“THE FARTHER REACHES OF HUMAN TIME”: RETROSPECT ON CARL SAUER AS PREHISTORIAN*

ABSTRACT. Carl Ortwin Sauer (1889–1975) is widely regarded as one of the most influential geographers of the twentieth century, admired particularly for his studies in cultural and historical geography. His contribution to the study of prehistory is less widely acknowledged, but, between 1944 and 1962, he published a series of speculative yet scholarly papers that contain many prescient insights into humanity's remote past and the relationships of our ancestors to the environments they occupied—and modified. In this essay, based on the Carl O. Sauer Memorial Lecture given at the University of California, Berkeley, in October 2001, I reflect on Sauer's contribution to the science of prehistory by examining, in the light of recent advances in knowledge, two major themes of Sauer's work: the early dispersal of Homo sapiens in the Old World, and the origins and prehistoric spread of agriculture.     

Environmental change during the Allerød and Younger Dryas reconstructed from Swiss tree-ring data

Annually resolved tree-ring width variations and radiocarbon ages were measured from a collection of 120 Lateglacial pine stumps excavated on the Swiss Plateau. These data – representing the oldest absolutely dated wood samples worldwide – extend the absolute tree-ring chronology from Central Europe by 183 years back to 12 593 cal. yr BP (10 644 cal. yr BC). They also yield a 1420-year floating chronology covering the entire Allerød and the early Younger Dryas (14 170–12 750 cal. yr BP). Radiocarbon data suggest a 250-year jump in the ¹⁴C reservoir correction around the time of the Allerød to Younger Dryas transition, although calendric dating of the floating chronology – by filling a ∼150 year gap – is necessary for confirmation. Various subgroups, based on the year of germination, were used to assess temporal changes in growth characteristics along the Allerød to Younger Dryas transition. Comparison of these Lateglacial data with a reference data set of living and historic pines from the Swiss Valais (AD 940–2000) revealed differences in both growth trend and level. The generally slower Lateglacial growth was likely influenced by higher geomorphic activity and severe climatic conditions. After removal of the biological age-trend, a strong common signal found in the tree-ring data suggests some skill in estimating interannual to multidecadal Lateglacial climatic variations.     

Pb-Pb single-zircon ages of granitoid boulders from the Vendian tillite of Wahlenbergfjorden, Nordaustlandet, Svalbard

There are three areas in eastern Svalbard where Vendian tillites are exposed: eastern Ny Friesland, western Nordaustlandet (north and south of Murchisonfjorden) and further east in inner Wahlenbergfjorden, near Aldousbreen. Clasts within the massive unmetamorphosed clay-mica-carbonate matrix of the tillites include carbonates, sandstones, siltstones, metavolcanics, schists and different granitoids, the metamorphic and igneous rock types being more frequent in the upper levels of the formation. Large granite boulders, up to 1 m in diameter, are known from the easternmost outcrop at Aldousbreen. Three granitoid boulders from the Aldousbreen outcrop, differing in petrography and chemistry. have been dated by the Pb-Pb singlezircon method. They yield ages of 2830 ± 5 Ma, 1802 ± 4 Ma and 1497 ± 26 Ma. These clasts also differ in petrography, chemistry and age from all known granitic rocks on Nordaustlandet, which have recently yielded Grenvillian (950-960 Ma) and Caledonian (ca. 410 Ma) ages. The concentration of large granitic clasts in the easternmost known tillite outcrops suggests derivation from the east and/or south. Possible areas include those beneath the ice of Austfonna and below the Carboniferous strata of southeastem Nordaustlandet. The apparent lack of a significant Grenvillian overprint suggests the possibility of a more distant source.     

A reconstruction of Holocene climatic changes from peat bogs in north-west Scotland

Continuous palaeoenvironmental sequences from three peat bogs located in north-west Scotland are presented which reveal palaeoclimatic changes during the Holocene, Peat cores were analysed lor humification. pollen and a range of other physical and palaeoecological data, and chronologies were constructed by radiocarbon dating. Reconstruction of past changes in bog hydrology formed the basis for palaeoclimatic interpretation. Four regional shifts to wetter bog conditions and two regional shifts to drier conditions are inferred. Best estimated aye ranges for the wet shifts span c . 5120 to 5070, 4020 to 3630, 3340 to 3270 and 940 to 800 cal BP, arid dry shifts are estimated at c . 4330 to 4120 and 1480 to 1340 cal BP. The most distinctive feature of the record is the oscillation from wetter to drier conditions around 4300 cal BP, followed by a return to wetter conditions after 4000 cal BP. This oscillation was probably caused by climatic changes which also had an inHuence on the dynamics of Scots pine within the region prior to, and during, its mid-Holocene phase of decline. Later shifts probably also reflect climatic changes, although evidence for human land-use complicates interpretation at c . 3300 cal BP. Potential causes are considered, including changes in the circulation of the North Atlantic Ocean during the mid-Holocene.     

Bedform distribution and inferred sand transport on Georges Bank, United States Atlantic Continental Shelf

Four bedform provinces have been identified on Georges Bank using sidescan-sonar and echo-sounding techniques: large sand waves superimposed on sand ridges, small sand waves, megaripples, and featureless seafloor. The large sand waves and sand ridges are found on the bank crest where the surface tidal currents are strongest. Areas of small sand waves and megaripples, formed where tidal currents are moderate in strength, border the area of large sand waves to the north and south. Featureless seafloor is found farthest from the bank crest where surface tidal currents are weakest. Sand-wave asymmetry and surface-sediment texture have been used to infer bedload transport paths on Georges Bank. In the large sand-wave area, bedforms indicate a clockwise transport around each of the linear north-west-striking sand ridges with slight convergence of the sand waves on the ridge crests. This transport pattern implies erosion from the troughs and accumulation on the sand ridges. The asymmetry of the small sand waves along the south side of Georges Bank indicates that sand is also transported southward away from the linear sand ridges on top of the bank. Although the asymmetry of megaripples could not be determined, the occurrence of megaripples between the small sand-wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand dispersal pattern is further supported by the surface sediments which become progessively finer to the north and SW away from the crest of Georges Bank.     

Petrology and Trace Element Geochemistry of the Honolulu Volcanics, Oahu: Implications for the Oceanic Mantle below Hawaii

The Honolulu Volcanics comprises small volume, late-stage (post-erosional)vents along rifts cutting the older massive Koolau tholeüticshield on Oahu, Hawaii. Most of these lavas and tuff of theHonolulu Volcanics have geochemical features expected of near-primarymagmas derived from a peridotite source containing Fo_87–89olivine; e. g. 100 Mg/(Mg + Fe²⁺) >65, >250 p. p. m. Ni,and presence of ultramafic mantle xenoliths at 18 of the 37vents. Consequently, the geochemistry of the alkali olivinebasalt, basanite, nephelinite and nepheline melilitite lavasand tuff of the Honolulu Volcanics have been used to deducethe composition of their mantle source and the conditions underwhich they were generated by partial melting in the mantle. Compositional trends in 30 samples establish that the magmaswere derived by partial melting of a garnet (<10 per cent)Iherzolite source, which we infer to have been carbon-bearing,from analogy with experimental results. This source was isotopicallyhomogeneous (Sr, Lanphere & Dalrymple, 1980; Pb, Sun, 1980;Nd, Roden et al., 1981), and we infer that the source was compositionallyuniform in all major-element oxides except TiO₂, in compatibletrace elements (Sc, V, Cr, Mn, Co and Ni), and in highly incompatibletrace elements (P, Th, La, Ce). However, the source appearsto have been heterogeneous in TiO₂, Zr, Hf, Nb, and Ta, elementsthat were not strongly incompatible during partial melting.Some nepheline melilitite samples may be derived from a sourcewith distinct Sc and heavy-rare-earth-elements (REE) abundances,or which had a phase or phases controlling the distributionof these elements. The relatively limited abundance range for several elements,such as Ti, Zr, Nb, is partly a consequence of the low degreesof melting inferred for the series (2 per cent for nephelinemelilitite, 11 per cent for alkali olivine basalt), which failedto exhaust the source in minor residual phases. We infer thatthese residual phases probably included phlogopite, amphibole,and another Ti-rich phase (an oxide?), but not apatite. In comparison with estimates of a primordial mantle compositionand the mantle source of mid-oceanic-ridge basalt the garnetperidotite source of the Honolulu Volcanics was increasinglyenriched in the sequence heavy REEs, Y, Tb, Ti, Sm, Zr, andHf all <P <Nd <Sr Ce <La <Nb Ta. A multi-stagehistory for the source of the Honolulu Volcanics is requiredbecause this enrichment was superimposed on a mantle that hadbeen previously depleted in incompatible elements, as indicatedby the relatively low ⁸⁷Sr/⁸⁶Sr ratio, high ¹⁴³Nd/¹⁴⁴Nd ratioand low contents of K, Rb, Ba, and Th. The composition of thesource of the Honolulu Volcanics differs from the source ofthe previously erupted tholeiitic shield. The modal mineralogyof the source of the Honolulu Volcanics is not represented inthe upper-mantle xenoliths, e. g. the garnet pyroxenite andolivine-poor garnet Iherzolite included within the lavas andtuff of the unit.     

Bedform climbing in theory and nature

Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.