Cal Before Present Research Articles

Marine04 Marine Radiocarbon Age Calibration, 0–26 Cal Kyr Bp

New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally ratified to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0–26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0–10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-resolution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5–26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue).

Charcoal and organic geochemical properties as an evidence of Holocene fires in tropical peatland, Central Kalimantan, Indonesia

A fire-history study of the tropical peatland of Central Kalimantan has been conducted using charcoal and organic geochemical analyses. A-153 cm core was taken from Kalampangan. Charcoal analysis and 14C dating were used to understand the fire pattern, and organic geochemical analysis was used to identify environmental changes caused by fires. The peat formed from 9472 - 8981 to 7624 - 7424 cal. yr BP (calendar years before present) coeval with the development of the wet and warm climate of the Southeast Asia region. Fire frequently occurred in the study area during the peat formation. The fire intensity must have been low to have allowed continous peat formation. The record indicated that two comparatively severe fires, or fire periods, occurred in the study area between ca. 7500 and ca. 7600 cal BP and between ca. 6400 and ca. 6500 cal. yr BP. In addition, several fires caused nitrogen loss from the peatland and destroyed the plant communities. Re-established plant communities gained nitrogen for photosynthesis from the atmosphere through bacterial fixation. As a consequence, low quality litters with more negative δ15N were produced.

The impacts of sea-level changes during latest Pleistocene and Holocene times on the morphology of the Ionian and Aegean seas (SE Alpine Europe)

The tentative coastline configuration in the Ionian and Aegean seas in three Late Pleistocene–Holocene time slices is given, based on the global eustatic sea-level curve in combination with local geological and geoarcheological data. At lowstand stage 2 (21 500 cal yr BP (years before present), sea level −120 m), extensive shelves existed in the northern Aegean Sea and, to a second degree, in the eastern Aegean Sea and in the northern and central parts of the Ionian Sea. Many islands were connected with each other and with the mainland. A great part of the periphery of gulfs that have an entrance less than 120 m deep was subaerially exposed, while in their central parts lakes were formed. At 11 500 cal yr BP (sea level −60 m) the exposed shelf was much restricted, most of the gulfs were overflowed by the advancing sea and only a few islands were still connected with the mainland. Finally, at 8000 cal yr BP and onward, the sea initially intruded the lowlands and the gulfs, but the subsequent sediment input by the rivers regressed the sea to its present position. Thus many human settlements and old cities that were maritime during Hellenistic or older times are now a few to tens of kilometers inland.

Holocene multidecadal and multicentennial droughts affecting Northern California and Nevada

Continuous, high-resolution δ 18O records from cored sediments of Pyramid Lake, Nevada, indicate that oscillations in the hydrologic balance occurred, on average, about every 150 years (yr) during the past 7630 calendar years (cal yr). The records are not stationary; during the past 2740 yr, drought durations ranged from 20 to 100 yr and intervals between droughts ranged from 80 to 230 yr. Comparison of tree-ring-based reconstructions of climate change for the past 1200 yr from the Sierra Nevada and the El Malpais region of northwest New Mexico indicates that severe droughts associated with Anasazi withdrawal from Chaco Canyon at 820 cal yr BP (calendar years before present) and final abandonment of Chaco Canyon, Mesa Verde, and the Kayenta area at 650 cal yr BP may have impacted much of the western United States.During the middle Holocene (informally defined in this paper as extending from 8000 to 3000 cal yr BP), magnetic susceptibility values of sediments deposited in Pyramid Lake's deep basin were much larger than late–Holocene (3000–0 cal yr BP) values, indicating the presence of a shallow lake. In addition, the mean δ 18O value of CaCO 3 precipitated between 6500 and 3430 cal yr BP was 1.6‰ less than the mean value of CaCO 3 precipitated after 2740 cal yr BP. Numerical calculations indicate that the shift in the δ 18O baseline probably resulted from a transition to a wetter (>30%) and cooler (3–5°C) climate. The existence of a relatively dry and warm middle-Holocene climate in the Truckee River–Pyramid Lake system is generally consistent with archeological, sedimentological, chemical, physical, and biological records from various sites within the Great Basin of the western United States. Two high-resolution Holocene-climate records are now available from the Pyramid and Owens lake basins which suggest that the Holocene was characterized by five climatic intervals. TIC and δ 18O records from Owens Lake indicate that the first interval in the early Holocene (11,600–10,000 cal yr BP) was characterized by a drying trend that was interrupted by a brief (200 yr) wet oscillation centered at 10,300 cal yr BP. This was followed by a second early-Holocene interval (10,000–8000 cal yr BP) during which relatively wet conditions prevailed. During the early part of the middle Holocene (8000–6500 cal yr BP), high-amplitude oscillations in TIC in Owens Lake and δ 18O in Pyramid Lake indicate the presence of shallow lakes in both basins. During the latter part of the middle Holocene (6500–3800 cal yr BP), drought conditions dominated, Owens Lake desiccated, and Lake Tahoe ceased spilling to the Truckee River, causing Pyramid Lake to decline. At the beginning of the late Holocene (∼3000 cal yr BP), Lake Tahoe rose to its sill level and Pyramid Lake increased in volume.

INTCAL98 Radiocarbon Age Calibration, 24,000–0 cal BP

The focus of this paper is the conversion of radiocarbon ages to calibrated (cal) ages for the interval 24,000–0 cal BP (Before Present, 0 cal BP = AD 1950), based upon a sample set of dendrochronologically dated tree rings, uranium-thorium dated corals, and varve-counted marine sediment. The 14C age–cal age information, produced by many laboratories, is converted to Δ14C profiles and calibration curves, for the atmosphere as well as the oceans. We discuss offsets in measured l4C ages and the errors therein, regional 14C age differences, tree–coral 14C age comparisons and the time dependence of marine reservoir ages, and evaluate decadal vs. single-year 14C results. Changes in oceanic deepwater circulation, especially for the 16,000–11,000 cal BP interval, are reflected in the Δ14C values of INTCAL98.