Previous   |    Next

doi:10.2204/iodp.proc.347.104.2015

Biostratigraphy

Diatoms

With the exception of one sample containing chrysophycean cysts, only diatoms were found at Site M0060 (Hole M0060A only), and they were identified to species level. The sampling resolution consisted of one sample at every core top for all recovered intervals. However, some intervals were also sampled at every section top. In order to provide an initial paleoecological framework for Site M0060, diatoms were classified with respect to salinity tolerance. Salinity tolerance classification follows the Baltic Sea intercalibration guides of Snoeijs et al. (1993–1998), which divide taxa into five groups: marine, brackish-marine, brackish, brackish-freshwater, and freshwater.

Overall, diatoms occur in very low abundances. For this reason, only two transects of each slide were viewed for samples that did not contain numerous whole valves. The presence of diatom fragments and whole valves is shown in Figure F4. All analyzed samples were barren to 7.5 mbsf. Diatoms were present at low abundances from 7.5 to 10.5 mbsf. A low-diversity freshwater assemblage comprising diatoms of the genera Aulacoseira, Cocconeis, and Martyana is present at 7.5 mbsf. Only a single valve of the brackish water taxa Martyana schulzii was recorded. At 9 and 10.5 mbsf, the assemblage is mainly composed of freshwater taxa from the genera Amphora, Aulacoseira, and Stephanodiscus. Brackish and marine taxa are also present but are fragmentary. This assemblage could be interpreted as freshwater with allochthonous brackish and marine diatom fragments. Alternatively, the assemblage may be brackish with robust freshwater diatoms washed in.

Between 10.5 and 132.9 mbsf, most of the samples are barren of whole and fragmentary diatom fossils. The total number of whole valves found in two slide transects for any sample from this interval did not exceed four. In the interval from 132.9 to 202.5 mbsf, all slides contain rare diatom fragments. Table T2 contains the diatom species, authorities, and habitat information for Site M0060, as well as presence/absence data for the interval between 7.5 and 10.5 mbsf. All Quaternary diatoms found in samples deeper than 10.5 mbsf co-occurred with Cretaceous coccolithophorids and rare fragments of Cretaceous diatoms from the genus Stephanopyxis (Table T3). These diatoms are therefore interpreted as redeposited.

Foraminifers

Results presented here include primarily those from samples taken from split-core sections during the Onshore Science Party (OSP) and supplementary information from core catcher samples studied offshore. A total of 152 samples (90 obtained offshore from core catchers and 62 onshore from split-core sections) were studied for foraminiferal biostratigraphy. Redeposited pre-Quaternary foraminifers occur intermittently throughout the site, indicated by a white, polished, or frosty surface and carbonate infilling (Rasmussen et al., 2005). Intervals with poor core recovery are not discussed in detail.

The shallowest sample (347-M0060A-3H-1, 11–13 cm; 0.13 mbsf) is dominated by Ammonia beccarii, Bulimina marginata, and Cibicides lobatulus (Table T4), which suggests a Holocene boreal marine assemblage (Seidenkrantz and Knudsen, 1993). The remaining upper part (upper 21 mbsf) of Site M0060 contains generally few benthic foraminifers (Fig. F5A), although taxonomic diversity is relatively high (4–13 species; Fig. F5C; Table T4). In this upper interval, the genus Elphidium is the most common benthic foraminifer with Elphidium excavatum clavatum generally composing the largest fraction of the assemblage (Fig. F5B) and B. marginata, Cassidulina reniforme (formerly crassa), and Elphidium williamsoni often present as accessories. Redeposited pre-Quaternary foraminifers occur occasionally. Taxonomic composition of this assemblage in combination with the presence of dropstones in this interval (see “Lithostratigraphy”) suggest cool-water marine conditions (Seidenkrantz, 1993b; Seidenkrantz and Knudsen, 1993). The presence of E. williamsoni may suggest shallow or intertidal waters (Knudsen et al., 2012) or transport of foraminifers from a nearby environment of this type.

Between 21 and 76 mbsf, the assemblage is of low diversity, and abundance varies from abundant to barren (Fig. F5A). E. excavatum clavatum and C. reniforme are often the only species present, with E. excavatum clavatum nearly always dominant. Redeposited foraminifers are rarely present. The dominance of E. excavatum clavatum and very low diversity suggest brackish and cold conditions (Seidenkrantz, 1993a; Seidenkrantz and Knudsen, 1993).

Between 76 and 99 mbsf, foraminiferal diversity increases (4–13 species; Fig. F5C) and foraminiferal abundance ranges from few to common. E. excavatum clavatum still typically composes the largest fraction of the assemblage (Fig. F5B), with Elphidium excavatum selseyensis, Elphidium spp., Haynesina spp., B. marginata, Islandiella helenae, C. reniforme, and Hyalinea balthica occurring frequently in low to moderate abundances. Redeposited pre-Quaternary foraminifers occur in more than half of the samples in this interval. The combination of subarctic (e.g., I. helenae) and boreal (e.g., H. balthica) species in this interval is an unrealistic combination, suggesting that some foraminifers are redeposited (Seidenkrantz and Knudsen, 1993). The characterization of this unit as slumped sands (see “Lithostratigraphy”) supports the interpretation that the sediments in this interval are indeed likely redeposited, although it is possible that occasional short periods of deposition in a marginal marine environment did occur.

Between 99 and 117 mbsf, sediment recovery was poor, preventing reliable environmental characterization.

Between 117 and 146 mbsf, foraminifers are very rare or absent, though a small number of Elphidium spp. and I. helenae specimens are present, as well as some pre-Quaternary redeposited specimens. Redeposition is additionally supported by the generally sandy grain size of the sediments and the occurrence of macroscopic shell fragments, which have been interpreted as derived from a deltaic environment (see “Lithostratigraphy”), although infrequent periods of sediment deposition in a cold and brackish environment could have occurred.

At 153.7–155.4 mbsf (Cores 347-M0060A-60H through 61H), foraminifers are abundant and the assemblage is among the most diverse found at Site M0060 (Fig. F5C). This interval contains few or no pre-Quaternary redeposited foraminifers and is dominated by H. balthica and B. marginata, with Uvigerina mediterranea, I. helenae (this taxon may possibly include Cassidulina laevigata), and Melonis barleeanus also occurring frequently. The fauna of this brief interval represents a typical marine boreal to lusitanian (i.e., warmer) assemblage (Seidenkrantz, 1993a; Seidenkrantz and Knudsen, 1993) and occurs in a thin, fine-grained, organic-rich, and weakly laminated subunit surrounded by diamicton; the surrounding diamicton may represent a delta slope environment in which periodic marine clay deposition occurred (see “Lithostratigraphy”). The thickness of this fine-grained unit is uncertain because of poor recovery and disturbed sediments above and below it. This interval may represent a short interstadial period or a longer warm period that has been partially eroded or poorly recovered at this site.

Between 160 and 190 mbsf, foraminifers are rare to nearly absent, but the assemblage is taxonomically diverse, with Elphidium tumidum (formerly Elphidium groenlandicum), Elphidium spp., and I. helenae dominant and pre-Quaternary redeposited foraminifers intermittently present. The foraminifers in this interval, which has been characterized as debris flow material in a deltaic environment including charcoal potentially of Jurassic age (see “Lithostratigraphy”), are likely redeposited.

Between 194 and 205 mbsf (Cores 347-M0060A-76H through 82P), foraminiferal abundance increases to generally common (ranging from very rare to abundant; Table T4) and the assemblage is among the most diverse at this site (Fig. F5C), although redeposited pre-Quaternary foraminifers occur. This interval is dominated by E. excavatum clavatum and B. marginata, but H. balthica, Nonionella labradorica, I. helenae (C. laevigata?), and Elphidium spp. also occur frequently. These species represent a marine boreal to lusitanian (i.e., warmer) assemblage and potentially greater water depths (Seidenkrantz, 1993a). Recovery in this interval is limited, and sediments are disturbed in some intervals. Core 347-M0060A-76H is a diamicton section and thus likely contains recently redeposited foraminifers, but the core below (347-M0060A-82P) is laminated silty sand and appears to contain in situ foraminifers.

Between 212 and 230 mbsf, foraminifers are rare to nearly absent and taxonomic diversity is low, with E. tumidum and other Elphidium spp. primarily occurring, as well as some redeposited pre-Quaternary specimens.

Ostracods

Ostracods were examined from 147 samples (including 90 core catchers) from Holes M0060A and M0060B during the OSP for Expedition 347. Samples were studied in the >125 µm fraction. Ostracods were present in 40 samples (Table T5).

Ostracod abundance per sediment volume from both holes is plotted in Figure F6. Ostracods mainly occur in the upper 20 mbsf (Holes M0060A and M0060B). Scattered occurrences of single valves are also observed over the entire record (Table T5). Ostracod abundance is the highest in the interval of 6–20 mbsf, ranging from 70 to 130 valves/20 cm³ sample. A high juvenile to adult ratio and good preservation in this interval allows us to assume in situ burial (Fig. F6). The interval of highest ostracod abundance corresponds to the lithostratigraphic unit of sandy clayey silt and fine–medium sand (see “Lithostratigraphy” and “Geochemistry”).

Ostracods of three different ecological groups occur at this site, and their abundance variations suggest that salinity and water depth decreased uphole in the studied interval. The ecological groups distinguished are freshwater, shallow-water marine, and North Atlantic taxa.

The samples from 6.08 mbsf (Hole M0060A) and 6.66 mbsf (Hole M0060B) contain a shallow-water brackish-marine assemblage. The most abundant are Sarsicytheridea punctillata, Acanthocythereis dunelmensis, Leptocythere spp., Elofsonella concinna, and Heterocyprideis sorbyana. These are typical shallow-water species found on Arctic and North Atlantic shelves (Cronin, 1981; Cronin et al., 2010; Stepanova et al., 2007). These samples also include the freshwater taxon Ilyocypris sp. The valves are well preserved and include juveniles. This assemblage may indicate that the studied site was located close to the coastline and the freshwater specimens were transported from the continent.

In the interval 10.66–17.27 mbsf (Hole M0060A), a similar shallow-water assemblage comprising the shallow-water marine and brackish water taxa is distinguished.

In the samples containing in situ ostracods at 20.56 mbsf (Hole M0060A) and 15.92 mbsf (Hole M0060B), the assemblage is dominated by marine taxa typical of North Atlantic waters and high salinity ≥26–30 (Cronin et al., 1995; Frenzel et al., 2010), such as Cytheropteron pseudomontrosiense, Cytheropteron arcuatum, Cytheropteron biconvexa, and Polycope spp.

Palynological results

For Site M0060, palynological analyses focused on Hole M0060A. One sample per 1–2 cores was examined for palynomorphs. In most samples, concentrations of palynomorphs are extremely low and show indications of severe degradation/oxidation. Additionally, most samples from Hole M0060A contain reworked tertiary palynomorphs, sometimes in higher concentrations than the presumably autochthonous/in situ palynomorphs encountered. Only a few samples contained enough palynomorphs in situ to yield statistically relevant results. We therefore do not show a detailed pollen diagram for this site but give a general overview on palynological findings (main pollen types, marine/terrestrial ratio, and pollen concentration; Fig. F7; Table T6; see PalyM0060.xls in PALYNOLOGY in “Supplementary material”). Bisaccate pollen was included in the reference sum used for calculations, but for some samples, this pollen type may be overrepresented because of transportation bias and its particular resistibility to oxidation (Cheddadi and Rossignol-Strick, 1995). Dinocyst content varied depending on the type of sediment. Sediments with marine components (e.g., foraminifers) contained significant numbers of dinoflagellate cysts.

From Hole M0060A, 25 sediment samples were analyzed. With the exception of the uppermost sample (0.75 mbsf), all analyzed samples indicate a relatively high degree of oxidation due to the rarity/absence of taxa particularly susceptible to oxidation-based degradation and the higher percentages of robust pollen types and spores.

0.75 mbsf

This sample is characterized by high pollen concentration (resulting in a relatively high counting sum), whereas freshwater algae and reworked pollen occur in low amounts or are absent (Fig. F7). Pollen types that are generally more susceptible to oxidation are present (e.g., Quercus; Cheddadi and Rossignol-Strick, 1995). Thus, this sample may better reflect the original ecosystems than samples from greater depths. Quercus is the dominating nonsaccate pollen type in this sample; Betula and Alnus are also frequent. These findings, in combination with the presence of herbal pollen (e.g., Asteraceae), the absence of Fagus and Tilia pollen, and low Corylus and Ulmus pollen percentages, indicate an earlier Holocene age for this sample. It is also the only sample containing enough organic-walled dinoflagellate cysts (dinocysts) to make statistically relevant inferences about the dinocyst assemblages. Operculodinium centrocarpum/Protoceratium reticulatum is the dominating cyst type (61%). Spiniferites sp. cysts are also frequent (26%). Lingulodinium machaerophorum is present but only in low percentages (4%). Relatively long spines of the Lingulodinium specimens encountered, compared to samples from Site M0059, may indicate either cold or relatively saline conditions (Mertens et al., 2009); however, the number of Lingulodinium specimens measured is too low to yield robust results. The sample at 0.75 mbsf is the only one in which the genus Ataxiodinium was encountered.

15.69–40.60 mbsf

This interval is characterized by relatively lower percentages of bisaccate (particularly Pinus) pollen grains (Fig. F7). High percentages of freshwater algae and a high reworked/in situ pollen ratio may indicate that during the time interval reflected by these samples, tertiary material was transported to the site by freshwater inflow from land. The lower bisaccate pollen percentages compared to the interval immediately deeper may thus be a transport signal indicating that fluvial transport played a more important role for this interval than eolian transport.

48.68–202.67 mbsf

This interval is characterized by high percentages of bisaccate (particularly Pinus) pollen grains (Fig. F7), lower percentages of freshwater algae, and a lower reworked/in situ pollen ratio compared to the interval between 15.69 and 40.60 mbsf. The amount of reworked pollen is still almost as high as, and in some cases even higher than, the amount of in situ pollen. Airborne pollen transport may have been of more importance for the sediments in this interval than for the overlying sediments because bisaccate pollen (particularly well suited for airborne transport) is very common.

Top of page   |    Previous   |    Next