Non-linear Holocene Climate Behavior Reconstructed from Icelandic Lake Sediment Linked to Both Explosive and Diffusive Volcanism

Because of the strong influence of oceanic heat transport on terrestrial climates throughout the northern North Atlantic, Iceland's climate should reflect key attributes of North Atlantic circulation changes throughout the Holocene. High-resolution paleoclimate reconstructions from well dated Icelandic lake sediment cores, synchronized using tephra layers and paleomagnetic secular variations, reveal concordant abrupt changes, each accompanied by dramatic increases in sediment accumulation rates indicating accelerated erosional processes. Orbitally forced summer insolation decreases are reflected in a first-order cooling trend, but this trend is dominated by abrupt changes in state in the catchments, including stepped increases in periglacial activity, decreases in vegetation cover, and glacier growth. Many of these stepwise changes are associated with dated Icelandic explosive tephra-producing eruptions such as the 10180 BP Saksunarvatn, ~7000 BP H5, ~4300 BP H4, ~3800 BP H-S and ~2980 BP H3, but also repeated diffusive basaltic eruptions accompanied by high sulfur loading in the atmosphere. The strongest disturbances occur during the last 2 ka, beginning about 550 AD with shorter periods of relative summer warmth such as between 950 and 1250 AD. However, this warmth was punctuated by multi-decadal cold intervals, the most apparent around 1150 AD. The transition into the Little Ice Age is recorded by initial summer cooling 1250-1300 AD, more severe drops in summer temperatures 1450 to 1550 AD, followed by cold spikes in the early 1600s, mid 1700s and mid 1800s AD, when peak summer cold was achieved. The lake systems record LIA cold perturbations that coincide with periods of known, severe explosive and effusive volcanism, with subsequent re-equilibration in a new state. We conclude from our paleoclimate studies that both explosive and effusive eruptions, if sustained long enough, may have acted as regional cooling triggers.