Postglacial relative sea-level reconstruction and environmental record from isolation Basins in NW Iceland

Abstract

Isolation basin methodology was successfully applied to a number of coastal basins in NW Iceland. Basin isolation was traced using a combination of bio-lithostratigraphy (diatoms) and other geo-chemical proxies (biogenic silica, loss-on-ignition, and sodium). Limited data was available to develop an event chronology which was based primarily the Saksunarvatn Ash, identified in many of the lake cores. 6 sea-level index points were identified from a staircase of isolation basins between 1m and 75m a.s.l. and used to reconstruct the first preliminary relative sea-level curve for northwest Iceland. The marine limit was tightly constrained around ca. 75m a.s.l. by the different litho- biostratigraphy of two basins The reconstruction of changes in relative sea-level suggest that relative sea-level fell from ca. 75m a.s.l at 12.8 cal. Ka BP to below 22.7m a.s.l sometime after 9.2 14c Ka BP (ca. 10.3 cal. Ka BP). This relative sea-level fall corresponds to an actual isostatic land uplift of ca. 100.4m at a rate of 4cm yr majority of basins investigated had evidence of isolation occurring before the onset of organic accumulation within the basin. This characteristic of isolation basins stratigraphy in Iceland was more pronounced in those basins that isolated earlier implying that climate may have been an influence. I speculate that a cold harsh climate, indicated by diatoms and low lake and catchment bio-productivity at the time of isolation, caused a time-lag between full hydrological and sedimentological isolation and the onset of organic deposition within the basin. It may prove difficult to radiocarbon date isolation contacts that are particularly organic-poor and alternative methods like tephrochronology may be additionally required in order to produce future Icelandic RSL data with a tight chronological control. Attempts to produce a record of environmental and climatic change from NW Iceland met with mixed success. High levels of background tephra prevented a climate signal being recorded by biogenic silica analysis. The Saksunarvatn Ash was well distributed across Vesfirõir facilitating correlation with other marine and terrestrial sites around the North Atlantic of palaeoenvironmental importance. Were the Saksunarvatn Ash was found deposited in clastic-rich, fresh water gyttja, it has been suggested that those areas were still experiencing cool temperatures during the early Pre-Boreal. Diatom, Loss-on-ignition and pH analysis clearly show the Pre-Boreal warming from ca. 10.1 cal. Ka. BP.