Geochemistry of labile organic matter in sediments and interstitial waters of ODP Sites 107-651 and 107-653

Sediment and interstitial water from Sites 651 and 653 (ODP Leg 107) were investigated by organic geochemical methods to characterize labile organic compound classes (amino compounds and carbohydrates) and to evaluate their progressive diagenetic and thermal degradation in deep-sea sediments. Downhole distribution of dissolved organic carbon (DOC) appears related to redox zones associated with bacterial activity and of diagenetic recrystallization of biogenic tests and not so much to organic matter concentrations in ambient sediments. DOC ranges from 250 to 8300 µmol/L (3-100.1 ppm). Amino acids contribute 10%-0.3% of DOC; carbohydrates range from 78 to 5 µmol/L. Rate of degradation of amino acids by thermal effects and/or bacterial activity at both sites (significantly different in sedimentation rates: average 41 cm/1000 yr in the top 300 m at Site 651, average 3.9 cm/1000 yr in the Pliocene/Quaternary sequence at Site 653 to 220 mbsf) is more dependent on exposure time rather than on the depth within the sediment column. Variability in neutral, acidic, and basic amino acid fractions of total amino acids (with a range of 1.1-0.02 µmol/g sediment; up to 2.5% of organic carbon) varies with carbonate content and by differences in thermal stability of amino acids. Distribution patterns of monosaccharides are interpreted to result from differences in organic matter sources, sedimentation rates, and the degree of organic matter decomposition prior to and subsequent to burial. Total particulate carbohydrates range from 1.82 to 0.21 µmol/g sediment and contribute about 8% to the sedimentary organic matter. Investigation of trace metals in the interstitial waters did not show any correlation of either DOC, amino compounds, or carbohydrates.

Palaeoenvironment records of six sediment cores collected in the Gulf of Carpentaria

The Gulf of Carpentaria is an epicontinental sea (maximum depth 70 m) between Australia and New Guinea, bordered to the east by Torres Strait (currently 12 m deep) and to the west by the Arafura Sill (53 m below present sea level). Throughout the Quaternary, during times of low sea-level, the Gulf was separated from the open waters of the Indian and Pacific Oceans, forming Lake Carpentaria, an isolation basin, perched above contemporaneous sea-level with outlet channels to the Arafura Sea. A preliminary interpretation is presented of the palaeoenvironments recorded in six sediment cores collected by the IMAGES program in the Gulf of Carpentaria. The longest core (approx. 15 m) spans the past 130 ka and includes a record of sea-level/lake-level changes, with particular complexity between 80 and 40 ka when sea-level repeatedly breached and withdrew from Gulf/Lake Carpentaria. Evidence from biotic remains (foraminifers, ostracods, pollen), sedimentology and geochemistry clearly identifies a final marine transgression at about 9.7 ka (radiocarbon years). Before this transgression, Lake Carpentaria was surrounded by grassland, was near full, and may have had a surface area approaching 600 km-300 km and a depth of about 15 m. The earlier rise in sea-level which accompanied the Marine Isotopic Stage 6/5 transgression at about 130 ka is constrained by sedimentological and biotic evidence and dated by optical- and thermoluminescence and amino acid racemisation methods.

(Table 2) Concentrations and D/L values for three amino acids in bulk foraminifera from ODP Sites 182-1126 and 182-1127

A number of studies have shown that methanogens are active in the presence of sulfate under some conditions. This phenomenon is especially exemplified in carbonate sediments of the southern Australian continental margin. Three sites cored during Ocean Drilling Program (ODP) Leg 182 in the Great Australian Bight have high concentrations of microbially-generated methane and hydrogen sulfide throughout almost 500 m of sediments. In these cores, the sulfate-reducing and methanogenic zones overlap completely; that is, the usual sulfate-methane transition zone is absent. Amino acid racemization data show that the gassy sediments consist of younger carbonates than the low-gas sites. High concentrations of the reduced gases also occur in two ODP sites on the margin of the Bahamas platform, both of which have similar sedimentary conditions to those of the high-gas sites of Leg 182. Co-generation of these reduced gases results from an unusual combination of conditions, including: (1) a thick Quaternary sequence of iron-poor carbonate sediments, (2) a sub-seafloor brine, and (3) moderate amounts of organic carbon. The probable explanation for the co-generation of hydrogen sulfide and methane in all these sites, as well as in other reported environments, is that methanogens are utilizing non-competitive substrates to produce methane within the sulfate-reducing zone. Taken together, these results form the basis of a new model for sulfate reduction and methanogenesis in marine sediments. The biogeochemical end-members of the model are: (1) minimal sulfate reduction, (2) complete sulfate reduction followed by methanogenesis, and (3) overlapping sulfate reduction and methanogenesis with no transition zone.

Seasonal variations in surface metabolite composition of Fucus vesiculosus and Fucus serratus sampled at outer Kiel Fjord (August 2012 - July 2013)

The chemical composition of surface associated metabolites of two Fucus species (Fucus vesiculosus and Fucus serratus) was analysed by means of gas chromatography-mass spectrometry (GC-MS) to describe temporal patterns in chemical surface composition. Method: The two perennial brown macroalgae F. vesiculosus and F. serratus were sampled monthly at Bülk, outer Kiel Fjord, Germany (54°27'21 N / 10°11'57 E) over an entire year (August 2012 - July 2013). Per month and species six non-fertile Fucus individuals were collected from mixed stands at a depth of 0.5 m under mid water level. For surface extraction approx. 50 g of the upper 5-10 cm apical thalli tips were cut off per species. The surface extraction of Fucus was performed according to the protocol of de Nys and co-workers (1998) with minor modifications (see Rickert et al. 2015). GC/EI-MS measurements were performed with a Waters GCT premier (Waters, Manchester, UK) coupled to an Agilent 6890N GC equipped with a DB-5 ms 30 m column (0.25 mm internal diameter, 0.25 mM film thickness, Agilent, USA). The inlet temperature was maintained at 250°C and samples were injected in split 10 mode. He carrier gas flow was adjusted to 1 ml min-1. Alkanes were used for referencing of retention times. For further details (GC-MS sample preparation and analysis) see the related publication (Rickert et al. submitted to PLOS ONE).

(Table 1) Concentrations of amino acids in sediments from DSDP Site 68-502

In this chapter, we will report on the amino acids in the total acid hydrolysate of eight sediment samples from Leg 68 Site 502. This site was located on a topographic high at a depth of 3051 meters in the Colombian Basin of the western Caribbean Sea. Four holes were cored at the site by means of the hydraulic piston corer to a maximum sediment depth of 218 meters. The composite section is a virtually continuous, undisturbed sediment record covering almost 8 million years from the Holocene to late Miocene. Age estimates for the section are based on excellent magnetostratigraphic and biostratigraphic records. Four lithostratigraphic units (A, B, C, and D) were recognized, based on differences in color and content of clay, ash, foraminifers, and siliceous microfossils (Prell, Gardner, et al., 1980): A, yellowish brown to light brownish gray foraminifer-bearing (> 10%) nannofossil marl; B, gray to olive gray foraminifer-bearing nannofossil marl with occasional ash beds; C, light gray to dark greenish gray calcareous clay and foraminifer-bearing (< 10%) nannofossil marl; D, pale green to grayish green calcareous, ash-bearing clay with siliceous microfossils. The calcium carbonate content of these sediments increases from about 27 to about 49% from late Miocene to middle Pliocene (about 3.6 Ma) and remains uniform at about 48 to 50% from that time throughout the Quaternary. The eight sediment samples for amino acid analyses came from the third (502B) and fourth (502C) holes at Site 502. Samples ranged in sub-bottom depth from 4.3 to 225 meters spanning time from 0.3 to 7.7 Ma.