HOLOCENE RECORDS OF PALEOCEANOGRAPHIC AND CLIMATIC VARIABILITY FROM THE N. ICELAND CONTINENTAL SHELF.
CASTANEDA, ISLA S.. Instaar.
Smith, Laryn M.. Instaar.
Kristjansdottir, Greta B.. Instaar.
Andrews, John T.. Instaar.
Anderson, David M.. Noaa and Instaar.
In recent years, an increasing number of high-resolution Holocene studies have focused attention on century to millennial scale climatic variations. These climatic variations are particularly evident in the North Atlantic and Arctic regions, as these areas are highly sensitive to both natural and human-induced climate change (Overpeck et al., 1997). In this study, we use oxygen isotope and total carbonate data from marine sediment cores to construct a high-resolution Holocene history of the N. Iceland region. Based on this data, we divide the Holocene history of the N. Iceland continental shelf into three main stages: 1) deglaciation of the NW Peninsula (until ~9000BP), 2) early Holocene warming (9000-5000BP), and 3) neoglacial cooling (5000BP-present).
In 1997, a series of piston and gravity cores were collected from the Icelandic continental shelf aboard the Bjarni Saemundsson (B997 cruise). In this study, nine cores from the N. Icelandic continental shelf are examined (B997-317, -321, -324, -327, -328, -329, -330, -331, and –332) as they contain records of Holocene climatic variability (Fig. 1). The lithofacies present in the majority of the cores examined in this study is an olive black sandy silt. This lithofacies represents the interval from ~8000-0BP. Another lithofacies, an olive black silty clay, is present in the older cores examined in this study and represents the interval from >9500-8000BP. Age-control in all cores is provided by AMS 14C dating of mollusks and foraminifera, and by tephrochronology. An important tephra marker, the Saksunarvatn Ash, has been identified in several of the cores examined in this study. In this study a 400-year marine reservoir correction is applied to all ages. In cores 329 and 332, applying a 400-year marine reservoir correction predicts the age of the Saksunarvatn ash as 9080 and 9000BP, respectively.
Of the nine cores examined in this study, only one core (B997-324) contained both planktic and benthic foraminifera in enough abundance to conduct oxygen isotope analysis on both types. In core 324, stable isotopes were analyzed on two benthic species, Melonis barleeanus (formerly M. zaandamae) and Turborotalia quinqueloba, and on the planktic species N. pachyderma (s). In cores 317, 321, and 327, the planktic species N. pachyderma (s) was used for oxygen isotope analysis as no benthic foraminifera occurred in enough abundance. In the nearshore cores (B997-328, -329, -330, -331, and –332), few if any planktic foraminifera were noted, therefore, oxygen isotope analyses were made on the epifaunal benthic species Cibicides lobatulus.
Cores 324, 329 and 332 extend back to >9500BP, and capture the deglaciation of the NW Peninsula of Iceland. During this interval, sedimentation rates are extremely high (>6m/kyr in the nearshore cores), and oxygen isotope values indicate a rapid warming (Fig. 2). Total carbonate values also indicate a rapid warming occurring during this interval. In cores 329 and 332, sedimentation rates slow somewhat after ~9000BP, and suggest that the deglaciation of the NW Peninsula was completed by ~9000BP. Additionally, oxygen isotopes indicate warming trends until ~9000BP, and slight cooling occur after this time. Sedimentation rates remain high in both cores 329 and 332 until ~8000BP, and the high sedimentation during this period may be due to increased erosion rates on land following the retreat of ice caps.
The interval from 9000-5000BP, is marked by a general warming trend (Fig. 2). This warming is recognized in the d18O and total carbonate records. Additionally, foraminiferal assemblage data from core 329 supports warming during this interval with numbers of benthics/gram reaching their highest values, and numbers of agglutinated foraminifera decreasing throughout the interval. Stotter et al. (1999) do not note any glacier advances during this time and Eiriksson et al. (2000) also characterize the interval from 9000-6200BP as a period when the Irminger Current had an increased influence on the N. Iceland shelf. Oxygen isotope values reach their lightest values in the interval from 7000-5000BP, suggesting that this was the warmest time of the Holocene. In core 330, following a broad light interval in the early Holocene, d18O values rapidly increase between approximately 5500-4000 BP and indicate a return to cooler conditions.
The interval from ~5000BP to the present is the interval best documented in this study. Within this interval, cores 328 and 330 have a high sampling resolution (approximately one sample per 64 years). Additionally, in these two cores, the total carbonate data correlates highly to the oxygen isotope data, suggesting that the d18O values are due to changes in temperature and not salinity. If significant changes in salinity had occurred, then we would expect that the d18O and total carbonate records would not exhibit the same trends.
In the past 5000BP, the bottom water temperatures of the N. Iceland continental shelf have varied considerably (Fig. 3). The overall trend over the past 5000BP is a cooling trend toward the present. In the past 5000BP, oxygen isotope and total carbonate data indicate cold periods at ~3800-3600BP, and from ~600-200BP (the Little Ice Age). Warmer intervals occurred at 3400-3200B, and at 1800-1200BP. In the interval from 3000-2000BP, lesser magnitude warming and cooling intervals are apparent. In the past 5000BP, several records used in our study show shifts in d18O values of up to 0.5 per mil. If we are correct in assuming that the d18O signal at several of our coring sites is responding mainly to changes in temperature, then in the past 5000BP, bottom water temperatures of the N. Iceland continental shelf have varied by up to 2 degrees C.
Eiriksson, J., Knudsen, K.L., Haflidason, H., and Henriksen, P., 2000a. Late-glacial and Holocene paleoceanography of the North Icelandic shelf. Journal of Quaternary Science: 15, 23-42.
Overpeck, J., Hughen, K., Hardy, D., Bradley, R., Case, R., Douglas, M., Finney, B., Gajewski, K., Jacoby, G., Jennings, A., Lamoureux, S., Lasca, A., MacDonald, G., Moore, J., Retelle, M., Smith, S., Wolfe, A., and Zielinski, G., 1997. Arctic Environmental Change of the Last Four Centuries: Science, v. 278, p. 1251-1256.
Poole, D.A.R., Trond, Dokken, M., Hald, M., and Polyak, L., unpublished paper. Stable isotope fractionation in recent benthic foraminifera from the Barents and Kara Seas.
St„tter, J., Wastl, M., Caseldine, C., and H¾berle, T., 1999. Holocene paleoclimatic reconstruction in northern Iceland: approaches and results. Quaternary Science Reviews: 18, 457-474.
Figure 1. Location of B997 cores examined in this study.
Figure 2. Oxygen isotope records from cores 329, 330, and 332 during the interval from >9000-5000BP.
Figure 3. Oxygen isotope data from cores 328 and 330 from 5000BP to the present. Oxygen isotope data has been smoothed in order to make trends in the data more visible.
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