Carin Andersson Dahl | Uni Research (original) (raw)

Papers by Carin Andersson Dahl

Chemical Geology, 2008

The potential of Mg in Arctica islandica as a climate proxy is explored through analysis of live-... more The potential of Mg in Arctica islandica as a climate proxy is explored through analysis of live-collected shells from Irvine Bay, NW Scotland. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis of the right hand valve from two specimens indicates that seasonal Mg/Ca variations do not correlate with seawater temperature. The highest Mg/Ca typically occurs at the annual growth

Geochemistry, Geophysics, Geosystems, 2008

1] Extraction of small aliquots (mg-mg) from aragonite samples such as corals, sclerosponges, biv... more 1] Extraction of small aliquots (mg-mg) from aragonite samples such as corals, sclerosponges, bivalves, and otoliths is required for bulk oxygen isotope analyses. However, aragonite is a metastable polymorph of calcium carbonate that undergoes transformation to calcite, which is the more thermodynamically stable polymorph of CaCO 3 . This polymorph transformation occurs via heating and/or stress, and such conditions may occur during drilling. During this polymorph transition, exchange with atmospheric or organic C and O is possible. Therefore extracting aliquots during drilling without modification or contamination via isotope exchange is imperative for accurate isotopic analysis. Using a micromill, aliquots were taken from aragonite cod otoliths that had been grown in the laboratory under stable temperature conditions. X-ray diffraction analysis showed that although 6% of the sample converts to calcite during drilling, there is no significant effect on d 18 O.

Geochemistry, Geophysics, Geosystems, 2009

1] The behavior of Sr in the aragonite shells of the bivalve Arctica islandica has been explored ... more 1] The behavior of Sr in the aragonite shells of the bivalve Arctica islandica has been explored to determine its potential for high-resolution environmental reconstruction. The structural state of Sr in the shell was investigated using X ray absorption fine structure (XAFS) and Sr/Ca variations across the prismatic layer in the umbo and outer shell were determined by secondary ionization mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) respectively. XAFS data suggest that Sr randomly substitutes for Ca within the shell aragonite. Sr/Ca in both the umbo and the outer shell exhibits significant seasonal variation but does not correlate with seawater temperature. Sr/Ca varies significantly between parallel transects across the same annual bands. Sr concentrations are highest in transects along the maximum growth axis of the outer shell but lowest in these transects in the umbo. Scanning electron microscopy indicates changes in crystal morphology both within and between transects. We do not, however, observe a clear relationship between shell growth rate and Sr/Ca. Fluctuations in Sr/Ca are most likely to be controlled by a complex interaction of biological and kinetic processes.

Quaternary Science Reviews, 2010

Marine isotope stage 3 (29e59 kyr BP) is characterised by rapid shifts from cold stadial to warm ... more Marine isotope stage 3 (29e59 kyr BP) is characterised by rapid shifts from cold stadial to warm interstadial periods, which may be linked to changes in the vigour of the Atlantic Meridional Overturning Circulation due to variable freshwater input by melting ice. Here we present two northern North Atlantic multi-proxy records of sea surface conditions that indicate warm (near) sea surface conditions during such ice-rafting events. We infer near surface temperature from planktonic foraminiferal counts, Mg/Ca and oxygen isotopes of left-coiling Neogloboquadrina pachyderma and from calcite content. Temperatures increased during ice-rafting and rose rapidly to interstadial values after ice-rafting ceased. This pattern is clearest during Heinrich Event 4, but also present throughout the other millennial scale ice-rafting events. It indicates that stadials in the Greenland ice-cores are concurrent with a (near) surface warming in the North Atlantic, which was probably restricted to the summer, as winter temperatures must have remained low for sediment-laden ice to reach the site. As similar warming during ice-rafting events is seen regionally in the northern North Atlantic it cannot be explained by a rerouting of the North Atlantic Current. Rather, we attribute it to a shoaling of a warm subsurface water mass that was formed as a result of decreased ventilation of the upper waters and a continued northward subsurface flow of warm water. Planktonic carbon isotopes support this suggestion showing coincident decreased ventilation during deposition of icerafted detritus (IRD). The absence of a clear meltwater spike in the d 18 O records during IRD input suggests that besides glacial freshwater, sea ice may have been responsible for the ventilation decrease and associated near surface heat built up. The proposed scenario is in agreement with modelling studies that require the release of heat trapped below the surface to restart the overturning circulation.

Multi-proxy paleoclimatic time series have been developed from IMAGES Sites and adjacent suppleme... more Multi-proxy paleoclimatic time series have been developed from IMAGES Sites and adjacent supplementary cores in the Eastern Norwegian Sea. The records cover the last 2000 years at decadal resolution, allowing for a detailed reconstruction of the surface hydrography of the main path of the northern limb of the north Atlantic circulation cell. Centennial to millennial scale events are recorded, such as the "Medieval Warm Phase" (MWP) and the "Little Ice Age" (LIA), which constitute the main long term century scale features. Superimposed on these are multidecadal variability of somewhat less amplitude. There is a close correspondance with continental records reflecting summer temperaure and winter precipitation in western Scandinavia over this period.SST changes are found to be in the range of 1-2 degrees. Significant land-sea correlation is observed. A cold phase in the early 20th Century, a series of cold phases in the LIA and two warm phases in the MWP are observ...

Climate of the Past Discussions, 2014

We present and examine a multi-sensor global compilation of mid-Holocene (MH) sea surface tempera... more We present and examine a multi-sensor global compilation of mid-Holocene (MH) sea surface temperatures (SST), based on Mg / Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst census counts. We assess the uncertainties originating from using different methodologies and evaluate the potential of MH SST reconstructions as a benchmark for climate-model simulations. The comparison between different analytical approaches (time frame, baseline climate) shows the choice of time window for the MH has a negligible effect on the reconstructed SST pattern, but the choice of baseline climate affects both the magnitude and spatial pattern of the reconstructed SSTs. Comparison of the SST reconstructions made using different sensors shows significant discrepancies at a regional scale, with uncertainties often exceeding the reconstructed SST anomaly. Apparent patterns in SST may largely be a reflection of the use of different sensors in different regions. Overall, the uncertainties associated with the SST reconstructions are generally larger than the MH anomalies. Thus, the SST data currently available cannot serve as a target for benchmarking model simulations. Further evaluations of potential subsurface and/or seasonal artifacts that may contribute to obscure the MH SST reconstructions are urgently needed to provide reliable benchmarks for model evaluations.

Micromilling Temperature a b s t r a c t Oxygen isotope ratios (d 18 O) were measured in micromil... more Micromilling Temperature a b s t r a c t Oxygen isotope ratios (d 18 O) were measured in micromilled samples of cod otoliths recovered from two Stone Age sites, one from Skoklefald, southeastern Norway and five from Skipshelleren, southwestern Norway. Discrete samples of otolith material were milled from the edge of the otolith towards the centre, each sample representing 3-4 weeks of growth. The d 18 O values showed seasonal signals, covering a period 1-2 years prior to the time of capture. Isotope ratios at the edge of the otoliths, which is the material deposited at the time of fish capture, corresponded to the coldest seasonal water temperatures, in late winter or early spring. These data provide independent evidence for the season of use of the Skoklefald and Skipshelleren sites.

Fremtidige klimaendringer vil vaere en kombinasjon av naturlig klimavariabilitet og et resultat a... more Fremtidige klimaendringer vil vaere en kombinasjon av naturlig klimavariabilitet og et resultat av menneskelig påvirkning (gjennom utslipp av klimagasser fra fossilt brensel og endret arealbruk). For å kunne skille mellom de menneskeskapte klimaendringene og de naturlige klimavariasjonene må vi vite hvordan klimaet har endret seg før den industrielle revolusjonen.

The Pliocene section of ODP Site 642B (Eastern Nordic Seas, 1286 m water depth) is studied to det... more The Pliocene section of ODP Site 642B (Eastern Nordic Seas, 1286 m water depth) is studied to determine the role of the Nordic Seas, as a gateway linking the North Atlantic and the Arctic, though the last time Earth's climate was close to equilibrium with a greenhouse gas forcing comparable to the present. The site is located underneath the present pathway of the Norwegian Atlantic Current, detecting changes in the strength of polar heat transport. A multi-proxy approach is used to characterize the Pliocene conditions through the water column. The predominant conditions changes at several occasions through the Pliocene, e.g. with summer mixed layer temperatures switching between periods with warmer than present by 1-2°C to colder than present by 1°C. Occasionally strong fresh water influence on the surface water is also indicated. Bellow the summer mixed layer, colder and/or saltier conditions than presently is seen through most of the Pliocene. The bottom water is occasionally ...

Marine Biology, 2004

A high-resolution sampling technique for stable-isotope measurements in otoliths is now available... more A high-resolution sampling technique for stable-isotope measurements in otoliths is now available, but little is known about the precision and accuracy of the results when using this technique. Otolith samples of pen-reared cod (Gadus morhua L.) with well-defined temperature history were obtained using this high-resolution sampling technique. Samples from different-aged cod otoliths were collected in two yearly increments corresponding to the calendar years 1994 and 1995, and the stable-oxygen (d 18 O) and -carbon (d 13 C) isotope compositions of the samples were measured. Otolith d 18 O values had a clear seasonal pattern in response to the seasonal water temperature. Otolith material deposited in the same calendar years of different-aged fish showed similar d 18 O values. High precision of the temperature estimate using otolith d 18 O values was obtained when comparing results from different otoliths. The accuracy of the temperature estimates, relative to measured mean monthly water temperature, was also high, but reduced sampling resolution in the otoliths significantly reduced the accuracy due to attenuation of the d 18 O signals. We found that the otolith d 13 C values had a cyclic pattern, roughly in phase with the d 18 O values. This was most likely caused by temperature-increased metabolism. Indications of age-specific otolith d 13 C values are also presented.

The Journal of Foraminiferal Research, 1997

In order to decipher past variations in sea-surface temperature in the western equatorial Pacific... more In order to decipher past variations in sea-surface temperature in the western equatorial Pacific Ocean, the relative abundances of planktic foraminifera from Ocean Drilling Program Site 806 have been analyzed from the time interval 3.9-2.4 Ma. Pliocene sea-surface temperatures were estimated from the planktic foraminiferal data using transfer function FP-12E and the modern analog technique. Results from these two methods indicate cooler sea-surface temperatures between approximately 3.35 and 3.05 Ma using the modern analog technique, and between 3.35 and 3.2 Ma and approximately 3.15 and 3.0 Ma using transfer function FP-12E. The apparent cooler periods coincide partly with an increase in planktic and benthic foraminiferal 6l80. Comparison between estimated sea-surface temperatures and species abundances indicate that the estimated changes in sea-surface temperature during this period could be the result of changes in the depth of the thermocline rather than a decrease in sea-surface temperature.

Geophysical Monograph, 2005

This paper presents an exploratory synthesis of quantitative temperature reconstructions during t... more 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 so uth 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.

Realistic models of climate change require accurate data of past seasonal temperature regimes. In... more Realistic models of climate change require accurate data of past seasonal temperature regimes. In many marine settings this is difficult to achieve using the marine sedimentary record. This study tests the feasibility to use archaeological cod otoliths to reconstruct changes in the seasonal temperature cycle in the Barents Sea. Potentially, stable oxygen isotope records from cod otoliths could be used to produce records of Holocene temperature change on a seasonal time scale. Human settlements along the coast of northern Norway have exploited the cod (Gadus morhua) population of the Barents Sea from the earliest occupation, through the middle ages, to the present day. A pilot project has been carried out in Bergen to exploit the availability of a collection of cod otoliths from archeological excavations that is housed by the Bergen Museum. Cod otoliths from archaeological excavations in northern Norway, 26 specimens from from Måsøy (Finnmark) and 17 specimens from Vanna (Troms), were selected for this study. These specimens were AMS radiocarbon dated and the ages are spanning approximately 1400 to 1780 AD. Most of the otoliths are from cod that were captured during the Little Ice Age. Seasonal growth patterns were identifiable in the archeological otoliths, comparable to those in modern otoliths. Micromilling was used to sample for stable oxygen isotopes over a 2-year growth period in each of a total of 43 fossil cod otoliths. There are large differences in the temperature ranges experienced by the fish over the 2-year period analyzed. For the Måsøy specimens the temperature range experienced by the fish is between 2.7 to 9.9 °C. The Vanna cod experienced temperature ranges between 1.7 to 7.2 °C. The maximum temperature ranges for both Vanna and Måsøy specimens are higher than the seasonal instrumental measurements for the 0-200 m depth interval in the Kola section in the Barents Sea. The cod otoliths in our study have not yet been divided into different age groups, which is necessary in order to be able to interpret the calculated temperature ranges because the annual temperature cycles experienced by fish at different ages are different. Another problem that needs to address is the annual migration of cod of northern Norway. It is known that both mature and immature cod undertake long seasonal migrations.