TEPHRAS IN CORE MD99-2269, NW-ICELAND SHELF: A STRATIGRAPHIC TOOL FOR ESTIMATING CHANGES IN RESERVOIR AGE OF SEAWATER

An abundance of traceable and dated tephra-markers 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. The objective of this study is to examine Holocene changes in reservoir age of seawater on the NW-Iceland shelf.

Tephra-markers and adjacent radiocarbon ages from core MD99-2269, a 2533 cm core from Reykjafjardarall, NW-Iceland shelf (365 m water-depth), are analyzed. The MD99-2269 site is situated under fluctuating influence of the warm Irminger Current from the south and the cold East Iceland Current from the north. Previous AMS radiocarbon dates indicate a remarkable linear sedimentation rate of 5 yrs/cm from present back to 12,278 +/- 518 cal BP.

Distribution maps of several Holocene tephras, particularly rhyolitic tephras from the Icelandic volcano Hekla, indicate a distribution of airborne tephra (< 0.5-1 cm layer thickness) over the MD99-2269 site. However, only the Saksunarvatn tephra (9,000 14C BP or 10,180 +/- 60 cal BP) is visible in the core. We have located several tephra-markers by grain counting in the >150 µm size fraction. 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. The counted grains were classified into: 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. 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. Samples from selected tephra peaks were geochemically analyzed at the Nordic Volcanological Institute in Iceland and the University of Colorado. Radiocarbon ages were obtained from the University of Arizona.