Increase In Sediment Accumulation Rates Research Articles

Strontium isotope stratigraphy and geochemistry of the late Neogene ocean

A curve describing the variation of the strontium isotopic composition of seawater for the late Neogene (9 to 2 Ma) was constructed from 87Sr/ 86Sr analyses of marine carbonate in five Deep Sea Drilling Project (DSDP) sites: 502, 519, 588, 590, and 593. The strontium isotopic composition of the oceans increased between 9 and 2 Ma with several changes in slope. From 9 to 5.5 Ma, 87Sr/ 86Sr values were nearly constant at ∼ 0.708925. Between 5.5 and 4.5 Ma, 87Sr/ 86Sr ratios increased monotonically at a rate of ∼ 1 × 10 −4 per million years. The steep slope during this interval provides the potential for high resolution strontium isotope stratigraphy across the Miocene/Pliocene boundary. The rate of change of 87Sr/ 86Sr decreases to near zero again during the interval 4.5–2.5 Ma, and ratios average 0.709025. The relatively rapid increase of 87Sr/ 86Sr between 5.5 and 4.5 Ma must be related to changes in the flux or average 87Sr/ 86Sr ratios of the major inputs of Sr to the oceans. Quantitative modelling of these inputs suggests that the increase was most probably caused by an increase in the dissolved riverine flux of strontium to the oceans, an increase in the average 87Sr/ 86Sr composition of river water, or some combination of these parameters. Modelling of this period as a transient-state requires a pulse-like increase in the input of 87Sr to the oceans between 5.5 and 4.5 Ma. Alternatively, the 5.5–4.5 Ma period can be modelled as a simple transition from one steady-state to another if the oceanic residence time of strontium was eight times less than the currently accepted value of 4 Ma. During the time interval of steep 87Sr/ 86Sr increase, other geochemical and sedimentologic changes also occur including an increase in sediment accumulation rates, a drop in the calcite compensation depth (CCD), and a decrease in the δ 13C of dissolved bicarbonate (i.e., “carbon shift”). The simplest mechanism to explain 87Sr/ 86Sr variation and these related geochemical changes is to invoke an increase in the dissolved chemical fluxes carried by rivers to the oceans. This, in turn, implies increased chemical denudation rates of the continents and shelves during the late Neogene. The increase in chemical weathering rates is attributed to increased exposure of the continents by eustatic regression, intensified glacial/interglacial cycles, and accelerated rates of global tectonism beginning at 5.5 Ma during the latest Miocene.

Shallowing-Upwards Sedimentation in a Coral Reef Lagoon, Great Barrier Reef of Australia

ABSTRACT Sediments in the 5-25 m deep lagoon of Davies Reef, a typical mid-shelf reef in the Central Great Barrier Reef, are described, based on data gleaned from 19 soft sediment cores (75 mm diameter, up to 7.5 m penetration), 22 surface sediment samples and air lift surveys. The Holocene sediment pile, which averages 5-10 m thickness, is entirely skeletal in origin and has a very poorly sorted, coral gravel-rich base which grades up into poorly sorted muddy sands with infaunal molluscan gravel. These muddy sands (mainly derived from the reef rim as opposed to being produced in situ on the lagoon floor) have been intensely bioturbated by callianassid shrimps, leaving the infaunal molluscan gravel patchily distributed and the bivalves disarticulated and preferentially oriented sub-hori ontally, concave-up. There are no vestiges of primary depositional sedimentary structures such as bedding or cross-bedding. Radiocarbon dating suggests that significant accumulation of muddy-sand in the center of the lagoon did not begin until about 2,500-3,000 yr BP, and that sediment accumulation rates have increased through time from an average of 1.4 mm/yr between 2,400 and 640 yr BP, to 3.4 mm/yr over the past 640 years. Interpreting these data, the coral gravel-rich base to the sediment pile represents a period of prolific coral growth across the top of Davies Reef Platform following flooding by the Holocene transgression but before the outer reef rim caught up with sea level to form a lagoon. When the rim did reach sea level (about 3,000-4,000 yr BP?), it started to shed large qua tities of mud and sand grade sediment into the center of the reef platform and this, combined with decreased water circulation, led to the demise of coral growth in the newly formed lagoon. As the area of reef rim at sea level increased, so too did the supply of sediment to the lagoon, which explains the observed increase in sediment accumulation rate through time. Using previously published data to aid speculation into the future development of the lagoon, a shallowing-upwards model for reef lagoon sedimentation is developed. In this model, a complete sequence consists of a fining-upwards base (reflecting the gradation from coral gravel up into bioturbated muddy sands) and a coarsening-upwards top (reflecting a gradation from bioturbated muddy sands up into a zone of shallow subtidal co al rubble and patch reefs, then inter- and supratidal windward storm shingle ramparts and leeward sand cays).

Historical Relationships between Phosphorus Loading and Biogenic Silica Accumulation in Bay of Quinte Sediments

Sediments from Warwick's 1972 Glenora-B core from the Bay of Quinte, Lake Ontario, were analyzed to compare the historical relationship between the accumulation of biogenic silica (BSi) and total phosphorus (TP). The similarities in patterns provide evidence that increased phosphorus (P) inputs caused increased diatom production and BSi accumulation. BSi accumulation increased soon after initial European settlement by French Sulpicians in 1669 and reached maximum levels during the early 1850s when forest clearance and erosion of the deforested drainage basin increased sediment accumulation rates 110-fold compared with rates before 1669. Maximum rates of BSi and TP accumulation increased 170-fold and 150-fold, respectively, during the same period. Ratios of BSi:TP were about sixfold greater in the sediments deposited after 1888 than in those deposited prior to 1669, indicating that the proportion of available P in TP inputs increased with increased disposal of domestic sewage into the bay as populations shifted to urban centers. Increases in BSi accumulation above the Ambrosia horizon (dated at 1852) indicate that sediments can be a significant sink for BSi. Although the onset of Si depletion cannot be confirmed with data from this core, there is clear evidence that BSi accumulation increased as the result of increased P inputs.

Limnological effects of peatland drainage and fertilization as reflected in the varved sediment of a deep lake

Stratigraphy of diatoms and chemistry in the surface sediment deposited at 35 m depth in Lake Polvijarvi was studied. The existence of annual laminations or varves in the sediment allowed a precise dating of the profile. Diatoms were analysed in 0.5 cm sequences; from 0 to 16.0 cm continuously and then intermittently every fourth 0.5 cm down to 44.0 cm. Sediment chemistry (loss-on-ignition, C, N, Fe, Mn, Mg, P, chlorophyll and carotenoids) was analysed from sediment surface down to 10.5 cm of altogether 33 subsamples, each containing 1–3 varves, and spanning the period 1921–1980. From 4.5 cm depth upwards the diatom concentration strongly increases, and the plankton diatom succession from Tabellaria flocculosa through Asterionella formosa to Melosira ambigua and Fragilaria crotonensis reflects a marked eutrophication of the lake. This algal succession occurs in pace with an increase in sediment accumulation rate and changes in sediment chemistry, which indicate increased allochthonous inputs and enhanced algal production in the lake. The change of the lake ecosystem is contemporaneous with extensive peatland draining and fertilizing that was carried out on its watershed during the past two decades. Existing chemical data from a number of lakes situated within the drainage area prove that at present the treated peatlands are the main source of nutrient loading of Lake Polvijarvi. A former period with indications of slightly increased productivity of the lake was dated by varve counting to AD 1690–1910 (35–12 cm). This period (characterised by Asterionella formosa) may coincide with that of the slash-and-burn cultivation in the area.

Observations on the recent history of Lough Neagh and its drainage basin

The present study traces the recent development of Lough Neagh from the microfossil record of the lake sediment. The history of vegetational change in the catchment area is discussed on the basis of pollen analytical information and by reference to primary and secondary documentary historical sources. Changes in the history of the lake itself are inferred from changes in the relative composition of diatom assemblages in the sediment and from changes in the calculated influx of diatoms to the sediment. The accumulation of sediment in the lake basin is discussed and radiometric data (14C,137Cs,210Pb) are used to construct a sediment chronology from which accumulation rates are calculated. It is shown that the rate has increased from about 0.08 cm a-1 (where a is the symbol for year) in A.D. 1700 to about 0.8 cm a-1today. Four major stages in the recent development of the lake are indicated: a pre-disturbance stage ( > 2000 B.C. toca. a.d. 1700), a stage of accelerated mineral inwash (fromca. A.D. 1700); and two stages of cultural eutrophication (fromca. 1915 and fromca. 1960 respectively). The eutrophication stages are characterized by increased sediment accumulation rates, increased diatom influx rates, and the emergence of Stephanodiscus as the most important diatom genus.