Decrease In Accumulation Rate Research Articles

Recent precipitation decrease across the western Greenland ice sheet percolation zone

Abstract. The mass balance of the Greenland Ice Sheet (GrIS) in a warming climate is of critical interest in the context of future sea level rise. Increased melting in the GrIS percolation zone due to atmospheric warming over the past several decades has led to increased mass loss at lower elevations. Previous studies have hypothesized that this warming is accompanied by a precipitation increase, as would be expected from the Clausius–Clapeyron relationship, compensating for some of the melt-induced mass loss throughout the western GrIS. This study tests that hypothesis by calculating snow accumulation rates and trends across the western GrIS percolation zone, providing new accumulation rate estimates in regions with sparse in situ data or data that do not span the recent accelerating surface melt. We present accumulation records from sixteen 22–32 m long firn cores and 4436 km of ground-penetrating radar, covering the past 20–60 years of accumulation, collected across the western GrIS percolation zone as part of the Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) project. Trends from both radar and firn cores, as well as commonly used regional climate models, show decreasing accumulation rates of 2.4±1.5 % a−1 over the 1996–2016 period, which we attribute to shifting storm tracks related to stronger atmospheric summer blocking over Greenland. Changes in atmospheric circulation over the past 20 years, specifically anomalously strong summertime blocking, have reduced GrIS surface mass balance through both an increase in surface melting and a decrease in accumulation rates.

A 2700-year annual timescale and accumulation history for an ice core from Roosevelt Island, West Antarctica

Abstract. We present a 2700-year annually resolved chronology and snow accumulation history for the Roosevelt Island Climate Evolution (RICE) ice core, Ross Ice Shelf, West Antarctica. The core adds information on past accumulation changes in an otherwise poorly constrained sector of Antarctica. The timescale was constructed by identifying annual cycles in high-resolution impurity records, and it constitutes the top part of the Roosevelt Island Ice Core Chronology 2017 (RICE17). Validation by volcanic and methane matching to the WD2014 chronology from the WAIS Divide ice core shows that the two timescales are in excellent agreement. In a companion paper, gas matching to WAIS Divide is used to extend the timescale for the deeper part of the core in which annual layers cannot be identified. Based on the annually resolved timescale, we produced a record of past snow accumulation at Roosevelt Island. The accumulation history shows that Roosevelt Island experienced slightly increasing accumulation rates between 700 BCE and 1300 CE, with an average accumulation of 0.25±0.02 m water equivalent (w.e.) per year. Since 1300 CE, trends in the accumulation rate have been consistently negative, with an acceleration in the rate of decline after the mid-17th century. The current accumulation rate at Roosevelt Island is 0.210±0.002 m w.e. yr−1 (average since 1965 CE, ±2σ), and it is rapidly declining with a trend corresponding to 0.8 mm yr−2. The decline observed since the mid-1960s is 8 times faster than the long-term decreasing trend taking place over the previous centuries, with decadal mean accumulation rates consistently being below average. Previous research has shown a strong link between Roosevelt Island accumulation rates and the location and intensity of the Amundsen Sea Low, which has a significant impact on regional sea-ice extent. The decrease in accumulation rates at Roosevelt Island may therefore be explained in terms of a recent strengthening of the ASL and the expansion of sea ice in the eastern Ross Sea. The start of the rapid decrease in RICE accumulation rates observed in 1965 CE may thus mark the onset of significant increases in regional sea-ice extent.

High-resolution trace element records of an ice core from the eastern Tien Shan, central Asia, since 1953 AD

[1] High-resolution records of trace elements (Ba, V, Cr, Mn, Co, Ni, Cu, As, Rb, Sr, Mo, Cd, Sn, Sb, Tl, Pb, Bi, Th, and U) quantified in an ice core recovered from the Miaoergou glacier in the eastern Tien Shan, central Asia, spanning the period 1953–2004 AD, provide the first comprehensive time series on characterizing the relative contributions from natural and anthropogenic sources to the deposition of trace elements in central Asia. It is suggested that rock and soil dust is the most important natural source for most of elements investigated. Slight decreases in concentrations (or fallout fluxes) of crustal elements, such as Ba, Mn, Rb, Th, U, and Sr are observed during recent decades, which may be due to decreases in dust emissions from source regions and a decrease of accumulation rate since 1980s. The increasing trends of median concentrations and crustal enrichment factors (EFc) of V, Cr, Co, Ni, Cu, and Mo, during the period 1953–2004 AD, are insignificant in comparison to their respective levels prior to 1953 AD. However, slight enhancements of both concentrations and EFc are observed for Cd, Sb, Pb, Bi, Tl, and Sn since 1950s. Such recent increases are likely to be attributed to enhanced anthropogenic emissions, such as metal smelting, mining, stationary fossil fuel combustion, and combustion of gasoline due to human activities in Eurasia, particularly Xinjiang in northwestern China, Russia, and Kazakhstan. Our study supports evidence that environmental contamination has become a global problem for Pb and Bi and a large-scale phenomenon for Cd, Sb, Tl, and Sn.

Organic geochemical records from Lake Malawi (East Africa) of the last 700 years, part II: Biomarker evidence for recent changes in primary productivity

Relatively few well-dated and high-resolution paleoclimate records of the past few centuries presently exist from tropical East Africa. Here, we examine the bulk and molecular geochemical records of two varved sediment cores from Lake Malawi, which together provide a continuous record of environmental variability in East Africa of the last 730 years. We observe a number of changes in the aquatic ecosystem of Lake Malawi, which are likely attributed to both natural climatic forcing and anthropogenic activities. Biomarkers of dinoflagellates (dinosterol) and bacterivorous ciliates (tetrahymanol) display increased accumulation rates from ~ 1900 AD to the present, while a simultaneous decrease in accumulation rates of diatom biomarkers (isololiolide/loliolide) is observed. Increased accumulation rates of retene, a compound derived from conifers, are also noted since ~ 1930 AD and likely reflect increased soil erosion due to deforestation of the Lake Malawi watershed. Spectral analysis of the high-resolution TOC record indicates a periodicity of 204 years, similar to the 206 year cycle noted in 14C and 10Be records, suggesting a link between East African climate and solar forcing.

The Valley-Fill Deposits of the Kłodnica River (Southern Poland): Environmental Drivers of Facies Changes from the Late Vistulian Through the Holocene

Lithological analyses and radiocarbon dating were used to elucidate the patterns and controls of Late Quaternary valley floor development of the Kłodnica River, the Upper Odra Basin. The research results were discussed with data obtained from valleys of rivers draining piedmont basins and highlands of southern Poland. In consequence, five stages of morpho-sedimentary evolution of the Kłodnica valley were distinguished. In the Late Vistulian a large-scale deposition of channel alluvium took place in the conditions of high river discharges. This sedimentary style probably still existed in the Early Pre-Boreal as long as open grass communities survived in the Kłodnica catchment. The next phase, in the Late Pre-Boreal and Boreal, is characterized by a significant increase in accumulation rate of biochemical facies. The considerable restriction of minerogenic deposition was connected with widespread of forest and gradual limitation of the river discharges. The third stage began at the decline of the Boreal and was defined by decrease of accumulation rate or even biogenic accumulation break. Synchronously, periodic increases of fluvial activity were noticed in the form of cutoffs of meander loops and overbank deposition in oxbows. The beginning of the fourth period took place not earlier than in the Early Sub-Boreal. This stage was distinguished by renewed peat growth/increase in biochemical accumulation rate and periodic increase in alluviation, generally taking place in the conditions of low channel-forming flows. The latest phase (from the Middle Sub-Atlantic till now) is characterized by common initiation of slope deposition and a rapid increase in fluvial sedimentation, especially overbank and tributary fan facies. An increase in minerogenic deposition occurred in response to human impact, which became more significant from the Roman Period and occurred on a large scale from the early Middle Ages. Older settlement phases, including intense settlement from the Hallstatt Period, were not clearly recorded in the Kłodnica valley fill.

Accumulation rates of volcaniclastic deposits on Loihi Seamount, Hawaii

AMS radiocarbon age dating of planktonic foraminifera in volcaniclastic deposits on Loihi Seamount yields ages ranging from 590 years before present (y BP) at 10 cm depth to 5,880 y BP at 1,007 cm depth in an 11-m-thick section exposed along inward facing, caldera-bounding faults on the eastern side of Loihi’s summit. The accumulation rate of the deposit was about 0.37 cm/y from 5,880 to 3,300 y BP and it consisted of subequal amounts of alkalic and tholeiitic fragments. The rate slowed dramatically at about 3,300 y BP to an average 0.04 cm/y and the particles that have accumulated since consist mostly of alkalic glass fragments. The decrease in accumulation rate could indicate a decrease in volcanic activity at Loihi beginning about 3,300 y BP. This lower level of activity appears to be continuing today.

Climate vs. anthropogenic changes in North Adriatic shelf sediments influenced by freshwater runoff

Land-ocean coupling was investigated for the Adige River-Adriatic Shelf system by analyzing precipitation, water discharge and suspended load records over the period 1922-2000 and by comparing them with the 210 Pbex and porosity depth profiles of 4 marine cores taken along the riverborne sediment transport pathway on the shelf. The results show that precipitation was affected only by minor variations in yearly and seasonal amounts but was accompanied by a significant decrease (increase) in the number of wet days (precipitation intensity). River discharge—which is strongly influenced by total precipitation as well as by the number of wet days (in contrast to the quantitatively less important precipitation intensity)—shows a continuous decrease throughout the whole period. A significant decrease in suspended load is recorded after the 1940s, and meteoro- logical and hydrological data suggest a major discontinuity over the same period, which was also recorded by marine sediments as a significant decrease in accumulation rates of fine materials. After the 1970s a further change in water discharge is observed in the river-system lowlands, recorded on the Adriatic shelf as intermittent sediment accumulation, and probably related to major floods. Changes seem to be mostly anthropogenic, such as progressive withdrawal of water and sediment for agricultural and civil uses, as well as damming for irrigation purposes and for generation of electricity.

Phyto_ and zooplankton paleofluxes during the deposition of sapropel S1 (eastern Mediterranean): Biogenic carbonate preservation and paleoecological implications

The relative and absolute abundances and accumulation rates of foraminifera and calcareous nannofossils were quantified in a box_core containing sapropel S1 from the Florence Rise area (SE of Cyprus; 2302 m water depth). The main objective of this study was to reconstruct in detail variations in paleoecological conditions of water masses during the deposition of this sapropel. In particular, we qualitatively evaluated the importance of carbonate dissolution on planktonic assemblages to better interpret the abundance profiles obtained from the two investigated taxa. Selective carbonate corrosion in the core is shown by 1) the decrease in shell mass weight of selected species of planktonic foraminifera, 2) the decrease in accumulation rate of dissolution_susceptible holococcoliths and coccolith species, and 3) the absence of pteropods. However, the presence of other relatively dissolution_susceptible coccolith species throughout all of the S1 interval suggests moderate dissolution. Florisphaera profunda shows a marked increase in paleofluxes within the sapropel coupled with a decrease in the accumulation rate of the upper_middle photic zone coccoliths, suggesting an ecological depth_separation of the water column, probably characterised by higher nutrient availability at depth and nutrient_depleted surface waters between ∼ 10 and 6.5 kyr BP. In the same interval Turborotalita quinqueloba and Globigerina bulloides, two foraminiferal species frequently occurring during periods of high fertility, increase in relative abundance. The maximum increase in relative abundance of Globigerinoides ruber (var. alba and rosea) marks the climatic optimum phase and the maximum stratification in surface water that occurred at the beginning of sapropel S1 deposition when the bottom waters were anoxic. An important change in foraminiferal assemblages occurs at ∼ 8 kyr BP and corresponds with a negative shift in CaCO 3, Ba and C org contents. This short interval marks the establishment of relatively less anoxic conditions in the bottom water, introducing the last phase of sapropel formation. After ∼ 6.5 kyr BP, a progressive re_establishment of normal oceanographic conditions occurred before the real end of the sapropel S1. This transition is well recorded by the reoccurrence and major accumulation rate of the mixing indicator foraminiferal species Globorotalia inflata and by the gradual decrease in abundance of F. profunda.

Recent Accumulation Rate Changes in South Greenland from Internal Layering

The burial depths of three internal layers detected by airborne radar sounding in the Dye-3 region of southern Greenland are used to infer past changes in the accumulation rate in the region. Averaged over the period since the deepest layer was deposited (∽1700 to ∽2000 yr BP) the ratio of accumulation rates at the western and eastern ends of the 60 km recorded length of the layers was 0.63, with accumulation increasing to the east. According to recent maps, the present-day value of this ratio is 0.78-0.87. Thus, the distribution of accumulation appears to have become more uniform, because of a decrease in accumulation rate at the eastern end of the flightline, an increase at the western end, or a combination of both. Observed patterns of thickness change and the displacement of the ice crest westward relative to the flow divide are both consistent with a recent decrease in accumulation rate on the eastern end of the segment of flightline considered.

Fruticose arboreal lichen biomass accumulation in an old-growth balsam fir forest

The standing biomass accumulation of lichens from the genera Alectoria, Bryoria, and Usnea was studied along two environmental gradients, altitude and height along the vertical tree axis, in an old-growth balsam fir (Abies balsamea (L.) Mill.) forest. Lichens were sampled from 50 trees in five sampling sites at an altitude of 720-1068 m asl and encompassing three vegetation belts (mountain, subalpine, and alpine). The lichen biomass accumulation rate was studied at branch and tree levels. The Gompertz equation was used to assess the relationship between time since lichen colonization and standing lichen biomass. The relationship between tree lichen biomass and age was not significant. Colonization delay was assumed to correspond to the age of the youngest branch supporting a given lichen taxon, and our results show that it increased from mountain to alpine belts. Variations in lichen accumulation were related to the branch position along the tree height and altitude. Lichen biomass accumulation rates decreased from tree top to base. We speculate that this reduction reflects a response to microclimatic gradients, aging of thalli, fragmentation, competition, succession, and caribou grazing. The accumulation rate was greater in the mountain belt (r = 0.063) than in the alpine belt (r = 0.027). In the subalpine belt, the relationship between lichen biomass and time was not significant (p > 0.05). The decrease in accumulation rate along the altitudinal gradient may be primarily explained by climatic conditions.Key words: arboreal lichens, lichen biomass accumulation, altitude effect, height in tree effect, Alectoria, Bryoria, Usnea.

Pliocene calcium carbonate sedimentation patterns of the Ontong Java Plateau: ODP Sites 804 and 806

Two deep-sea cores from the northeastern slope of the Ontong Java Plateau, Ocean Drilling Program Site 806 (2520 mbsl) and Site 804 (3861 mbsl), has been studied with the purpose of investigating Pliocene (Site 806, 2.4–3.8 Ma; Site 804, 1.8–4.4 Ma) sub- and supralysoclinal changes in calcium carbonate accumulation, based on CaCO 3, coarse-fraction and faunal data. At the shallower site, Site 806, there is a slight decrease in carbonate content and a distinct long-term trend in decreasing accumulation rate throughout the studied interval. Small-scale changes in density, hence accumulation rate, are related to grain-size changes due to winnowing at this site, but the overall pattern of decreasing carbonate accumulation seems to be the result of both winnowing and a decrease in productivity. The deeper site, Site 804, exhibits a general increase in carbonate content after about 3.15 Ma as well as a long-term increase in mass accumulation rate throughout the Pliocene interval studied. Detailed comparison of the records of density and carbonate content emphasizes the strong relationship between carbonate content and accumulation rate at this site. The observed pattern of decreasing accumulation rate at shallow depth parallel to enhanced preservation at greater depths may be explained by a decrease in the production of North Atlantic Deep Water (NADW). A reduction of the volume of NADW pumped into the Pacific would reduce the supply of nutrients and deep-water CO 2. This could account for both the decrease in accumulation rate at the shallower site, due to decreased productivity, and the enhanced preservation at the deeper site, due to a decrease in the corrosiveness of deep waters.

Preservation of fossil diatoms in Atlantic sediment cores: control by supply rate

Dissolution of fossil diatoms in piston cores from the North and equatorial Atlantic Ocean is generally severe only in areas of low primary productivity, and hence low fluxes of diatoms to the sea floor. In silica-rich sediments, initial dissolution increases the silica content of pore waters, which acts as a buffer against continued dissolution. Thus down-core decrease in accumulation rates reflects relative loos compared to initial supply rates, rather than absolute dissolution. Most records show either very strong or very weak impact by post-depositional dissolution. An empirical model suggest that in pelagic sediments, dissolution measurably affects diatom accumulation only when supply is below approximately 2.5 × 10 8 diatom fragments cm −2 1000 y −1, or 6.4 × 10 6 whole diatoms cm −2 1000 y −1. Other factors may also influence the preservation of siliceous microfossils; however, water depth has no discernible effect on the relative severity of opal dissolution in sediments of the abyssal Atlantic.

Accumulation of modern sediment in Quinault submarine canyon

Twenty box cores have been studied from Quinault submarine canyon, a major pathway between the Washington continental shelf and the deep sea for silt and clay originating from the Columbia River. Sediment accumulation rates (on a 100-yr time scale) are calculated from profiles of excess 210Pb and of 137Cs. Accumulation rates decrease with increasing distance downcanyon: from 14 × 10 −2 g cm −2 yr −1 in the upper canyon (lower head) to 3 × 10 −2 g cm −2 yr −1 in the lower canyon. In the upper canyon (lower head), channel-floor areas have a higher mean accumulation rate ( 12 × 10 −2 g cm −2 yr −1 ) than channel-wall areas ( 5 × 10 −2 g cm −2 yr −1 ). The difference in accumulation rate between channel-floor and channel-wall areas indicates preferential accumulation (i.e., focusing) in topographic lows. The decrease in accumulation rate (from high rates in the channel floors of the upper canyon to low rates in the lower canyon) may be caused by progressive depletion of a nepheloid-layer source and/or may be an artifact of sediment focusing in the upper canyon. The silty clay and clayey silt accumulating in the canyon show no significant trend in grain size or clay mineralogy with depth in cores or with distance downcanyon. The lack of sorting probably results from the transport of sediment in aggregate form, with a nearly random distribution of size and mineralogy within aggregates. Quinault Canyon is accumulating approximately 5.5 × 10 5 MT of sediment yr −1. The lithogenic component (64%) accounts for 1–4% of the Columbia River discharge. Similarity of sediment-trap deposition rates (including storm events) and 210Pb accumulation rates indicates that the vertical flux is a major input of sediment to Quinault Canyon and that storms are important in the accumulation of sediment on a 100-yr time scale.

Some questions and answers about the accumulation of fine-grained sediment in continental margin environments

Accumulation rates (100-yr time scale) in proximal regions of allochthonous dispersal systems range from centimeters to millimeters per year, depending primarily on fluvial discharge of sediment. A general decrease in accumulation rate occurs from proximal to distal regions, and the across-shelf maximum rate is in the mid-shelf region. Sedimentary structure generally changes from stratified in proximal deposits to homogeneous in distal deposits. Although the content of sand and coarse silt commonly decreases along dispersal systems, progressive fining of clay and fine silt is not as obvious. Deposition rates (100-day time scale) in proximal deposits can be significantly more rapid (centimeters per month) than longer-term accumulation rates. Rapid sedimentation does not necessarily produce mass movement.

Annihilation of edge dislocation dipoles by a glide mechanism

A glide mechanism for the annihilation of an edge dislocation dipole is proposed. It can operate in many crystal structures. Evidence is taken from transmission electron microscope investigations of plastically deformed NaCl. The mechanism can explain the decrease in accumulation rate of primary dislocations as observed during deformation beyond stage I of the work-hardening curve.

Neogene Turbidite Sedimentation in Komandorskiy Basin, Western Bering Sea

Sediments underlying the abyssal plain in Komandorskiy basin were recovered at Deep Sea Drilling Project Site 191 in the east-central part of the basin (168.1°E). Most sedimentary fill is diatomaceous silty clay. Interbedded in the silty clay are layers of turbidite sand and volcanic ash. The sand layers range in composition from volcanic lithic sand to chert-rich sand containing as much as 40% quartzose debris. The sand probably originates on the continental shelf in the westernmost Bering Sea, and is deposited by turbidity currents crossing the abyssal plain. Chert-rich sand may originate on the shelf between Mys Olyutorskiy and Ostrov Karaginskiy, and volcanic lithic sand may originate farther south toward Mys Kamchatskiy. Below 500 m, sediments of the abyssal plain are altered extensively. The silty clay is compacted to mudstone, and sand layers are graywacke. Porosity in the sand layers is destroyed completely, and the deepest sand layer has textures that approach those of semischist. The graywacke layers contain considerable quantities of volcanic and sedimentary lithic debris, and may have originated on the continental shelf in the Komandorskiy Islands. Neogene accumulation rates in Komandorskiy basin were about 60 m/m.y.-1 in the late Miocene, increasing to 300 m/m.y.-1 in late Pliocene and early Pleistocene time. These high rates were maintained by extensive continental-shelf erosion during late Cenozoic sea-level transits, and by exceedingly high diatom productivity in the overlying waters of the Bering Sea. A slight decrease in accumulation rates in the late Pleistocene may be the result of a decline in diatom productivity. Unlithified Neogene deposits of Komandorskiy basin are distal representatives of contemporaneous deposits in continental-margin basins and have some characteristics of potential reservoir rocks. Their more proximal counterparts at shallower-water depths deserve increased attention as possible hydrocarbon sources.