean Drilling Program Site 1092 (ODP Leg 177) drilled in the sub-Antarctic sector of the Southern Ocean. The results are compared with the stable isotope results from nearby Site 704 (ODP Leg 114). Oxygen isotope data show that minimum values are about 0.5‰ less than those of the Holocene, which is consistent with the results from Site 704, indicating only minor deglaciation of Antarctica during the studied interval. Oxygen isotope data from both Site 1092 and Site 704 are slightly higher relative to Pacific values during several intervals which could be related to the contribution of warm, saline North Atlantic Deep Water (NADW). Comparisons of benthic carbon isotope gradients between sites located in the North Atlantic, sub-Antarctic sector of the Southern Ocean, and Pacific indicate that at times, the gradient between the Southern Ocean and the Pacific evolved differently than the Atlantic–Pacific gradient. This suggests that variations in NADW strength alone might not be responsible for the observed carbon isotope values in the Southern Ocean.