Sedimentology of cores from the Iberian margin


Autoria(s): Löwemark, Ludvig
Cobertura

MEDIAN LATITUDE: 37.192751 * MEDIAN LONGITUDE: -8.544351 * SOUTH-BOUND LATITUDE: 36.041667 * WEST-BOUND LONGITUDE: -10.680000 * NORTH-BOUND LATITUDE: 41.488300 * EAST-BOUND LONGITUDE: -7.071667 * DATE/TIME START: 1993-08-12T00:00:00 * DATE/TIME END: 1997-05-04T18:14:00

Data(s)

28/07/2016

Resumo

Bioturbation in marine sediments has basically two aspects of interest for palaeo-environmental studies. First, the traces left by the burrowing organisms reflect the prevailing environmental conditions at the seafloor and thus can be used to reconstruct the ecologic and palaeoceanographic situation. Traces have the advantage over other proxies of practically always being preserved in situ. Secondly, for high- resolution stratigraphy, bioturbation is a nuisance due to the stirring and mixing processes that destroy the stratigraphic record. In order to evaluate the applicability of biogenic traces as palaeoenvironmental indicators, a number of gravity cores from the Portuguese continental slope, covering the period from the last glacial to the present were investigated through X-ray radiographs. In addition, physical and chemical parameters were determined to define the environmental niche in each core interval. A number of traces could be recognized, the most important being: Thalassinoides, Planolites, Zoophycos, Chondrites, Scolicia, Palaeophycus, Phycosiphon and the generally pyritized traces Trichichnus and Mycellia. The shifts between the different ichnofabrics agree strikingly well with the variations in ocean circulation caused by the changing climate. On the upper and middle slope, variations in current intensity and oxygenation of the Mediterranean Outflow Water were responsible for shifts in the ichnofabric. Larger traces such as Planolites and Thalassinoides dominated in coarse, well oxygenated intervals, while small traces such as Chondrites and Trichichnus dominated in fine grained, poorly oxygenated intervals. In contrast, on the lower slope where calm steady sedimentation conditions prevail, changes in sedimentation rate and nutrient flux have controlled variations in the distribution of larger traces such as Planolites, Thalassinoides, and Palaeophycus. Additionally, distinct layers of abundant Chondrites correspond to Heinrich events 1, 2, and 4, and are interpreted as a response to incursions of nutrient rich, oxygen depleted Antarctic waters during phases of reduced thermohaline circulation. The results clearly show that not one single factor but a combination of several factors is necessary to explain the changes in ichnofabric. Furthermore, large variations in the extent and type of bioturbation and tiering between different settings clearly show that a more detailed knowledge of the factors governing bioturbation is necessary if we shall fully comprehend how proxy records are disturbed. A first attempt to automatize a part of the recognition and quantification of the ichnofabric was performed using the DIAna image analysis program on digitized X-ray radiographs. The results show that enhanced abundance of pyritized microburrows appears to be coupled to organic rich sediments deposited under dysoxic conditions. Coarse grained sediments inhibit the formation of pyritized burrows. However, the smallest changes in program settings controlling the grey scale threshold and the sensitivity resulted in large shifts in the number of detected burrows. Therefore, this method can only be considered to be semi-quantitative. Through AMS-^C dating of sample pairs from the Zoophycos spreiten and the surrounding host sediment, age reversals of up to 3,320 years could be demonstrated for the first time. The spreiten material is always several thousands of years younger than the surrounding host sediment. Together with detailed X-ray radiograph studies this shows that the trace maker collects the material on the seafloor, and then transports it downwards up to more than one meter in to the underlying sediment where it is deposited in distinct structures termed spreiten. This clearly shows that age reversals of several thousands of years can be expected whenever Zoophycos is unknowingly sampled. These results also render the hitherto proposed ethological models proposed for Zoophycos as largely implausible. Therefore, a combination of detritus feeding, short time caching, and hibernation possibly combined also with gardening, is suggested here as an explanation for this complicated burrow.

Formato

application/zip, 23 datasets

Identificador

https://doi.pangaea.de/10.1594/PANGAEA.863490

doi:10.1594/PANGAEA.863490

Idioma(s)

en

Publicador

PANGAEA

Direitos

CC-BY: Creative Commons Attribution 3.0 Unported

Access constraints: unrestricted

Fonte

Supplement to: Löwemark, Ludvig (2001): Biogenic traces as palaeoceanographic indicators in Late Quaternary sediments from the SW Iberian margin. Berichte-Reports, Institut für Geowissenschaften, Universität Kiel, 14, 138 pp, doi:10.2312/reports-ifg.2001.14

Palavras-Chave #>63 µm; Acc rate TOC; Accumulation rate per year, organic carbon; Age, 14C calibrated; Age, 14C milieu/reservoir corrected (-400 yr); Age, comment; Age, dated; Age, dated material; Age, dated standard error; Age dated; Age std e; Analytical method; automatically detected; Burrow fragments, pyritized; Burrow pyrit; CaCO3; Calcium carbonate; Carbon, organic, total; Comm; Counting; Counting >250 µm fraction; Dated material; Depth; DEPTH, sediment/rock; Digital imaging; G. bulloides d18O; GIK/IfG; Globigerina bulloides, d18O; Gravity corer (Kiel type); Gravity corer (Russian type); h = horizointal, v = vertical, + = vertical and horizontal, 0 = random; Ice rafted debris; Institute for Geosciences, Christian Albrechts University, Kiel; IRD; ka B.P.; ka B.P.; most probable intercept, rounded to nearest hundred; Lab no; M39/1; M39/1_08-3; M39/1_16-3; M39/1_22-4; M39/1_29-4; M39/1_29-7; M39/1_29-8; M39/1_36-2; M39/1_58-2; M39008-3; M39016-3; M39022-4; M39029-4; M39029-7; M39029-8; M39036-2; M39058-2; Mass spectrometer Finnigan MAT 251; Meteor (1986); Method; Orientation; PO200-10-21-1; PO200-10-28-2; POS200/10; POS200/10_21-1; POS200/10_28-2; Poseidon; Reference; Reference/source; RGC; Sample, optional label/labor no; Sedimentation rate; Sed rate; Size fraction > 0.063 mm, sand; size fraction >250 µm; size fraction 150-250 µm; SL; TOC; Water content of wet mass; Water wm
Tipo

Dataset