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RECENT studies on the changes in composition of interstitial water during its separation from sediments1–3prompted us to evaluate the effect of squeezing on the concentration of ferrous iron within the interstitial water. Sediment samples were taken in northern Chesapeake Bay (38°56′ N and 76°25′ W) with a Benthos gravity corer. Layers of sediment were immediately extruded from the plastic coring tube and loaded into nylon squeezer barrels. The interstitial water was squeezed from the sediment through a 0.22 µm Millipore filter by the pressure of nitrogen acting against a rubber diaphragm4. The water drained directly into a calibrated test tube containing a solution of o-phenanthroline, buffered at pH 3.6, and the iron content was determined colorimetrically5.

Recent theoretical1–6and observationa7–10demonstrations of depression in surface temperature of the Earth due to explosive volcanic eruptions have provided a more exact basis for speculations that explosive volcanicity may have had a climatic impact11–15during periods over the past 2 Myr. In this letter, broad volcanic–climate relationships are assessed over a longer period by comparing the dates of global explosive phases and of volcanic–plutonic phases with paleotemperature and glaciation records from the Neogene. Analysis of these records indicates significant relationships between the explosive and volcanic–plutonic phases and Neogene temperature decline and also a correspondence between the three greatest explosive and volcanic-plutonic phases and glacial expansion. The second highest global explosive peak was coincident with recently suggested dates of around 3.2 Myr for the initiation of the late Cenozoic glacial age. Further testing of these relationships is necessary to resolve inconsistencies in the late-middle to early-late Miocene and in the late Pliocene.

A combination of geophysical, biological and glaciological information supports the idea of a 2,600 year solar cycle.

DURING the late and post Pleistocene, cyclic oscillations in global temperature seem to have occurred, superimposed on the predominant temperature trends determined by the advance and retreat of the mid-latitude ice sheets. Oscillation periods of around 2,600 yr (refs. 1–3) and of around 400 and 2,400 yr (ref. 4) have been proposed. Since these cycles were noted further palaeotemperature data have been obtained and are here included in an analysis of cyclic temperature patterns over the past 20,000 yr.

AN instantaneous glaciation model for the formation of the large Pleistocene ice sheets has been proposed by Flohn1and by Ives et al2. It depends on the sudden buildup of a permanent snow cover over sub-Arctic upland plateaux including Baffin Island, Ungava, Labrador and Keewatin, Melville Peninsula, and the Finnmark Plateau and upland Sweden. The crucial event in this sudden snow buildup is the survival of snow over a large area for a single summer which then results in a series of feedback reactions leading to the establishment of permanent snowfields and, subsequently, icefields. I suggest here that such a survival could have resulted from one or several closely spaced massive volcanic ash eruptions.

THERE may be a connection between the periodicity of glacial advance and the occurrence of volcanic eruptions. Lamb1has presented a graph which shows a similar curve for Arctic sea ice off Iceland and great Icelandic volcanic eruptions since 870 ADand Bray2found the number of maximum advances of alpine glaciers since 1680 ADis significantly related (χ2= 7.8, P< 0.01) to intervals with a volcanic dust veil index of 1,000 or greater. The volcanic dust estimates have now been extended to 1500 AD(ref. 1) and in this paper I analyse the relationship between these estimates and records of maximum glacial advance from 1500 ADto the present.

CLOSE relationships between the timing of volcanic eruption and of glaciation in the past millennia have been noted1–3together with the possibility that the three main Neoglacial ice advances were contemporaneous with the three well dated volcanic activity periods in the Southern Andes3. Here, I show that glacial advance phases in the Western Northern, Eastern Northern and Southern Hemispheres are mainly synchronous with volcanic activity periods in New Zealand, Japan and southern South America over the past 40,000 yr (the period of reasonably accurate 14C measurements).

MOST post-Pleistocene glaciation has apparently been confined to four phases at intervals of around 2,600 years1. The exact timing of these phases will be examined in relation to recent carbon-14 age corrections and additional glacial chronologies.

KING-HELE1predicted that the intensity of the present sunspot cycle (beginning about late 1964) may resemble the relatively high activity cycles with maxima in 1870 and 1949 and that the solar activity of the forthcoming maximum would reach the level which prevailed in late 1959. This prediction of a fairly high level of sunspot activity for the present cycle can be supported by circumstantial evidence from three periods A.D. 1100–1203, 1724–98 and 1914–64, which have remarkable similarities in various aspects of solar activity, and in terrestrial climate, glacial activity and tree growth.