HOLOCENE HEKLA TEPHRAS: A STRATIGRAPHIC TOOL FOR ESTIMATING CHANGES IN RESERVOIR AGE OF SEAWATER. CORE MD99-2269, NW-ICELAND SHELF
KRISTJANSDOTTIR, GRETA B . INSTAAR and Department of Geological Sciences, University of Colorado, Boulder, CO.
An abundance of traceable and dated tephra layers from Icelandic volcanoes offer a unique opportunity for land-sea correlations and determination of reservoir age of seawater around Iceland. An offset of tephra markers from an established radiocarbon-based age model can be interpreted as a change in reservoir age of the water. A study by Eiriksson et al. (2000) on the central N-Iceland shelf has suggested an increase in the reservoir age of seawater by 130 yrs during late Holocene (3000-850 cal BP). This is tentatively related to increasing influence of colder (and older) water from the east Iceland current on the N-Iceland shelf. The objective of this study is to examine whether similar changes in reservoir age of sea water occurred on the NW-Iceland shelf.
Core MD99-2269 is a 2533 cm long marine sediment core located in 365 m water depth in the Reykjafjardarall basin, NW Iceland shelf. The core, which was cored during the international IMAGES V cruise in 1999, is located in an important oceanographic location with fluctuating influence of the warm Irminger Current coming from the south and the cold East Iceland Current coming from the north. The core consists of olive-grey silty-clay with ice rafted debris at the base. Nine AMS radiocarbon dates, calibrated by assuming a constant 400 yr reservoir age and CALIB 4.3, indicate a remarkable linear sedimentation rate of 5 yrs per cm from present back to 12,278 +/- 518 cal BP. We have recently submitted two more samples for AMS radiocarbon dating to verify the age model.
Two major tephra markers have been identified via geochemistry in the core and core catcher. The basaltic Saksunarvatn tephra (9,000 14C BP or 10,180 +/- 60 cal BP) is found as a distinct layer at 2118-2121 cm, whereas shards of rhyolitic Vedde (10,300 14C BP or 11980 +/- 80 cal BP) type composition where found in the core catcher. The Saksunarvatn tephra falls right on the linear age model of core MD99-2269. Andrews et al. (subm) has previously noted that a 400 yr reservoir correction is appropriate for this location during deposition of the Saksunarvatn tephra.
Distribution maps of several Holocene tephras, particularly rhyolitic tephras from the Icelandic volcano Hekla (Hekla 1, Hekla 3, Hekla 4, and Hekla 5, dated at 846, 3008, 4201, and 6950 cal BP respectively), indicate a distribution of airborne tephra (< 0.5-1 cm layer thickness) over the location of core MD99-2269. However, only the Saksunarvatn tephra marker is visible in the core. Other tephra markers have to be located by grain counting under a microscope. Grains were counted in samples previously prepared for foraminifera analyses, using the size fraction >150 Ám. The samples were split until they could evenly cover a counting tray, then 1 cm2 squares were randomly selected for counting until at least 300 grains were counted. Mud clumps and fecal pellets were counted, but not included in the total count of 300 grains.
Much of the sediment on the Iceland shelf is tephra and it is crucial to be able to distinguish primary tephra layers from reworked tephra. Primary tephra layers are assumed to consist of an abundance of fresh tephra shards. The fresh tephra was differentiated from the reworked tephra mainly on the basis of having sharp, clean edges whereas the reworked tephra has duller edges. The fresh tephra also tends to look shiny under the microscope compared to the matter reworked tephra. The counted grains were classified into the following categories: 1) Fresh dark-colored tephra, 2) Fresh light-colored tephra, 3) Other lithics, 4) Biogenic material, and 5) Other material. The first two categories were intended to identify peaks in fresh tephra grains, which presumably coincide with primary tephra layers. In contrast, other lithics included material not associated with primary tephra layers such as rock fragments, crystal fragments, and reworked tephra. Biogenic material included foraminifera, diatoms, and mollusk fragments (sponge spicules were not counted), whereas other material consisted mainly of greenish organic material and a few unidentified grains.
Once peaks of fresh tephra have been identified samples are taken for geochemical analyses. A few preliminary samples have been analyzed, but they show enrichment in Fe and Al, and depletion in Na and K relative to any published geochemical tephra analyses. The preliminary samples were not taken from the tephra peaks and we are hoping to get better analyses when analyzing tephra from the peak samples.
Andrews, J. T., Geirsdottir, A., Hardardottir, J., Principato, S., Krisjansdottir, G. B., Helgadottir, G., Gronvold, K., Drexler, J. & Sveinbjornsdottir, A. (subm.) Distribution, age, sediment magnetism, and geochemistry of the Saksunarvatn (10.18 ▒ cal ka) ash in marine, lake, and terrestrial sediments, NW Iceland.
Eiriksson, J., Knudsen, K. L., Haflidason, H. & Heinemeier, J. (2000) Chronology of late Holocene climatic events in the northern North Atlantic based on AMS 14C dates and tehpra markers from the volcano Hekla, Iceland. Journal of Quaternary Science, 15(6), 573-580.
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