Basis Of Radiocarbon Dates Research Articles

Sedimentary Facies Patterns and Geologic History of a Holocene Marine Transgression

Studies of Holocene sediments in coastal Delaware show complex sediment distribution patterns resulting from lateral and vertical movement of successive environments of deposition over a Pleistocene unconformity. These sediments are infilling a drowned topography with a local relief of 70 ft and possibly up to 125 ft eroded on highly variable Pleistocene sediments. Identification of the Pleistocene surface remains a problem. However, it may be recognizable at the unconformity as a soil zone or intermixture of firm marsh clay-silts with Pleistocene sands, as well as on the basis of radiocarbon dates. Larger depositional features forming around eroding Pleistocene headlands and infilling the estuaries include characteristic shoreline environments, such as spits, dunes, baymouth barriers, an intermeshing network of tidal deltas, nearshore marine erosional-depositional sands and gravels, and lagoons or estuaries with fringing Spartina, Distichlis, and Phragmites marshes, which form the westernmost edge of the transgressive units. The thickness and areal extent of the sedimentary bodies are to a large degree controlled by the morphology of the Pleistocene unconformity. A large portion of the Holocene sedimentary units is being eroded by the transgressing Atlantic Ocean. Cores of sediment under the shallow lagoons, such as Rehoboth, Indian River, and Assawoman Bays, and in the fringing marsh environment, show that the depositional units are thin, highly irregular in areal extent, extremely variable in thickness, and difficult to project. Sedimentary processes active in the shallow bays include shoreline marsh erosion and the formation of thin, possibly ephemeral, beach-dune washover complexes consisting of clean, well-sorted sand, with typical beach and washover sedimentary structures. These wash-over beaches are an anomaly completely surrounded by Spartina marshes on the landward side and extremely muddy sands grading into dark gray lagoonal muds on the bay side. It appears that distinctive sedimentary structures and sediment size-sorting relationships, such as those that characterize the larger, more common sedimentary units of the coastal area, may be formed in miniature at the very thin edge of transgression and may lead to considerable confusion in the interpretation of sediments of this type in the geologic record.

Radiocarbon dating

The cosmic ray production of new atoms in m atter is the basis of radiocarbon dating. In this case the atom is carbon of mass fourteen made from the most abundant atom in air—nitrogen of mass fourteen. Radiocarbon— carbon-14, 14 C—lasts 8300 years on the average (see note on radioactive decay on p. 10 for explanation of ‘half life’ and ‘average life’) before reverting in radioactive decay to nitrogen-14 and during this time it enters all living things as well as sea water and air. Chemically carbon dioxide (the product of the combustion of carbon with air—which is 20% oxygen) is the food of life and presumably the freshly produced 14 C atom burns sooner or later (probably in a few days, although this time is not at all well known) to 14 CO 2 a which is mixed with the ordinary carbon dioxide (0.03% in air) which contains mainly non-radioactive carbon atoms of masses 12 and 13 in abundances of 99% and 1%, respectively. The process which converts CO 2 a into plants—photosynthesis—is the way in which the radio­ carbon is introduced into living beings, for all life on Earth so far as is known either is a plant or lives off plants. In principle and in theory one could understand if organisms were to live off fossil (or primeval, if there be any) organic matter and that radiocarbon dating would not work for them. They would not be in touch with the cosmic rays through recent photosynthesis and the long time that coal or oil has been underground would have required that the original radiocarbon (assuming the cosmic rays were working when the coal and oil deposits were made) would long since have disappeared.

Pleistocene man in South America

Abstract South America was occupied by or before 14,000 B.P. Five different lithic traditions can be assigned to the late Pleistocene on the basis of radiocarbon dates and stratigraphy, and two others date to the very end of the Pleistocene or the earliest Holocene. The four earliest traditions, predating 11,000 B.P., may have been brought in by different groups of migrants from North America and, ultimately, Asia. The later traditions either show clear North American affinities or else appear as local developments in South America, but probably do not hark back to Asian antecedents.

Transgressive Facies Patterns in Delaware Coastal Area: ABSTRACT

Studies of Recent sediments in coastal Delaware show complex sediment-distribution patterns resulting from lateral and vertical movements of successive environments of deposition over a Pleistocene unconformity. Recent sediments are infilling a drowned topography with a local relief of 70 ft and possibly up to 125 ft eroded on highly variable Pleistocene sediments. Identification of the Pleistocene surface is a problem. However, it may be recognizable as a soil zone or intermixture of marsh clay with Pleistocene sands at the unconformity; it also may be recognized on the basis of radiocarbon dates. Larger depositional features forming around eroding Pleistocene headlands and infilling the estuaries include characteristic depositional shoreline environments, such as spits, dunes, baymouth bars, an intermeshing network of tidal deltas, nearshore marine erosional-depositional sand and gravel, and the bays or estuaries and fringing Spartina and Distichlis marshes which form the westernmost edge of the transgressive units. The thickness and areal extent of the sedimentary bodies are controlled largely by the morphology of the Pleistocene unconformity. A large part of these Recent sedimentary units is being eroded by the transgressing Atlantic Ocean. Cores of sediment taken under the shallow bays, such as Rehoboth, Indian River, and Assawoman Bays, and in the fringing marsh environment, show that the depositional units are thin, highly irregular in areal extent, extremely variable in thickness, and difficult to project. Sedimentary processes active in the shallow bays include shoreline marsh erosion and the formation of thin, possibly ephemeral, beach-dune complexes consisting of clean, well-sorted sand having typical beach and dune sedimentary structures. They are anomalous in that they are completely surrounded by Spartina marshes on the landward side and extremely muddy sand grading into dark gray lagoonal mud on the bay side. It appears that distinctive sedimentary structures, and sediment size-sorting relations such as those hat characterize the larger more common sedimentary units of the coastal area may be formed in miniature at the very thin edge of transgression and may lead to considerable confusion in the interpretation of sediments of this type in the geologic record. End_of_Article - Last_Page 537------------

Epiphasic Carotenoids in Post‐Glacial Lake Sediments

The presence of α‐ and β‐carotenes in surface mud from Lower Linsley Pond, Connecticut, and Little Round Lake, Ontario, was demonstrated by comparing the chromatographic positions and absorption spectra of the isolated compounds with authentic specimens of the same compounds prepared from carrots. Seven other carotenes were isolated from the Lower Linsley Pond mud, three of which were probably myxoxanthin, rhodoviolascin, and lycopene.Carotenoids were found in sediment cores taken from Upper Linsley Pond and Bethany Bog, Connecticut. On the basis of radiocarbon dates and pollen zonations the age of the oldest sediments in these cores was estimated to be approximately 11,000 years. Carotenoids were present in the organic sediments at all depths, but were present in traces or not at all in the basal clay‐silts. Five carotenoid zones of the paper chromatograms were estimated separately using an arbitrary concentration scale: carotene (consisting of α‐ and β‐carotenes and perhaps their cis‐isomers as well), myxoxanthin (?), rhodoviolascin (?), and two unidentified epiphasic carotenoids termed compounds A and B. In the core from Bethany Bog, rhodoviolascin was maximal in the upper part, carotene in the middle, and myxoxanthin and compound B in the middle and lower parts. Myxoxanthin, a pigment of the blue‐green algae, was present only in sediments of the mesotrophic phase of the history of Bethany Bog, and not in those of the dystrophic (bog) phase. The same pigments obtained from the Bethany Bog core were found in a core from Upper Linsley Pond, but the changes in concentration with depth were less striking. A new yellow pigment was present in the Upper Linsley Pond samples.Data on the biochemical composition of mud may be important in determining the kind and amounts of plant tissues incorporated into the mud.