Showing papers on "Varve published in 1995"

Sediment fluxes and varve formation in Santa Barbara Basin, offshore California

Abstract: Results from a year-long sediment-trap study provide the first direct observations of seasonal variability in sediment fluxes to the sea floor in Santa Barbara Basin, offshore California, and are used to evaluate varve formation in this basin. Sediment fluxes throughout the year are dominated by lithogenic material; biogenic sediments (primarily silica) are of secondary importance. We conclude that the combination of high lithogenic fluxes and low biogenic fluxes during the fall-winter period results in the deposition of dark laminae. The onset of upwelling and high surface productivity in the spring results in high biogenic silica fluxes that last into the summer. The relative contribution of silica to the total sediment flux is highest during this spring-summer period, and we suggest that light laminae are formed at that time. This is consistent with previous suggestions that sediment input is the primary factor controlling varve formation in this basin.

Evaluating the mobility of137Cs,239+240Pu and210Pb from their distributions in laminated lake sediments

Abstract: Post-depositional mobility of137Cs,239+240Pu and210Pb was assessed in six small lake basins by comparing sedimentary nuclide profiles with their known fallout history. Laminae couplets, when present, were determined to be varves because the137Cs and239+240Pu 1963 fallout peaks are present in laminae couplets corresponding to years 1962–1964. There is no evidence of mobility of210Pb, because 1) mass accumulation rates based on210Pb agree with those based on137Cs and239+240Pu peak depths and with those based on varve counts, and 2)210Pb ages agree with varve ages. Significant mobility of137Cs is evident from the penetration of137Cs to depths 15–20 cm deeper than239+240Pu. Deep penetration of137Cs in spite of a sharp gradient below the peak is interpreted by a numerical model to suggest that137Cs is present in two distinct forms in these sediments, 67–82% as an immobile form and 18–33% reversibly adsorbed with a K d of approximately 5000. The profiles can be interpreted equally well assuming a small portionof the total137Cs was present as an extremely mobile phase (K d ≪5000) in the months to years following peak fallout, slowly becoming more strongly adsorbed. High NH 4 + concentrations in porewaters may enhance diffusion of the mobile form of137Cs, but not of the immobile form of137Cs that defines the sharp gradient. Mobility of137Cs is likely also enhanced by the low clay content and the high porosity of these sediments. Thus the first detection of137Cs in the sediments cannot automatically be assumed to correspond to a date of 1952 (initial testing of thermonuclear weapons), although the depth of the peak can be assumed to correspond to 1963 (the year of maximum fallout from testing of thermonuclear weapons).239+240Pu is a more reliable sediment chronometer than137Cs because it is significantly less mobile.

Non-Glacial Varved Lake Sediment as a Natural Timekeeper and Detector on Environmental Changes

Abstract: 年縞(non-glacial varve)を用いた高精度年代測定法は,第四紀の年代決定法の中で最も有力な方法の一つである.本論文では,第1にスウェーデン年縞編年学(Swedish Varve Chronology)を例として取りあげ,年縞研究の現状と問題点をレビューした.その結果,1.最新年縞年代と現在との関係,2.年縞の計数方法および,3.年縞でない葉理の付加および年縞の消失が,編年に大きな影響を与えることが指摘できた.第2に,福井県三方五湖の水月湖における数本の堆積物コアから得られた,過去16,000年間にわたる年縞堆積物を用いた編年学について詳しく述べた.この年縞堆積物では歴史記録から年代をさかのぼる計数,および広域テフラや泥流堆積層による対比や計数の検証が行われつつあり,Younger Dryas期や最終氷期極相期の編年が数年単位で明らかにできる可能性を指摘した.第3に,集水域が狭く,周辺からの粗粒砕屑物が流入しない水月湖の細粒堆積物中の鉄鉱物や粘土鉱物組成の変動が,大陸起源の風成塵変動,日本海の海水準変動および若狭湾周辺の降水量変動を反映しており,年縞計数の結果から過去8,830年間の変動を1年～数年単位で明らかにした.

Sedimentological proof and dating of the Early Holocene volcanic eruption of Ulmener Maar (Vulkaneifel, Germany)

Abstract: A minerogeniclayer occurs in early postglacial organic sediments from five maar lakes (West Eifel Volcanic Field, Germany). The mineralogy and stratigraphic position of this tephra suggests that it is related to the youngest German volcano, Ulmener Maar, nearby. Radiocarbon dating of wood from the base of the Ulmener Maar Tephra at two locations provide ages in agreement with an accelerator mass spectrometer 14C date for the minerogenic layer from sediments of Lake Holzmaar situated 13 km south-west of Ulmener Maar. The mean radiocarbon age is 9 560 years BP. Dating by varve chronology provides an age of 10017 years VT (varve time in years before 1950) or 10 895 years corrected VT. Based on palynology the Ulmener Maar Tephra was deposited at the end of the Preboreal. High values of natural remnant magnetization intensity, typical of pyroclastic material, confirm that this minerogenic layer differs in composition from other clastic deposits of the sedimentary record. Geochemical analyses reveal increased values of total trace elements for the Laacher See Tephra and Ulmener Maar Tephra. An isopach map based on thickness variations of the Ulmener Maar Tephra at five investigated maar lakes indicates that the tephra was mainly transported to the south west.

The younger dryas cold event-was it synchronous over the north atlantic region?

Abstract: Determined independently from annually laminated ice cores and lake sediments, and German pines, the calendar ages of Younger Dryas (YD) boundaries significantly disagree with one another. 14C dates, plotted vs. calendar ages for samples from different sediments, also reveal distinct offsets. The adjustment of varve chronologies to synchronize the boundaries of the YD nearly cancels the discrepancies between 14C data, and supports the synchronism of the YD cold period over the North Atlantic region. However, the exact timing of the event cannot be estimated in this way.

A varve-based calibration of the Bridge River tephra fall

Abstract: A 1 cm thick layer of Bridge River tephra is preserved in a core of varved lake sediments recovered from Hector Lake, Alberta. The varve sequence extends from the core top to well below the tephra layer and has bseen calibrated to calendar years. A varve count yields a calendar age of 2332 BP for the ash layer, with a potential error of about ±50 years. This age is about 30 years younger than the current best radiocarbon-based estimate of the time of the eruption. However, in light of the potential errors in both the varve count and the radiocarbon age, the two ages are not inconsistent. The tephra contains no biotite. In the past, biotite-free Bridge River tephra has been associated with a possible late (ca. 2000 BP) tephra fall, but the tephra from Hector Lake is clearly associated with the main (2300–2400 BP) eruptive event.

Holocene vegetation of the Van area : Palynological and chronological evidence from Söğütlü, Turkey

Abstract: Evidence of the terrestrial pollen precipitation in the Van area, obtained from the marsh of Sogutlu, is compared with the lacustrine record from Lake Van, eastern Turkey. The radiocarbon chronology of the vegetation history of the Van area is compared with that from varve counting. The first method produced the most probable results, supported by palynological investigations in other parts of Turkey and Iran. The combination of radiocarbon dating and palynological correlation dates the sediments of Lake Van 1100 to 2200 years older than the varve dating. The indicator value of Cerealia-type pollen as evidence of prehistoric agriculture is discussed and compared with results from observations on modern crops. In the present study, Cerealia-type pollen may have been produced by wild grasses growing in the marsh.

Late Weichselian to early Holocene sedimentation in a steep fjord/valley setting, Visdalen, Edgeøya, eastern Svalbard: glacial deposits, alluvial/colluvial‐fan deltas and spit‐platforms

Abstract: The Visdalen valley, situated at the northwestern corner of Edgeoya, was investigated with respect to lithostratigraphy and depositional environments of the Quaternary sediments. Eight major lithostratigraphic units are recognised of which seven were deposited during the Late Weichselian to early Holocene glaciation, deglaciation and the subsequent emergence of the area, and one unit deposited prior the last glaciation. Till deposition from a west-flowing glacier was followed by glaciomarine and later marine deposition of fine-grained sediments. Coarse-grained colluvial and alluvial-fan deltas were deposited along the mountainsides in the Visdalen palaeo-bay, and distal sediment gravity-flow deposits from these deltas were interbedded with the glaciomarine-marine sediments. A spit-platform (riegel) was built up across the Visdalen bay contemporaneously with the alluvial fan-deltas. Its formation was time-transgressive, with its highest part in the south close to the marine limit at 85 m a.s.l. and its lowest part in the north at ca 65 m a.s.l. The sediment source was alluvial and colluvial debris, which was entrained by longshore currents along the more exposed coast south of Visdalen and transported northwards to the final place of deposition. The bulk part of the riegel ridge is composed of progradational successions of steep foresets dipping towards NW, N and NE, and clearly rejects an earlier ice-contact model. Datings suggest that the fan-delta deposition and the riegel formation ended before 9,000 BP. A meltwater-fed lagoon with a highest level at >50m a.s.l. was formed behind the riegel ridge in which, according to varve counting, glaciolacustrine sedimentation lasted more than 250 years and occurred within the time span 9,000-8,500 BP. Gradual uplift of the area resulted in drainage of the glaciolacustrine lagoon. Beachface processes and fluvial down-cutting took place during the emergence of the area.

Palaeomagnetic cave stratigraphy of sediments from Hellemofjord, northern Norway

Abstract: SUMMARY Subglacial deposited sediments from three deep karst caves in Nordland, northern Norway, carry single-component palaeomagnetic directions interpreted to represent discrete records of palaeosecular variations (PSV) defining a clockwise rotation of the vector with time. The results suggest that preserved sediment sections commenced accumulating at the highest altitude in this cave system, consistent with an inverse stratigraphic sedimentation. Anomalously low inclinations are interpreted to represent a detrital remanent magnetization acquired at the time of deposition and affected by inclination error. The preservation of the inclination error in these sterile sediments is attributed to the absence of processes that may cause post-depositional alignment of magnetic grains. Non-systematic directional scatter is also attributed to poor smoothing of geomagnetic field variations in these cave sediments compared with lacustrine deposits. The ahnost closed PSV loop may represent discrete periods of sedimentation during one or, alternatively, several subglacial events. The PSV loop exhibits amplitudes in declination and inclination comparable with Holocene or Weichselian lacustrine PSV records. The minimum duration of sediment accumulation may be of the order of 400 to 1000 yr. If the investigated sediments represent succeeding records of geomagnetic secular variations during the last Late Weichselian glacial retreat, the reasonably good correlation with the PSV records from the Torreberga (Southern Sweden) varved clay sequence (12 200-10 200 bp) and UK lacustrine sediments (0-10 000 bp) tentatively suggests that the Ragge Javre Raigi sediments accumulated during the final Weichselian glacial retreat between 10 900 and 9 800 bp.