Radiocarbon dating holocene, general considerations
Research on the present coccolith distribution shows that there is maximum productivity in zones of oceanic upwelling, notably at the subpolar convergence and the equatorial divergence. During a transgressive stage the saltwater wedge at river mouths causes a back-up, and the estuary becomes much more sluggish thalassostatic.
The Table shows the comparative dates of radiocarbon years and those obtained by other means. By radiocarbon dating the transgressive and regressive phases have been shown to be correlative in time. In some relatively recent continental deposits, dating club sa obsidian a black glassy rock of volcanic origin can be used for dating. One of the most important means is carbon dating.
Obsidian weathers slowly at a uniform rate, and the thickness of the weathered layer is measured microscopically and gauged against known standards to give a date in years. In a number of cases there is evidence from buried beach deposits that the foundations of the inner dunes are older strandlines that were established when the sea was somewhat higher than today. In The Netherlands the basal Holocene is buried in the fluvial deposits of the lower Rhine. From evidence outside the areas of subsidence, it seems likely that the worldwide eustatic sea level rise reached its maximum sometime between and bp many workers consider the date to be about bp. The preceding and substantially longer sequence of alternating glacial and interglacial ages is the Pleistocene Epoch.
At the same time, in the tide flat areas the Calaisian was followed by the Dunkirk stage, or Dunkerquian. The marine beds alternate with continental deposits that represent river or swamp environments. The amplitudes of the fluctuations and the finite curve are less easily established. Others, in Alaska, proposed a Holocene section beginning at bp. With deglaciation the marginal bulge should slowly collapse.
Smoothed versions have been offered by several other workers. Subsequently, they became stabilized by a brief eustatic regression, while the high water table permitted the growth of the Lower Peat. Hand in hand with melting, the sea level responded so that, as the ice began to retreat from its former terminal moraines, the sea began to invade the former coastlands.
The Netherlands area is subsiding also, but here the pattern is complicated by the long-term negative tectonic trend of the North Sea Basin and the Rhine delta. As sea level rose, it paused or fluctuated at various stages, leaving erosional terraces, beach deposits, and other indicators of the stillstand. Certain pitfalls have been discovered in tree-ring analysis, however. In addition to the marine-limit strandlines, there are row upon row of lower beach levels stretched out across Scandinavia, around Hudson Bay, and on other Arctic coasts.
During the latest glacial stage the subpolar zone was displaced toward the equator, but with the subsequent warming of waters it shifted back to the borders of the polar regions. Some melting occurs, but there is a very great melt-retardation factor in high-latitude ice sheets high albedo or reflectivity, short melt season, and so forth. Broadly comparable patterns occur in other areas, from France and Britain to Texas, Oregon, and Brazil.
The postglacial eustatic rise had to traverse the North Sea Plain and advance up the English Channel several hundred kilometres before it reached the Netherlands area. In the case of mid-latitude ice sheets, however, once melting starts, the ice disappears at a tremendous rate.
Fortunately, in areas that were depressed by the load of glacial ice there has been progressive postglacial uplift crustal rebound that has led to the exposure of the nearshore deposits. In both the Mississippi and Rhine deltas, sediments that represent the earliest marine Holocene are missing. In North America that fulcrum seems to run across Illinois to central New Jersey and then to swing northeastward, paralleling the coast and turning seaward north of Boston. In the Scandinavian region the fulcrum crosses central Denmark to swing around the Baltic Sea and then trends northeastward across the Gulf of Finland north of St.
The sediments must lie seaward on the shelf margin, and the oldest marine layers are found to rest directly upon the late Pleistocene river silts and gravels. An important regressive phase seems to have initiated each new dune belt.
Is the Holocene rate higher or lower than during the last glacial stage? The Holocene represents the most recent interglacial interval of the Quaternary period. Six major fluctuations are recognizable in both the Mississippi and Rhine deltas.
Of these, Antarctica and Greenland have relatively high latitude situations and do not easily become deglaciated. These strandlines are dated and distinctive and do not grade into each other. There were, however, two retarding factors for glacial-age currents.
This boundary marks the very beginning of warmer climates that occurred after the latest minor glacial advance in Scandinavia. In addition, certain foraminiferal species tend to change their coiling direction from a left-handed spiral to a right-handed spiral at this time.
Because water-saturated sediment has a tendency to compact under further sediment loading, there is an additional built-in mechanism that adds to the subsidence in such areas. This in turn would have been appreciably less effective in driving major oceanic current gyres. In The Netherlands, in spite of subsidence, the western coastline became more or less stabilized about bp with the beginning of the formation of the Older Dunes alternating with interdune soils. From the climatic viewpoint these two parameters are often inversely related in different regions. The distribution of the carbonate plankton bears on the problem of rates of oceanic circulation.
This corresponds to a rise in sea level of five centimetres per year. Turbidity currents, generated on a large scale during the low sea-level periods, became much less frequent following the rise of sea level in the Holocene. Nature of the Holocene record The very youthfulness of the Holocene stratigraphic sequence makes subdivision difficult.
This technique not only offers an additional means of dating but also contains a built-in documentation of climatic characteristics. It seems likely that this fulcrum shifted inward toward the former glacial centre during the early part of the Holocene. Most striking scenically are the coasts with Holocene terraces undergoing tectonic uplift. In this deltaic setting Holocene sequences are found that are quite different from those in the postglacial uplifted regions.
In The Netherlands the barrier beaches re-formed close to the present coastline, and widespread tidal flats developed to the interior. In addition to foraminiferans in the globigerina oozes, there are nannoplankton, minute fauna and flora consisting mainly of coccolithophores.
On a subsiding coast there tends to be an alternation in importance between two types of associated sedimentary facies. The subsequent warming trend was marked by the Finiglacial retreat in northern Scandinavia, the Ostendian early Flandrian marine transgression in northwestern Europe. The boundary often is marked by a slight change in colour.
The age of a tree that has grown in any region with a seasonal contrast in climate can be established by counting its growth rings. The nearer the former centre of the ice sheet, the higher the marine limit.
In certain favourable situations, particularly in the drier, low latitudes, tree-ring records sometimes document and year sunspot cycles. Sometimes, as in a very severe season, a growth ring may not form. It was officially endorsed by the U.
In these places the highest ancient shoreline that is now preserved is known as the marine limit. Passing inland, the lines of equal uplift isobases are positive, whereas seaward they are negative.
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