Delta 66Zn ratios and age of carbonates from ODP Site 138-849 (Table 1)

The carbonate fraction of sediment core ODP 849, leg 138, located in the eastern equatorial Pacific, mostly consisting of coccoliths, was separated and analyzed for its Zn isotopic composition. The overall variation in Zn isotopic composition, as determined by multiple-collector, magnetic-sector, inductively coupled plasma mass spectrometry, was found to be on the order of 1? (expressed in delta66Zn, where deltaxZn=[(xZn/64Zn)sample/(xZn/64Zn)standard -1]*10**3 and x=66, 67 or 68) over the last 175 ka. The analytical precision was 0.04 per mil and the overall reproducibility was usually better than 0.07 per mil. The Zn isotopic composition signal exhibits several marked peaks and a high-frequency variability. A periodogram of the delta66Zn signal showed two periodicities of 35.2 and 21.2 ka. We suggest that the latter is caused by the precession of the Earth's axis of rotation. The periodogram exhibits a minimum at 41.1 ka, thus showing that the Zn isotopic composition is independent of the obliquity in the eastern equatorial Pacific. The range of delta66Zn values observed for the carbonate fraction of ODP 849 overlaps with the range observed for Fe-Mn nodules in the world's oceans, which suggests that seawater/carbonate Zn isotope fractionation is weak. We therefore assume that most of the Zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean. The ODP 849 delta66Zn record seems to follow the changes in the insolation cycles. Changes in the late summer/fall equatorial insolation modulate the intensity of the equatorial upwelling, hence the mixing between deep and surface waters. We propose that during decreased summer/fall equatorial insolation, when a steep thermocline can develop (El Niño-like conditions), the surface waters cannot be replenished by deep waters and become depleted in the lighter Zn isotopes by biological activity, thus resulting in the progressive increase of the delta66Zn values of the carbonate shells presumably in equilibrium with surface seawater.

Geochemistry and Hg isotopic composition of mid-Pleistocene sapropel of ODP Hole 161-974C

The concentrations of mercury (Hg) and other trace metals (Ni, Cu, Zn, Mo, Ba, Re, U) and the Hg isotopic composition were examined across a dramatic redox and productivity transition in a mid-Pleistocene Mediterranean Sea sapropel sequence. Characteristic trace metal enrichment in organic-rich layers was observed, with organic-rich sapropel layers ranging in Hg concentration from 314 to 488 ng/g (avg = 385), with an average enrichment in Hg by a factor of 5.9 compared to organic-poor background sediments, which range from 39 to 94 ng/g Hg (avg = 66). Comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers Hg to the seafloor. Near complete scavenging of Hg from the water column renders the sapropel Hg isotopic composition representative of mid-Pleistocene Mediterranean seawater. Sapropels have an average d202Hg value of -0.91 per mil ± 0.15 per mil (n = 5, 1 SD) and D199Hg value of 0.11 per mil ± 0.03 per mil (n = 5, 1 SD). Background sediments have an average d202Hg of -0.76 per mil ± 0.16 per mil (n = 5, 1 SD) and D199Hg of 0.05 per mil ± 0.01 per mil (n = 5, 1 SD), which is indistinguishable from the sapropel values. We suggest that the sapropel isotopic composition is most representative of the mid-Pleistocene Tyrrhenian Sea.

Nd isotope composition of fossil fish teeth of ODP Hole 119-738B

We report an optimized method for extracting neodymium (Nd) from fossil fish teeth with a single-stage column (125 µl stem volume; LN Resin, Eichrom Industries, Darien Illinois) for isotopic analysis by multi-collector inductively coupled mass spectrometry (MC-ICMPS). Three reference materials (basalt: BCR-2, BHVO-2; phosphate: fossil bone composite) and splits of fossil fish teeth samples previously processed with existing two-stage column methods were processed using the single-stage column method. 143Nd/144Nd values of reference materials agree within error with published values, and the values for fish teeth correspond with sample splits processed with two-stage columns. Precision to ± ~0.23 epsilon-Nd was achieved for 30 ng Nd samples of reference materials, and Nd isotope measurements of fossil fish tooth sample replicates as small as 7 ng Nd were reproducible within long term instrumental uncertainty. We demonstrate the utility of the new method with the first high resolution Nd isotope record spanning the ~40.0 Ma middle Eocene Climatic Optimum, which shows an excursion of 0.65 epsilon-Nd during the peak warming at the study site (Ocean Drilling Program Leg 119, Site 738; 30 kyr sample spacing from 40.3 to 39.6 Ma). LN Resin is already used in standard methods for separating Nd, and Nd isotopes are routinely measured by MC-ICPMS with high efficiency inlet systems. Our innovation is a single, small volume LN Resin column for Nd separation. The streamlined approach results in a 10X increase in sample throughput.

Molecular dissolved organic matter composition in lakes across Sweden as relative intensities of FT-ICR-MS peaks and PARAFAC components and optical indices

Whether intrinsic molecular properties or extrinsic factors such as environmental conditions control the decomposition of natural organic matter across soil, marine and freshwater systems has been subject to debate. Comprehensive evaluations of the controls that molecular structure exerts on organic matter's persistence in the environment have been precluded by organic matter's extreme complexity. Here we examine dissolved organic matter from 109 Swedish lakes using ultrahigh-resolution mass spectrometry and optical spectroscopy to investigate the constraints on its persistence in the environment. We find that degradation processes preferentially remove oxidized, aromatic compounds, whereas reduced, aliphatic and N-containing compounds are either resistant to degradation or tightly cycled and thus persist in aquatic systems. The patterns we observe for individual molecules are consistent with our measurements of emergent bulk characteristics of organic matter at wide geographic and temporal scales, as reflected by optical properties. We conclude that intrinsic molecular properties are an important control of overall organic matter reactivity.

Dissolved organic matter composition derived from FT-ICR-MS analyses during HEINCKE cruise HE361

Dissolved organic matter (DOM) is the main substrate and energy source for heterotrophic bacterioplankton. To understand the interactions between DOM and the bacterial community (BC), it is important to identify the key factors on both sides in detail, chemically distinct moieties in DOM and the various bacterial taxa. Next-generation sequencing facilitates the classification of millions of reads of environmental DNA and RNA amplicons and ultrahigh-resolution mass spectrometry yields up to 10,000 DOM molecular formulae in a marine water sample. Linking this detailed biological and chemical information is a crucial first step toward a mechanistic understanding of the role of microorganisms in the marine carbon cycle. In this study, we interpreted the complex microbiological and molecular information via a novel combination of multivariate statistics. We were able to reveal distinct relationships between the key factors of organic matter cycling along a latitudinal transect across the North Sea. Total BC and DOM composition were mainly driven by mixing of distinct water masses and presumably retain their respective terrigenous imprint on similar timescales on their way through the North Sea. The active microbial community, however, was rather influenced by local events and correlated with specific DOM molecular formulae indicative of compounds that are easily degradable. These trends were most pronounced on the highest resolved level, that is, operationally defined 'species', reflecting the functional diversity of microorganisms at high taxonomic resolution.