Elliott C. Spiker

The effects of early diagenesis on the chemical and stable carbon isotopic composition of wood

Abstract Studies of modern and ancient buried wood show that there is a linear correlation between carbohydrate content and the stable carbon isotope composition as carbohydrates are preferentially degraded during early diagenesis. As the carbohydrate content decreases, the δ 13 C value of the degraded wood decreases 1 to 2 per mil, approaching the value of the residual lignin. These results indicate that carbohydrate degradation products are lost and not incorporated into the aromatic structure as lignin is selectively preserved during early diagenesis of wood. These results also indicate that attempts to quantify terrestrial inputs to modern sedimentary organic matter based on δ 13 C values should consider the possibility of a 1 to 2 per mil decrease in the δ 13 C value of degraded wood.

Carbon isotope fractionation of sapropelic organic matter during early diagenesis

Abstract Study of an algal, sapropelic sediment from Mangrove Lake, Bermuda shows that the mass balance of carbon and stable carbon isotopes in the major organic constituents is accounted for by a relatively straightforward model of selective preservation during diagenesis. The loss of 13 C-enriched carbohydrates is the principal factor controlling the intermolecular mass balance of 13 C in the sapropel. Results indicate that labile components are decomposed leaving as a residual concentrate in the sediment an insoluble humic substance that may be an original biochemical component of algae and associated bacteria. An overall decrease of up to about 4‰ in the δ 13 C values of the organic matter is observed as a result of early diagenesis.

Contrasting magma types and steady-state, volume-predictable, basaltic volcanism along the Great Rift, Idaho

The Great Rift is an 85-km-long, 2- to 8-km-wide volcanic rift zone in the Snake River Plain, Idaho. Three latest Pleistocene to Holocene basaltic lava fields, Craters of the Moon, Kings Bowl, and Wapi, are located along the Great Rift. The Craters of the Moon lava field is a composite of more than 60 lava flows, 25 cinder cones, and at least 8 eruptive fissure systems. It covers 1,600 km2 and contains ∼30 km3 of lava flows and associated pyroclastic deposits. Field, radiocarbon, and paleomagnetic data show that the Craters of the Moon lava field formed in eight eruptive periods, each of which was typically several hundred years or less in duration and was separated from others before and after by intervals of several hundred to several thousand years. The first eruptive period began ∼15,000 yr B.P., and the latest eruptive period ended ∼2100 yr B.P. The small Kings Bowl lava field (3.3 km2, 0.01 km3) and the larger Wapi lava field (330 km2, 6 km3) both formed ∼2250 yr B.P. Three magma types have fed flows along the Great Rift. The types are (1) a contaminated type that has a SiO2 range of ∼49%–64% and commonly shows petrographic evidence of contamination, (2) a fractionated type that has a SiO2 range of ∼44%–54% and shows no evidence of contamination and whose chemical and mineralogical variation can be accounted for mainly by crystal fractionation, and (3) a Snake River Plain type that has a SiO2 range of ∼45%–48%, shows little evidence of fractionation, and is represented by Kings Bowl–Wapi flows and olivine basalts of the Snake River Plain. The contaminated and fractionated magma types were erupted at the Craters of the Moon lava field, and the Snake River Plain magma type was erupted at the Kings Bowl and Wapi lava fields. These relations imply that the magma reservoirs are spatially isolated. The magma output rate for the Craters of the Moon segment of the Great Rift was constant at ∼1.5 km3/1,000 yr for the period from 15,000 to 7000 yr B.P. The rate increased to ∼2.8 km3/1,000 yr from 7000 to 2000 yr B.P., mainly as a result of the addition of contaminated magma to the nearly constant output rate of fractionated magma. The Craters of the Moon segment of the Great Rift has experienced quasi-steady-state, volume-predictable volcanism for the last 15,000 yr. The recurrence interval of eruptive activity for the Craters of the Moon lava field ranges from several hundred to ∼3,000 yr. Because the present interval has lasted ∼2,100 yr, another eruptive period seems likely to occur within the next 1,000 yr. The steady-state, volume-predictable relationship suggests that 5–6 km3 of lava will be erupted in the next eruptive period.

Isotopic evidence for organic matter oxidation by manganese reduction in the formation of stratiform manganese carbonate ore

Abstract Unlike other marine-sedimentary manganese ore deposits, which are largely composed of manganese oxides, the primary ore at Molango (Hidalgo State, Mexico) is exclusively manganese carbonate (rhodochrosite, Mn-calcite, kutnahorite). Stable isotope studies of the carbonates from Molango provide critical new information relevant to the controversy over syngenetic and diagenetic models of stratiform manganese deposit formation. Negative δ 13 C values for carbonates from mineralized zones at Molango are strongly correlated with manganese content both on a whole rock scale and by mineral species. Whole rock δ 13 C data fall into three groups: high-grade ore = −16.4 to −11.5%.; manganese-rich, sub-ore-grade = −5.2 to 0%.; and unmineralized carbonates = 0 to +2.5%. (PDB). δ 18 O data show considerable overlap in values among the three groups: +4.8 to −2.8, −5.4 to −0.3%., and −7.4 to +6.2 (PDB), respectively. Isotopic data for individual co-existing minerals suggest a similar separation of δ 13 C values: δ 13 C values from calcite range from −1.1 to +0.7%. (PDB), whereas values from rhodochrosite are very negative, −12.9 to −5.5%., and values from kutnahorite or Mn-calcite are intermediate between calcite and rhodochrosite. 13 C data are interpreted to indicate that calcite ( i.e . unmineralized carbonate) formed from a normal marine carbon reservoir. However, 13 C data for the manganese-bearing carbonates suggest a mixed seawater and organic source of carbon. The presence of only trace amounts of pyrite suggests sulfate reduction may have played a minor part in oxidizing organic matter. It is possible that manganese reduction was the predominant reaction that oxidized organic matter and that it released organic-derived CO 2 to produce negative δ 13 C values and manganese carbonate mineralization.

Radiocarbon studies of latest Pleistocene and Holocene lava flows of the Snake River Plain, Idaho: Data, lessons, interpretations

Abstract Latest Pleistocene-Holocene basaltic lava fields of the Snake River Plain, Idaho, have been dated by the radiocarbon method. Backhoe excavations beneath lava flows typically yielded carbon-bearing, charred eolian sediment. This material provided most of the samples for this study; the sediment typically contains less than 0.2% carbon. Charcoal fragments were obtained from tree molds but only from a few backhoe excavations. Contamination of the charred sediments and charcoal by younger carbon components is extensive; the effects of contamination were mitigated but appropriate pretreatment of samples using acid and alkali leaches. Twenty of the more than 60 lava flows of the Craters of the Moon lava field have been dated; their ages range from about 15,000 to about 2000 yr B.P. The ages permit assignment of the flows to eight distinct eruptive periods with an average recurrence interval of about 2000 yr. The seven other latest Pleistocene-Holocene lava fields were all emplaced in short eruptive bursts. Their 14 C ages (yr B.P.) are: Kings Bowl (2222± 100), Wapi (2270 ± 50), Hells Half Acre (5200 ± 150), Shoshone (10,130 ± 350), North Robbers and South Robbers (11.980 ± 300), and Cerro Grande (13,380 ± 350).

Dissolved organic matter in anoxic pore waters from Mangrove Lake, Bermuda

Abstract Dissolved organic matter and dissolved inorganic chemical species in anoxic pore water from Mangrove Lake, Bermuda sediments were studied to evaluate the role of pore water in the early diagenesis of organic matter. Dissolved sulphate, titration alkalinity, phosphate, and ammonia concentration versus depth profiles were typical of many nearshore clastic sediments and indicated sulphate reduction in the upper 100 cm of sediment. The dissolved organic matter in the pore water was made up predominantly of large molecules, was concentrated from large quantities of pore water by using ultrafiltration and was extensively tudied by using elemental and stable carbon isotope analysis and high-resolution, solid state 13C nuclear magnetic resonance and infrared spectroscopy. The results indicate that this material has a predominantly polysaccharide-like structure and in addition contains a large amount of oxygen-containing functional groups (e.g., carboxyl groups). The 13C nulcear magnetic resonance spectra of the high-molecular-weight dissolved organic matter resemble those of the organic matter in the surface sediments of Mangrove Lake. We propose that this high-molecular-weight organic matter in pore waters represents the partially degraded, labile organic components of the sedimentary organic matter and that pore waters serve as a conduit for removal of these labile organic components from the sediments. The more refractory components are, thus, selectively preserved in the sediments as humic substances (primarily humin).

Hudson River: Evidence for extensive migration on the exposed continental shelf during Pleistocene time

Analyses of seismic-reflection profiles collected off New Jersey reveal a large buried channel that splits from the topographic Hudson shelf valley beneath the inner shelf and extends southward for at least 80 km. The buried valley has a flat bottom, a width of 2 to 17 km, and a relief of 3 to 15 m, all features similar to those of the Hudson shelf valley. The buried valley apparently is an ancestral pathway of the Hudson River that has been filled with heterogeneous fluvial deposits and capped by an additional 10 to 30 m of sediments. Vibracores from over or near the ancestral valley show that interbedded marine sand and mud layers constitute the upper part of the sedimentary fill. Radiocarbon ages, geotechnical properties, and micro-paleontological analyses of the core sediments indicate that the valley was formed and filled sometime prior to 28,000 yr ago and then was exposed subaerially during at least one sea-level regression. These results are the first clear subbottom evidence that the ancestral Hudson River flowed south of the Hudson shelf valley on the exposed continental shelf during Pleistocene time.

Speciation and isotopic composition of sedimentary sulfur in the Everglades, Florida, USA

Abstract We have studied the sulfur speciation and isotopic composition of two peat cores from Water Conservation Area 2A (WCA 2A) in the Florida Everglades. Core site E1 is affected by agricultural runoff from the Hillsboro Canal which drains the Everglades Agricultural Area; Core site U3 is distant from the canal and relatively unaffected by agricultural runoff. Depth profiles of the total sulfur content of both cores show fairly constant levels (∼0.7 wt.%) below about 25–30 cm depth in Core E1 and below 40–45 cm in Core U3. Above these depths, total sulfur increases to as much as 1.52 wt.% in Core E1 and 1.74 wt.% in Core U3, suggesting that more sulfur has entered the sediments and/or that more sulfur is being retained in recent times at both sampling sites. The changes in total sulfur content with depth in Core E1 correlate with changes in total phosphorus that have been observed in other studies at core sites near the Hillsboro Canal. This correlation of total sulfur with phosphorus with depth is not seen in Core U3 located away from the canal, possibly because phosphorus is more effectively retained than sulfur in the organic sediment near the canal. Organic-sulfur (OS) concentrations are at least twice as high as the disulfide-sulfur (DS) concentrations in the upper parts of both cores suggesting that iron is presently limiting the amount of disulfide minerals formed in these sediments. The degree of pyritization (DOP) in the upper parts of the cores suggest that sulfide mineralization is limited by the availability of highly reactive iron during the earliest stages of diagenesis. Positive δ 34 S values for reduced sulfur forms in both cores indicate a relatively restricted sulfate reservoir, consistent with nearly complete reduction of the sulfate available in the sediment at any given time. Differences between the two cores appear in the δ 34 S values for the near-surface sediments. The DS δ 34 S values in the upper 10.0 cm of sediment are more positive at site E1, with a mean δ 34 S value of +12.9 per mil, than at site U3, with a mean δ 34 S value of +2.9 per mil. These results may indicate that increased rates of organic deposition due to nutrient loading near the canal have increased the rate of sulfate reduction at the E1 site in recent times. Acid-volatile-sulfide (AVS) concentrations are lower than DS and OS concentrations by at least a factor of 10. Increasing δ 34 S values for AVS with increasing depth in both cores suggests ongoing reduction of a limited porewater sulfate reservoir after deposition. The disulfide and organic-sulfur δ 34 S values diverge from the δ 34 S values for AVS with depth, suggesting that most of the transformation of AVS into disulfide minerals or incorporation of sulfur into organic matter occurs in the near-surface sediments. A comparison of organic-sulfur δ 34 S values in the dominant flora at the U3 site (sawgrass leaves and periphyton) with organic-sulfur δ 34 S values at the top of the U3 core indicates that there was early incorporation of an isotopically light sulfide species into the organic matter.

Stratigraphy, structure, absolute age, and paleontology of the upper Pleistocene deposits at Sankaty Head, Nantucket Island, Massachusetts

The Sankaty Head cliff of Nantucket Island, Massachusetts, exposes drift of at least two glaciations and interglacial marine deposits. Radiocarbon, amino-acid-racemization, and uranium-thorium analyses were used to determine the absolute ages of the beds. The results indicate that (1) the Sankaty Sand correlates with oxygen-isotope stage 5 (Sangamonian), (2) the underlying drift is older than stage 5 (Illinoian or older), and (3) the overlying drift is Wisconsinan in age. Ostracodes and molluscs within the Sankaty Sand indicate that the marine climate during deposition of the lower part was somewhat warmer than the present climate off Sankaty Head and that the marine climate during the deposition of the upper part was as cold as or somewhat colder than the present climate. The paleoenvironmental data support a stage 5 (Sangamonian) age for the marine deposits.

Petroleum Pollutants in Surface and Groundwater as Indicated by the Carbon-14 Activity of Dissolved Organic Carbon

The 14C activity of dissolved organic carbon (DOC) can be used to distinguish between the fossil organic carbon due to petrochemical effluents and modern organic carbon due to domestic wastes and natural decaying organic matter. Rivers polluted by petrochemical effluents show varying amounts of depression of the DOC 14C activity, reflecting concentrations of 14C-deficient fossil carbon of as much as about 40 percent of the total DOC.