Joseph S. Stoner

Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12 000 cal yr: long-term North Atlantic oceanographic variability and solar forcing

Giant piston core MD99-2269 recovered 25 m of sediment in Hunafloaall, a deep trough on the North Iceland margin fronting the Iceland Sea, and the site of a shelf sediment drift. The rate of sediment accumulation is 2 m/kyr (5 yr/cm); the core terminated in the Vedde tephra (V12 cal ka). The sediment was sampled at between 5 and 50 yr/ sample, including rock magnetic, grain-size, and sediment properties. Data reduction was carried out using principal component analysis. Two PC axes for the 5-yr/sample magnetic data are strongly correlated with measures of coercivity (ARM20 mT/ARM) and magnetic concentrations (kARM). In turn ARM20 mT/ARM is highly correlated (negatively) with grain-size and the mean size of the sortable silt fraction. Analyses of the two PC axes with MTM spectral methods indicate a series of significant (s 99%) periodicities at millennial to multidecadal scales, including those at V200, 125, and 88 yr which are associated with solar variability. We also document a strong correlation between the sediment magnetic properties and the D 18 O on benthic foraminifera on the North Iceland inner shelf. We hypothesize that the links between variations in grain-size, magnetic concentrations, and solar forcing are controlled by atmospheric and oceanographic changes linked to changes in the relative advection of Atlantic and polar waters along the North Iceland margin. Today these changes are associated with variations in the deep convection in the Greenland and Iceland Seas. The precise linkages are, however, presently elusive although a combination of coarser sediments and low D 18 O values define a Holocene thermal maximum between V8 and 6 cal ka. @ 2003 Elsevier Science B.V. All rights reserved.

Chapter Three Magnetic Stratigraphy in Paleoceanography: Reversals, Excursions, Paleointensity, and Secular Variation

Publisher Summary This chapter focuses on the magnetic stratigraphy in paleoceanography. Magnetic stratigraphy rests on the idea that the recorded magnetization of a rock reflects the behavior of the geomagnetic field. In the simplest case, the natural remanent magnetization (NRM) of sediment is aligned with the (geo)magnetic field and is a function of its intensity and direction at the time of deposition. In practice, many factors may work to modify the original geomagnetic input signal. Under favorable circumstances and with detailed diligence, some of these effects can be separated and others avoided so that an accurate paleomagnetic record is recovered. Paleomagnetism has been working to keep pace and a significant new understanding of geomagnetic field behavior during times of constant polarity has begun to emerge. Essentially, it has been found that high amplitude, high frequency variations in the Earths magnetic field occur over large spatial scales even during times of constant polarity. New magnetostratigraphic opportunities over a range of temporal and spatial scales are emerging. The chapter also outlines the practical aspects of reconstructing the paleomagnetic record from marine sediments, and discusses some of the recent observations on the Quaternary geomagnetic record that are being made and their uses as a stratigraphic tool for paleoceanographic research.

A high-resolution Holocene sediment record from Húnaflóaáll, N Iceland margin: centuryto millennial-scale varability since the Vedde tephra

MD99-2269 is a 25 m long core from Hunaflóadll, a deep trough which runs northward towards the shelf break from the narrow neck of land which joins the Northwest Peninsula of Iceland with the ‘mainland’. The core was recovered from the thickest part of an elongate shelf sediment body, underlain by glacial diamictons with ages 26 ka BP. 3.5 kHz profiles indicate the presence of a reflector, RI, which can be traced over long distances. Twelve AMS radiocarbon dates and the presence of the Saksunarvatn tephra (which is the origin for the RI reflector at 21 m core depth) define a linear depth/age relation with an accumulation rate of 200 cm/ky. The date on the core top is ‘modem’. X-radiographs indicate that the sediments are principally massive. bioturbated muds. The data were measured at resolutions of between 5 and 40 yr/sample. Our primary data represent aspects of (a) multisensor track measurements, (b) colour, (c) grain size, (d) sediment properties and (e) sediment magnetic properties. Principal component analysis (PCA) was used to simplify the analysis of these five data sets, and resulted in nine significant axes compared to over 70 original variables. The explained variance on the first axes varied between 47 and 96%. These time series were reduced to a common 50-yr sample interval which were then also subject to PCA. Using the resulting PC scores we present three primary axes of environmental variability in Hunaflóaall that have century-scale quasi-periodicities, and demonstrate that these are highly correlated with specific ‘raw’ variables. We focus on variables linked to changes in grain size and productivity and show regional coherence (cores B997-321 and 330) in the coercivity/grain size measure ARMJ(20)/ARMJ(0), which have a close similarity to the detrended A 14C series. This provides a potential link to the vigour of the thermohaline circulation to the north of the Iceland shelf in the Iceland and Greenland Seas. Our data indicate increased variability in our proxies during the Neoglacial period.