A 2000 yr record of climate variations reconstructed from Haukadalsvatn, West Iceland

geirsdóttir, a., miller, g.h., thordarson, t., and ólafsdóttir, k.b.

Abstract

Study Location

Haukadalsvatn (3.3 km², elevation 32 m asl) at the head of Hvammsfjördur, western Iceland, occupies a narrow, elongated, glacially eroded basin, with a maximum depth of 42 m. The largest part of the catchment (172 km²) lies above 500 m asl.

Climate Proxy

Total organic content (TOC) , biogenic silica (BSi), C:N ratio, and δ¹³C:

• BSi is commonly considered a measure of primary lake productivity but is also influenced by the rate of minerogenic sedimentation. C:N ratio and ∂13C are independent proxies that reflect the proportion of terrestrial versus aquatic organic matter. Based on these two parameters, TOC concentrations over the past 2 ka primarily reflect the influx of soil organic matter input. In summary, we interpret BSi as a proxy for aquatic primary productivity, which is primarily related to spring temperature when diatom blooms occur in Haukadalsvatn, whereas TOC is a function of landscape instability and soil erosion by eolian activity.

Results

High BSi and low TOC values between ca. 900 and 1200 AD is coincident with the Medieval Warm Period (MWP), and suggests generally warmer seasons and landscape stability with little soil erosion. But this interval is punctuated by several short, cold perturbations, suggesting the MWP was not an interval of uninterrupted spring/summer warmth. BSi declines in a two-step pattern at the onset of the Little Ice Age with an initial cooling around 1250 to 1300 AD. A major disruption between 1450 and 1500 AD results in nearly a century of severe soil erosion and cold summers, and is the start of the main cold phase of the LIA. Even during the LIA, periods of relatively warm summers occurred, especially in the late 1600s and early 1700s. Recovery from the LIA began in the mid 1800s. The two steps into the LIA coincide with the intervals of greatest atmospheric loading of volcanic aerosols, supporting earlier suggestions that explosive volcanism may have acted as a trigger to LIA cooling, if accompanied by changes in system state that involved large positive feedbacks.

References

• Mann ME, Jones PD (2003) Global surface temperatures over the past two millennia. Geophys Res Letters 30: doi: 10.1029/2003GL017814
• Moberg A, Sonechkin DM, Holmgren K, Datsenko NM, Karlén W (2005) Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617