Pelagic Processes Research Articles

The Annual Cycle of Kiel Bight Plankton: A Long-Term Analysis

Over the past decade, the annual cycle of the major pelagic processes in relation to environmental factors and species composition of the plankton has been studied intensively at a fixed station in Kiel Bight. A series of sequential phases, differentiated according to characteristic properties, succeed each other in a recurring pattern each year. The following phases have been differentiated: the spring diatom bloom, the late spring copepod maximum, the summer stratification, the fall blooms and the winter dormancy. Each phase represents a particular pattern of biogenous element cycling, both within the pelagic system and between the pelagic and benthic systems. Each phase is also characterized by a spectrum of dominant species, many of which do not recur each year. Greatest variation is found amongst bloom diatoms, whereas large, slow-growing species such as the Ceratia and most metazooplankton are highly recurrent. Variation in species composition is not related to long-term trends since the past century, in spite of the considerable increase in anthropogenic nutrient input to the Bight. Short-term events appear to determine occurrence of fast-growing species, many of which have benthic resting stages in their life histories. It is concluded that more attention should be paid to life history strategies of species if the mechanisms of seasonal succession are to be elucidated. Long-term observations on appearance or absence of the various species in relation to environmental properties can provide clues as to the nature of these life history strategies.

Depositional model for calcilutites: Scaglia Rossa limestones, Umbro-Marchean Apennines

Summary The calcilutites of the mid-Cretaceous to uppermost Eocene Scaglia Rossa Formation in Italy have previously been interpreted as a continuous sequence of slowly deposited pelagic limestones. However, regional considerations of thickness, sedimentation rates and facies distribution, together with detailed analysis of composition, texture and sedimentary structures suggest that turbidity currents have played an important part in the deposition of, at least, the middle (Maastrichtian) member. Turbiditic calcilutites are very fine-grained and massive, whereas the pelagites are coarser-grained, mottled and bioturbated. There is, however, a continuous gradation of facies and processes between these two end-members, that we believe is applicable to many other modern and ancient deep-water carbonate systems. The common overlap of turbiditic and pelagic processes in fine-grained ‘pure’ carbonates arises partly from their relative lack of flocculation compared with muddier sediments.

Sources of Periplatform Carbonates: Northwest Providence Channel, Bahamas

ABSTRACT Sources and quantities of periplatform carbonate sediments have been determined in Northwest Providence Channel, Bahamas, by an end-member method employing geochemical, textural, petrographic, and SEM data. The Holocene sediment over most of this open seaway, 200 to 2,000 m deep, between Great Bahama Bank and Little Bahama Bank, is a drape of largely fine grained lime mud 50 cm thick. Shallow platform sources contribute 75-90 percent; the remainder is planktonic foraminifera, pteropods, and coccoliths. Mud (< 62 µm) from each bank and the planktonic source are each geochemically distinct and can be quantified. The two shallow platform sources dominate sedimentation in the deep basin. Trend-surface maps of mud derived from each bank reveal no windward-leeward asymmetry in cont ast to reported sand-transport patterns. This indicates that the fines are swept off both platforms by storm and tidal exchange. Concentric distribution of fines relative to source indicates no offset by currents, suggesting that fecal pelleting must rapidly remove fine sediment from the water column. Canyon valleys and intercanyon highs have equal thicknesses of Holocene sediment, which indicates that pelagic processes of deposition presently dominate over gravity-flow processes. Confirmation of this depositional process is that the deposition rate increased from 2 cm/1,000 years to 10 cm/1,000 years when postglacial sea level flooded the bank tops. Thus 80 percent of the present sedimentation rate results from bank-top contribution. Furthermore, the calculated mass of mud traceable to Little Bahama Bank agrees excellently with the independent estimate of overproduction there. In cores, bank sediments resulting from the most recent sea-level highstand sharply overlie planktonic sediments, writing a sea-level history in alternating stable and unstable mineral suites.

Sedimentation and Accumulation of organic carbon in the Angola Basin and on Walvis Ridge: Preliminary results of Deep Sea Drilling Project Leg 75

Leg 75 investigated two periods of accumulation of sediments rich in organic matter. In the Upper Cretaceous section, more than 200 beds of gray or black shale and mudstone rich in organic carbon (as much as 18%) are intercalated within a sequence of red and green mudstones that contain less than 1% organic carbon. The black shale beds are 1 to 62 cm thick and constitute less than 10% of the section in the 20 cores in which they were recovered. Both turbiditic and pelagic processes were involved in the deposition of these black shale beds. A combination of factors, including increased productivity and warm saline bottom waters, probably resulted in a delicate balance between oxic and anoxic conditions that periodically favored the accumulation and preservation of organic matter in the basinal sediments. The Pliocene-Pleistocene sediments are thoroughly bioturbated, and there is no evidence of persisting anoxia in the overlying water in spite of the high organic carbon content of the sediments. These two different periods of accumulation of organic matter both resulted in significant concentration of organic carbon (>2%, maximum 18%), but the Pliocene-Pleistocene sediments contain the greater mass of organic carbon per unit volume of the solid sediment.

Morphology and Sediments of Three Ocean Basin Entry Points, Hatteras Abyssal Plain

ABSTRACT Three canyon systems which enter into the northern one third of the Hatteras Abyssal Plain are the principal sources of Late Pleistocene and Holocene sediment for the entire abyssal plain. The Hatteras Canyon System enters as a point source via a small fan built directly on the ocean basin floor. The Wilmington Canyon mouth is somewhat wider and disgorges its load from a lower continental rise fan. The Hudson Canyon empties across the 110-km wide Sohm Gap, a feature which is on the true ocean basin floor. Because sedimentary processes are different, each basin entry point has produced unique topographic features, as well as unique sediment distribution patterns and sedimentary structures. The relative importance of turbidity current, pelagic and contour current processes, is variable rom system to system. Southward flowing contour currents contribute very little, if any, sediment to the Hatteras Fan, but are an important sediment source for fine materials on the seafloor at the mouth of the Wilmington Canyon and Sohm Gap. Provenance is also responsible for differences in the sediments in each canyon system. As an example, the Hatteras Canyon material is much more calcareous than material from the other two basin entry points, because of the relatively calcareous nature of the continental shelf source area near Cape Hatteras.

WENLOCK AND LUDLOW SEDIMENTATION IN THE AUSTWICK AND HORTON-IN-RIBBLESDALE INLIER, NORTH-WEST YORKSHIRE

Summary The Wenlock and Ludlow rocks of the Austwick and Horton-in-Ribblesdale Inlier are the result of turbidite and pelagic sedimentation in the Silurian geosyncline. The Austwick Formation consists of units of several fine-grained greywacke turbidites alternating with units of mudstones. The turbidites exhibit typical Bouma sequences. However, whilst possessing many distal features they have a high proximality index based on Bouma divisions. Palaeo-current data indicate flow towards the north-west for the turbidity currents, whilst the turbidites actually thin north-eastwards. The mudstones show evidence of reworking by bottom currents. Bioturbation, although uncommon in this formation, is persistent throughout the area in one bed. The Arcow Formation consists of distal calcareous turbidites with a highly bioturbated pelitic division. Current ripple lamination indicates a palaeo-current from the north-west, and the beds thin and die out towards the west. Siltstones and mudstones deposited by normal pelagic processes, by bottom currents and by extremely weak turbidity currents make up the Horton Formation. The turbidites of the Studfold Formation were deposited by currents from the north-west. They are poorly exposed but are very similar to those of the Austwick Formation.