Delivery Of Organic Matter Research Articles

Hydrogen emission in meteors as a potential marker for the exogenous delivery of organics and water.

We detected hydrogen Balmer-alpha (H(alpha)) emission in the spectra of bright meteors and investigated its potential use as a tracer for exogenous delivery of organic matter. We found that it is critical to observe the meteors with high enough spatial resolution to distinguish the 656.46 nm H(alpha) emission from the 657.46 nm intercombination line of neutral calcium, which was bright in the meteor afterglow. The H(alpha) line peak stayed in constant ratio to the atmospheric emissions of nitrogen during descent of the meteoroid. If all of the hydrogen originates in the Earth's atmosphere, the hydrogen atoms are expected to have been excited at T = 4400 K. In that case, we measured an H(2)O abundance in excess of 150 +/- 20 ppm at 80-90 km altitude (assuming local thermodynamic equilibrium in the air plasma). This compares with an expected <20 ppm from H(2)O in the gas phase. Alternatively, meteoric refractory organic matter (and water bound in meteoroid minerals) could have caused the observed H(alpha) emission, but only if the line is excited in a hot T approximately 10000 K plasma component that is unique to meteoric ablation vapor emissions such as Si(+). Assuming that the Si(+) lines of the Leonid spectrum would need the same hot excitation conditions, and a typical [H]/[C] = 1 in cometary refractory organics, we calculated an abundance ratio [C]/[Si] = 3.9 +/- 1.4 for the dust of comet 55P/Tempel-Tuttle. This range agreed with the value of [C]/[Si] = 4.4 measured for comet 1P/Halley dust. Unless there is 10 times more water vapor in the upper atmosphere than expected, we conclude that a significant fraction of the hydrogen atoms in the observed meteor plasma originated in the meteoroid.

Meteor Storms as a Window on the Delivery of Extraterrestrial Organic Matter to the Early Earth

The unique rarefied flow and flash heating in meteors creates physical conditions that can change exogenous organic matter into unique prebiotic molecules. with the exception of rare giant comet impacts, most infalling matter at the time of the origin of life was deposited in the atmosphere during the meteor phase. Much new data has been obtained from observations in the Leonid Multi-Instrument Aircraft Campaign; a series of NASA and USAF sponsored Astrobiology missions that explored the 1998–2002 Leonid meteor storms. Here, we provide an overview of some of this recent insight, which provides a framework in which the prebiotic chemistry can be studied.

The Ecological Patterns of Benthic Invertebrates in a Great Lakes Coastal Wetland

Benthic macroinvertebrates were sampled in the summer of 1997 using a standard D frame kick net along a transect across the Peshtigo wetland, a river delta wetland on the coast of Green Bay, Lake Michigan, to describe the spatial and temporal dynamics of the invertebrate community. Various abiotic factors, including sedimentation rates determined from 210Pb and 137Cs as a proxy for delivery of riverine organic matter, were also measured to determine which factors influenced these dynamics. Significant decreasing gradients in dissolved oxygen and pH with distance from the river, coupled with trends in sedimentation rates, chloride, and sum nitrate (nitrate + nitrite), revealed that riverine water was mixing with wetland water up to 100 m from the wetland-river interface. Aboveground primary production and total invertebrate densities exhibited Weibull type distributions, with highest rates and numbers occurring 20 to 100 m from the Peshtigo River. Invertebrate densities were largely represented by Asellus sp. isopods (12–53%) and exhibited highest numbers in September. Invertebrate diversity at the genus level linearly decreased with distance from the river based on the Simpson's index of diversity (r 2 = 0.60, p < 0.05) and the Shannon-Wiener function (r 2 = 0.73, p < 0.01). Patterns observed suggest that there is an “optimal” zone for benthic invertebrates in the Peshtigo wetland 20 to 60 m from the Peshtigo River that is protected from high-energy events (e.g., storms, boating) in the Peshtigo wetland by a buffer zone (0 to 20 m) but is close enough to benefit from replenished levels of dissolved oxygen, nutrients, and organic matter delivered via the Peshtigo River.

Sedimentary record of marine and terrigenous organic matter delivery to the Shatsky Rise, western North Pacific, over the last 130 kyr

Biomarker and carbon isotopic analyses of piston core samples were undertaken to reconstruct changes over the last 130 kyr in the delivery of organic matter from marine plankton and terrestrial sources to sediments on the Shatsky Rise in the western North Pacific. Marine and terrigenous organic matter delivery was reconstructed from alkenone and n-alkane concentrations using multiple regression analysis, which gave results consistent with a δ 13C mass balance model. Marine organic carbon comprises more than 86% of the total organic carbon throughout the sediment record. Fluxes of marine organic carbon were higher in glacial times, indicating that marine production increased in these periods. We conclude that in addition to the southward shift of the Subarctic front, eolian dust supply at this location played an essential role in enhancing paleoproduction during the last glacial maximum.

Applications of organic geochemistry to paleolimnological reconstructions: a summary of examples from the Laurentian Great Lakes

The organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions, evaluate histories of climate change, and assess impacts of humans on local ecosystems. The elemental, isotopic, and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area, and they serve as proxies of organic matter delivery and accumulation. Sedimentary records from the North American Great Lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions. The records of these lakes date from retreat of the Laurentide ice sheet around 12 ka, include the mid-Holocene Hypsithermal, and show consequences of recent human changes. Low C org/N total ratios indicate that most of the sediment organic matter in the Great Lakes is from algal production, yet changes in biomarker molecule compositions also show that varying amounts of land-plant organic matter have been delivered to the lakes. Elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative δ 13C values in the organic matter of sediments that were deposited in the 1960s and 1970s. Increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the Great Lakes. Accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the Great Lakes. Emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising, yet some potentially important measurements remain underutilized.

Synchroneity of meltwater pulse 1a and the Bølling warming: New evidence from the South China Sea

A twofold decrease in long-chain n -alcane ( n -nonacosane) concentrations in a downcore record from the northern South China Sea indicates a rapid drop in the supply of terrigenous organic matter to the open South China Sea during the last deglaciation, paralleled by an equally rapid increase in sea-surface temperatures, corresponding with the Bolling warming at 14.7 ka. The sudden drop in terrigenous organic matter delivery to this marginal basin is interpreted to reflect a short-term response of local rivers to rapid sea-level rise, strongly implying that the Bolling warming and the onset of meltwater pulse (MWP) 1a are synchronous. This phase relation contrasts with the widely cited onset of this MWP 1a ca. 14 ka, and implies that previous studies postulating a weakening of deep-water formation in the North Atlantic due to massive meltwater discharge during MWP 1a need to be reevaluated.

Reduced sulfur in euxinic sediments of the Cariaco Basin: sulfur isotope constraints on organic sulfur formation

Reduced sulfur accumulation in Holocene and latest Pleistocene euxinic marine sediments from the Cariaco Basin, Venezuela, was investigated to constrain the timing and possible pathways of organic matter (OM) sulfurization. Data were collected for a diverse suite of sulfur species, including concentrations and sulfur isotope compositions of pore-water sulfide, pore-water sulfate, pyrite sulfur, total organic sulfur (TOS), kerogen sulfur (KS), and polar bitumen sulfur (PBS). Results suggest that there was a period during which almost no diagenetic pyrite formed in the sediments of the Cariaco, coincident with a shift from high to lower sedimentation rates and a concomitant change in the delivery of organic matter to the sediments. The sulfur isotope composition of organic matter was predicted based on assumed pathways using weighted isotopic mass balance calculations and compared to measured isotope values for organic sulfur. These results indicate that organic sulfur is derived primarily from pore-water sulfide, with minor contributions from primary bio-sulfur (e.g., in proteins derived from algae and bacteria). The predicted sulfur isotope values of organic sulfur compounds (OSC) suggest that pore-water sulfide is the ultimate source of reduced sulfur for incorporation into organic matter. It is possible, however, that reactive sulfur intermediates such as elemental sulfur or polysulfides react directly with organic matter. These intermediate sulfur species are likely formed through partial oxidation of sulfide by anaerobic sulfide-oxidizing microbes living in the sediments.

Riparian aquatic interaction simulator (RAIS): a model of riparian forest dynamics for the generation of large woody debris and shade

Streams depend on riparian forests to supply many important functions such as delivery of large woody debris (LWD), organic matter, and nutrients into the aquatic ecosystem and to provide shade to help maintain cool water. Both forests and streams are dynamic, and inputs from the riparian forest are constantly replenished as organic matter and nutrients are processed and transported. Forest stand dynamics have been intensively studied and foresters have developed a good understanding of tree growth and mortality to support commercial forest management. In recent years, the relationships between aquatic functions and adjacent forest stand characteristics have been increasingly quantified by ecologists. Management of riparian forests to protect aquatic functions and water quality has received considerable attention in the Pacific Northwest and elsewhere. For scientific knowledge to be useful to decision-makers, the complex set of riparian relationships, increasingly well understood individually, need to be collectively and objectively linked in a form that can be easily used by scientists and non-scientists alike to develop management strategies for riparian forests. In this paper we describe an analytical system that quantitatively links widely used forest growth forecasting systems for coastal Pacific Northwest forest types to the riparian ecological functions of large woody debris recruitment and shade. The riparian aquatic interaction simulator (RAIS), with its user-friendly interface, allows managers to forecast aquatic functions for up to 300 years. RAIS provides these forecasts over a range of critical input variables and produces realistic estimates of riparian functions when compared with published research. RAIS is available at http://www.weyerhaeuser.com/rais.html

Geochemical evidence for variations in delivery and deposition of sediment in Pleistocene light–dark color cycles under the Benguela Current Upwelling System

Distinctive light–dark color cycles in sediment beneath the Benguela Current Upwelling System indicate repetitive alternations in sediment delivery and deposition. Geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid-Pleistocene intervals from ODP Sites 1082 and 1084. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1%. Concentrations of CaCO 3 vary inversely to TOC and Al, which suggests that both carbonate dissolution and terrigenous dilution contribute to the light–dark cycles. Opal concentrations are independent of both TOC and CaCO 3, therefore eliminating diatom production and lateral transport of shelf material as causes of the light–dark cycles. δ 13C org and δ 15N tot values do not vary across light–dark sediment intervals, implying that the extent of relative nutrient utilization did not change. The stable δ 15N tot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the TOC-rich layers. Parallel changes in concentrations of indicator trace elements and TOC imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen. Cu, Ni, and Zn are enriched in the darker sediment as a consequence of greater organic matter delivery. Redox-sensitive metals vary due to loss (Mn and Ba) or enrichment (Mo) under reducing conditions created by TOC oxidation. Organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution, sulfate reduction and the concentration of metals. Thus, export production is considered ultimately responsible for the generation of the color cycles.

Late-Quaternary supply of terrigenous organic matter to the Congo deep-sea fan (ODP site 1075): implications for equatorial African paleoclimate

Late-Quaternary sections (about 1 Ma) from the Congo deep-sea fan (ODP Leg 175, site 1075) were used to reconstruct the terrigenous organic matter supply to the easternmost equatorial Atlantic Ocean. Variations in quantity and quality of the riverine organic matter reflect the interaction between the paleoclimatic development within the continental catchment area and the paleoceanographic conditions in the Congo river plume. To characterize the delivery of organic matter from terrigenous and marine sources, we used elemental and bulk carbon isotopic analyses, Rock-Eval pyrolysis, lignin chemistry, and organic petrology. High-amplitude fluctuations occurring about every 15–25 ka reveal a mainly precessional control on organic sedimentation. Results from Rock-Eval pyrolysis indicate a mixed kerogen type III/II, as would be anticipated in front of a major river. Fluctuations in Tmax from Rock-Eval pyrolysis demonstrate pronounced cyclic changes in the delivery of low- and high-mature organic matter. Contribution of the low-mature organic fraction was strongest during warm climates supporting enhanced marine production offshore of the Congo. Organic petrological observations confirm the existence of abundant terrigenous plant tissues, both non-oxidized (vitrinite) and oxidized (inertinite). Charcoal-like organic matter (fusinite) is attributed to periods of increased bush fires in the continental hinterland, and implies more arid climatic conditions. Results from ratios of specific phenolic lignin components suggest that terrigenous organic matter in Late-Quaternary sections of site 1075 mainly derives from non-woody angiosperm tissue, i.e., grasses and leaves. Correlation between the amount of specific lignin phenols and the bulk δ13Corg signature fosters the conclusion that an appreciable amount of the terrigenous organic fraction derives from C4 plant matter. This may cause an underestimation of the terrigenous proportion of bulk organic matter when assessments are based on bulk carbon isotopic signatures alone.

Organic carbon isotope ratios (δ 13 C) of Arctic Amerasian Continental shelf sediments

Organic matter origins are inferred from carbon isotope ratios (δ13C) in recent continental shelf sediments and major rivers from 465 locations from the north Bering-Chukchi-East Siberian-Beaufort Sea, Arctic Amerasia. Generally, there is a cross-shelf increase in δ13C, which is due to progressive increased contribution seaward of marine-derived organic carbon to surface sediments. This conclusion is supported by the correlations between sediment δ13C, OC/N, and δ15N. The sources of total organic carbon (TOC) to the Amerasian margin sediments are primarily from marine water-column phytoplankton and terrigenous C3 plants constituted of tundra taiga and angiosperms. In contrast to more temperate regions, the source of TOC from terrigenous C4 and CAM plants to the study area is probably insignificant because these plants do not exist in the northern high latitudes. The input of carbon to the northern Alaskan shelf sediments from nearshore kelp community (Laminaria solidungula) is generally insignificant as indicated by the absence of high sediment δ13C values (–16.5 to –13.6‰) which are typical of the macrophytes. Our study suggests that the isotopic composition of sediment TOC has potential application in reconstructing temporal changes in delivery and accumulation of organic matter resulting from glacial–interglacial changes in sea level and environments. Furthermore, recycling and advection of the extensive deposits of terrestrially derived organic matter from land, or the wide Amerasian margin, could be a mechanism for elevating total CO2 and pCO2 in the Arctic Basin halocline.

A geochemical record of eutrophication and anoxia in Chesapeake Bay sediments: anthropogenic influence on organic matter composition

Abstract Organic and inorganic geochemical indicators were examined in a 3-m core collected from the mesohaline region of Chesapeake Bay (CB) to determine how sources of organic matter (OM) have changed during the preceding three centuries of increasing anthropogenic influence in this region. This study also establishes the history of eutrophication and anoxia/hypoxia and relates these processes to changes in OM deposition and preservation and to historical events within the Bay's watershed. The sediment record shows that a marked increase in organic carbon (35%–50%), biogenic silica (18%) and total sulfur (42%) occurs between 1934 and 1948. This transition is likely due to increasing anoxic/hypoxic bottom water conditions as indicated by an abrupt change in sulfur speciation. Lipid biomarker distributions indicate that a substantial change in the sources of OM deposited since 1934 has also occurred. Biomarker compounds derived from phytoplankton and microbial sources show a 2- to 4-fold increase in their abundance relative to total organic carbon (TOC) between 1948 and 1975. Using both diagenetic models and information on lipid reactivity, an effort is made to distinguish compositional changes due to changes in OM delivery (both quantity and quality) from changes that may be due to OM degradation. It appears that enhanced OM production in the mesohaline region of CB has contributed to the observed changes in quantity and character of OM preserved in sediments from this site. Increased inorganic fertilizer application and human population growth in the watershed are coincident with the onset of eutrophic and hypoxic conditions in CB, suggesting that anthropogenic activities within estuarine watersheds may exert a substantial influence on carbon cycling processes in estuaries and potentially the coastal ocean.

Pyrolysis of simple amino acids and nucleobases: survivability limits and implications for extraterrestrial delivery

The idea of extraterrestrial delivery of organic matter to the early Earth is strongly supported by the detection of a large variety of organic compounds in the interstellar medium, comets, and carbonaceous chondrites. Whether organic compounds essential for the emergence and evolution of life, particularly amino acids and nucleic acid bases found in the meteorites, can be efficiently delivered by other space bodies is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. In the present study we estimated survivability of simple amino acids (glycine, Lalanine, α-aminoisobutyric acid, L-valine and L-leucine), purines (adenine and guanine) and pyrimidines (uracil and cytosine) under rapid heating to temperatures of 400-1000°C under N2 or CO2 atmosphere. We have found that most of the compounds studied cannot survive the temperatures substantially higher than 700°C; however at 500600°C, the recovery can be at a percent level (or even 10%-level for adenine, uracil, alanine, and valine). The final fate of amino acids and nucleobases during the atmospheric deceleration and surface impacts is discussed depending on such factors as size of the space body, nature and altitude of the heating, chemical composition of the space body and of the atmosphere.

Sedimentary organic matter record of recent environmental changes in the St. Marys River ecosystem, Michigan–Ontario border

Lake George, located in the St. Marys River, has been heavily impacted by human-induced environmental changes over the past century. The effects of human impacts starting in the late nineteenth century and of natural, gradual diagenesis can be distinguished in the bulk organic matter and molecular contents of the sedimentary record. Organic carbon concentrations increase from 0.5% in sediments deposited 200 years ago to ∼4% in recent sediments. A fourfold increase in organic carbon mass accumulation rates accompanies the change in concentrations. Elevated C/N ratios in near-modern sediments indicate that increased delivery of land-derived organic matter has been responsible for much of the recent increases in sedimentary organic carbon. Organic δ 13C and δ 15N values change significantly and coincidentally with the environmental changes, reflecting depressed algal productivity since the introduction of industrial effluents to the aquatic system, increased delivery of land-derived organic matter and some impacts of acid rain. Increases in microbial and petroleum hydrocarbon contributions occur in sediments deposited since 1900. Fatty acid distributions provide evidence of substantial microbial reworking of organic matter throughout the sedimentary record.

Transport of extraterrestrial biomolecules to the Earth: Problem of thermal stability

The idea of extraterrestrial delivery of organic matter to the early Earth is especially attractive at present and is strongly supported by the detection of a large variety of organic compounds, including amino acids and nucleobases, in carbonaceous chondrites. Whether these compounds can be delivered by other space bodies is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. In the present study we estimated survivability of simple amino acids (α-aminoisobutyric acid, L-alanine, L-valine and L-leucine), purines (adenine and guanine) and pyrimidines (uracil and cytosine) under rapid heating to temperatures of 400 to 1000 °C under N 2 or CO 2 atmosphere. We have found that most of the compounds studied cannot survive the temperatures substantially higher than 700 °C; however at 500–600 °C, the recovery can be at a per cent level (or even 10%-level for adenine, uracil, alanine, and valine). Implications of the data for extraterrestrial delivery of the biomolecules are discussed.

Pyrolytic Behavior of Amino Acids and Nucleic Acid Bases: Implications for Their Survival during Extraterrestrial Delivery

The idea of extraterrestrial delivery of organic matter (by comets, asteroids, meteorites, and interplanetary dust particles) to the early Earth is very popular at present. A strong argument for its favor is the detection of a large variety of organic compounds, including amino acids and nucleic acid bases, in carbonaceous chondrites. Whether these compounds can be delivered by other space bodies is unclear and depends primarily on capability of the biomolecules to survive high temperature regimes during atmospheric deceleration and impacts to the terrestrial surface. Although some indirect estimates of simple biomolecules' survivability have been reported, there is an evident lack of experimental data. In the present study we demonstrate that some simple amino acids, purines, and pyrimidines do not completely decompose even under volatilization at 500°C in a nitrogen atmosphere at normal pressure, with the percentage of survival of the order of 1–10%. In the case of amino acids, several types of condensation products form (piperazine-2,5-diones, bicyclic amidines, hydantoins, etc.) with total yields in the same percentage range, preserving amino acid residues intact and being able to release free amino acids upon hydrolysis. Taking into account the property of amino acids as well as nucleic acid bases to sublime in vacuum under temperatures of about 200°C, one should expect that the biomolecules in the dust particles actually do not experience the temperatures as much as 400–500°C and rapidly sublime during the atmospheric passage, dissipating in the upper atmosphere. The biomolecules' survival during catastrophic airbursts of comets is also possible, but very unlikely for asteroidal impacts to the terrestrial surface (at least for those resulting in complete pulverization and evaporation of the projectiles).

Early Holocene climatic instability in Japan: organic geochemical evidence in sediment cores from Lake Biwa, Lake Kizaki and the Japan Sea

Sediment cores from Lake Biwa (Shiga Prefecture) and Lake Kizaki (Nagano Prefecture) provide evidence of variations in the climate of Japan subsequent to the end of the last glacial period. Concentrations of organic carbon increase in sediments deposited after the Pleistocene–Holocene boundary, as wetter climates washed in more soil nutrients and aquatic productivity increased. Organic matter C/N ratios simultaneously decrease, indicating smaller proportions of land-derived plant matter. Lake Biwa C/N ratios and organic carbon isotope ratios fluctuate markedly between 10 and 4 Kya, suggesting that both delivery of land-derived organic matter and production of lake-derived organic matter were variable during this period of time. Similar variability in biotic paleoceanographic indicators has been reported in sediments from the Oki Ridge in the Japan Sea and has been interpreted to record early Holocene fluctuations in the intensity of the Tsushima Current. Such fluctuations in this warm-water current are likely to have caused concordant fluctuations in climate on Honshu, a relationship that explains the variability in rainfall evident in the organic matter of the lake sediments.

Sedimentary record of sources and accumulation of organic matter in Pyramid Lake, Nevada, over the past 1,000 years

We have examined the organic matter contents of two 210Pb‐dated cores of sediment from Pyramid Lake for consequences of the historic environmental and limnological changes that have occurred since modem settlement of this region began in the 1850s. Agricultural diversion of Truckee River water has diminished the volume of this saline, terminal lake by one‐fourth since 1905. Organic C:N ratios, organic d13C values, and terrigenous:aquatic n‐alkane ratios indicate that the proportion of land‐derived organic matter in sediments has decreased as river flow has decreased. Algal productivity also appears to have decreased since 1930. Some prehistorical variations in the delivery of organic matter are evident in older sediments. Elevations in the proportion of land plant wax and in C:N ratios in sediments deposited A.D. 1200–1400 suggest that contributions of land‐derived organic matter were enhanced at this time. Proportions of biomarker n‐alkanes indicate that grasses were more important than trees and shrubs as the dominant source of land plant waxes during this period, suggesting an interlude of regionally wetter climate.

Observation of indigenous polycyclic aromatic hydrocarbons in 'giant' carbonaceous antarctic micrometeorites.

Two-step laser desorption/laser ionization mass spectrometry (microL2 MS) was used to establish the nature and mass distribution of polycyclic aromatic hydrocarbons (PAHs) in fragments of fifteen 'giant' (approximately 200 microns) carbonaceous Antarctic micrometeorites (AMMs). Detectable concentrations of PAHs were observed in all AMMs showing a fine-grained matrix. The range of integrated PAH signal intensities varied between samples by over two orders of magnitude. No evidence of contamination whilst in the Antarctic environment could be found. The dramatic variation of both PAH signal intensities and mass distributions between AMMs along with comprehensive contamination checks demonstrates that particles are not exposed to terrestrial PAHs at or above detection limits, either subsequent, during or prior to collection. Comparison of the observed PAH distributions with those measured in three carbonaceous chondrites [Orgueil (CI1), Murchison (CM2) and Allende (CV3)] under identical conditions demonstrated that marked differences exist in the trace organic composition of these two sources of extraterrestrial matter. In general, AMMs show a far richer distribution of unalkylated 'parent' PAHs with more extended alkylation series (replacement of -H with -(CH2)n-H; n = 1, 2, 3 ...). The degree of alkylation loosely correlates with a metamorphic index that represents the extent of frictional heating incurred during atmospheric entry. A search for possible effects of the chemical composition of the fine-grain matrix of host particles on the observed PAH distributions reveals that high degrees of alkylation are associated with high Na/Si ratios. These results, in addition to other observations by Maurette, indicate that 'giant' micrometeorites survive hypervelocity (> or = 11 km s-1) atmospheric entry unexpectedly well. Because such micrometeorites are believed to represent the dominant mass fraction of extraterrestrial material accreted by the Earth, they may have played a significant role in the prebiotic chemical evolution of the early Earth through the delivery of complex organic matter to the surface of the planet.

Glacial–interglacial variations in Quaternary production of marine organic matter at DSDP Site 594, Chatham Rise, southeastern New Zealand margin

CaCO 3 and total organic carbon concentrations, organic matter C/N and carbon isotope ratios, and sediment accumulation rates in late Quaternary sediments from DSDP Site 594 provide information about glacial–interglacial variations in the delivery of organic matter to the Chatham Rise offshore of southeastern New Zealand. Low C/N ratios and nearly constant organic δ 13C values of −23‰ indicate that marine production dominates organic matter supply in both glacial and interglacial times during oxygen isotope stages 1 through 6 (0–140 ka) and 17 through 19 (660–790 ka). Increased organic carbon mass accumulation rates in isotope stages 2, 4, 6, and 18 record enhanced marine productivity during glacial maxima. Excursions of organic δ 13C values to ca. −29‰ in portions of isotope stage 2 suggest that the local concentration of dissolved CO 2 was occasionally elevated during the last glacial maximum, probably as a result of short periods of lowered sea-surface temperature. Dilution of carbonates by clastic continental sediment generally increases at this location during glacial maxima, but enhanced delivery of land-derived organic matter does not accompany the increased accumulation of clastic sediments.