Variations In Sediment Accumulation Rates Research Articles

How much organic carbon have UK lakes stored in the Holocene? A preliminary estimate

Temperate lake sediments store a substantial amount of organic carbon (OC) over millennia. Despite the importance of quantifying terrestrial carbon budgets for Nature-based Solutions, the long-term accumulation of OC in European temperate lakes is poorly constrained. In this study, we analyzed 30 lake sediment records to generate a preliminary first-order estimate of Holocene OC accumulation rate (OCAR) and OC storage in UK lakes. We also examined the environmental variables that influence OCAR and produced synthesized Holocene records of %OC and z-scores of log-transformed OCAR and sediment accumulation rate (SAR) at 500-year resolution. Based on our estimation, we report an average Holocene OCAR of 7.4 ± 5.5 g C m−2 yr−1 and a Holocene total OC storage of 0.24 ± 0.18 Pg C in UK lakes. Apart from latitude, no relationship was found between the average Holocene OCAR and the various environmental variables (i.e. temperature, precipitation, surface area, catchment area, depth, altitude, and geology type). During the Holocene, OCAR closely resembles variations in SAR, whereas the increase in %OC is likely explained by the warming climate. Early Holocene variations in OCAR were primarily climate-driven. In contrast, the anthropogenic impact on the landscape exerted a predominant influence on OC burial during the middle-late Holocene. Our results improve the current understanding of terrestrial carbon budgets in the UK and demonstrate the under-appreciated importance of lakes as long-term OC stores.

Trench floor depositional response to glacio-eustatic changes over the last 45 ka, northern Hikurangi subduction margin, New Zealand

ABSTRACT Glacio-eustatic cycles lead to changes in sedimentation on all types of continental margins. There is, however, a paucity of sedimentation rate data over eustatic sea-level cycles in active subduction zones. During International Ocean Discovery Program Expedition 375, coring of the upper ∼110 m of the northern Hikurangi Trough Site U1520 recovered a turbidite-dominated succession deposited during the last ∼45 kyrs (Marine Isotope Stages (MIS) 1–3). We present an age model integrating radiocarbon dates, tephrochronology, and δ18O stratigraphy, to evaluate the bed recurrence interval (RI) and sediment accumulation rate (SAR). Our analyses indicate mean bed RI varies from ∼322 yrs in MIS1, ∼49 yrs in MIS2, and ∼231 yrs in MIS3. Large (6-fold) and abrupt variations in SAR are recorded across MIS transitions, with rates of up to ∼10 m/kyr occurring during the Last Glacial Maximum (LGM), and <1 m/kyr during MIS1 and 3. The pronounced variability in SAR, with extremely high rates during the LGM, even for a subduction zone, are the result of changes in regional sediment supply associated with climate-driven changes in terrestrial catchment erosion, and critical thresholds of eustatic sea-level change altering the degree of sediment bypassing the continental shelf and slope via submarine canyon systems.

Refined geochronology and revised stratigraphic nomenclature of the Upper Cretaceous Wahweap Formation, Utah, U.S.A. and the age of early Campanian vertebrates from southern Laramidia

The Western Interior of North America preserves one of the most complete successions of Upper Cretaceous marine and non-marine strata in the world; among these, the Cenomanian-Campanian units of the Kaiparowits Plateau in southern Utah are a critical archive of terrestrial environments and biotas. Here we present new radioisotopic ages for the Campanian Wahweap Formation, along with lithostratigraphic revision, to improve the geological context of its fossil biota. The widely accepted informal stratigraphic subdivisions of the Wahweap Formation on the Kaiparowits Plateau are herein formalized and named the Last Chance Creek Member, Reynolds Point Member, Coyote Point Member, and Pardner Canyon Member (formerly the lower, middle, upper, and capping sandstone members respectively). Two high-precision U-Pb zircon ages were obtained from bentonites using CA-ID-TIMS, supported by five additional bentonite and detrital zircon LA-ICP-MS ages. Improved geochronology of the Star Seep bentonite from the base of the Reynolds Point Member via CA-ID-TIMS demonstrates that this important marker horizon is over a million years older than previously thought. A Bayesian age-stratigraphic model was constructed for the Wahweap Formation using the new geochronologic data, yielding statistically robust ages and associated uncertainties that quantifiably account for potential variations in sediment accumulation rate. The new chronostratigraphic framework places the lower and upper formation boundaries at 82.17 +1.47/-10.63 Ma and 77.29 +0.72/-0.62 Ma, respectively, thus constraining its age to the first half of the Campanian. Additionally, a holistic review of known vertebrate fossil localities from the Wahweap Formation was conducted to better understand their spatio-temporal distribution including revised ages for early members of iconic dinosaur lineages such as Tyrannosauridae, Hadrosauridae, and Centrosaurinae. Chrono- and lithostratigraphic refinement of the Wahweap Formation and its constituent biotic assemblages establishes an important reference for addressing questions of Campanian terrestrial paleoecology and macroevolution, including dinosaur endemism and diversification throughout western North America. • New high-precision age for the Star Seep bentonite extends age of the Wahweap Fm. • Subdivisions of the Wahweap Formation are formally recognized. • A detailed age-stratigraphic model was generated including uncertainty. • Robust ages with uncertainty produced for any stratigraphic level in the formation. • Oldest members of key dinosaur lineages found to be older than previously thought.

Astronomical cycles and variations in sediment accumulation rate of the terrestrial lower Cretaceous Xiagou Formation from the Jiuquan Basin, NW China

The Lower Cretaceous Xiagou Formation in Qingxi Sag consists of more than 1000 m of lacustrine sediments, including dolomitic mudstone/argillaceous dolomite and sandstone/mudstone, which is considered to be the most important source rock in the Jiuquan Basin. Power spectra, evolutionary fast Fourier transform (FFT), and correlation coefficient analyses of the gamma-ray (GR) series from four wells, including Liu 8 well, Long 14 well, Q2-36 well and Liu 9 well, through the Xiagou Formation reveal strong astronomical signals of long eccentricity (405 kyr), short eccentricity (100 kyr), obliquity (37 kyr) and precession (22 kyr). These provide an excellent opportunity to establish a floating astronomical time scales (ATS) then to determine variations in the sediment accumulation rate (SAR). Based on the floating ATS of the Xiagou Formation, the Xiagou Formation covers a 10.3 myr period depositional duration, and depositional duration of three members in the Xiagou Formation are 3.5 ± 0.1 myr, 3.1 ± 0.1 myr and 3.65 ± 0.15 myr, respectively. The high-resolution SARs of the Xiagou Formation vary from 8.5 cm/kyr to 19.8 cm/kyr. The different lithologies in the Xiagou Formation exhibit the same variation tendency in SARs, but the mechanisms of astronomical forcing that affect the SARs are different. In addition, the correlation between total organic carbon (TOC) and SARs can be used to investigate the impact of the SAR on organic matter abundance and conclude that relatively high SAR can protect organic matter and prevent oxidation, but rapid SAR can also dilute organic matter abundance of source rock.

Reply to: "Comment on: Linked variations in sediment accumulation rates and sea-level in Guanabara Bay, Brazil, over the last 6000 years by Alberto G. Figueiredo Jr., Mauro B. de Toledo, Renato C. Cordeiro, José M.O. Godoy, Fabiano T. da Silva, Sérgio C. Vasconcelos, Ricardo A. dos Santos [Palaeo 415 (2014) 83–90]", by André Kirchner and Udo Nehren in 2015

Reply to: "Comment on: Linked variations in sediment accumulation rates and sea-level in Guanabara Bay, Brazil, over the last 6000 years by Alberto G. Figueiredo Jr., Mauro B. de Toledo, Renato C. Cordeiro, José M.O. Godoy, Fabiano T. da Silva, Sérgio C. Vasconcelos, Ricardo A. dos Santos [Palaeo 415 (2014) 83–90]", by André Kirchner and Udo Nehren in 2015

Is mercury from small-scale gold mining prevalent in the southeastern Peruvian Amazon?

There is an ongoing debate on the fate of mercury (Hg) in areas affected by artisanal and small-scale gold mining (ASGM). Over the last 30 years, ASGM has released 69 tons of Hg into the southeastern Peruvian Amazon. To investigate the role of suspended matter and hydrological factors on the fate of ASGM-Hg, we analysed riverbank sediments and suspended matter along the partially ASGM-affected Malinowski-Tambopata river system and examined Hg accumulation in fish. In addition, local impacts of atmospheric Hg emissions on aquatic systems were assessed by analysing a sediment core from an oxbow lake. Hg concentrations in riverbank sediments are lower (20–53 ng g−1) than in suspended matter (∼400–4000 ng g−1) due to differences in particle size. Elevated Hg concentrations in suspended matter from ASGM-affected river sections (∼1400 vs. ∼30–120 ng L−1 in unaffected sections) are mainly driven by the increased amount of suspended matter rather than increased Hg concentrations in the suspended matter. The oxbow lake sediment record shows low Hg concentrations (64–86 ng g−1) without evidence of any ASGM-related increase in atmospheric Hg input. Hg flux variations are mostly an effect of variations in sediment accumulation rates. Moreover, only 5% of the analysed fish (only piscivores) exceed WHO recommendations for human consumption (500 ng g−1). Our findings show that ASGM-affected river sections in the Malinowski-Tambopata system do not exhibit increased Hg accumulation, indicating that the released Hg is either retained at the spill site or transported to areas farther away from the ASGM areas. We suspect that the fate of ASGM-Hg in such tropical rivers is mainly linked to transport associated with the suspended matter, especially during high water situations. We assume that our findings are typical for ASGM-affected areas in tropical regions and could explain why aquatic systems in such ASGM regions often show comparatively modest enrichment in Hg levels.

Comment on: “Linked variations in sediment accumulation rates and sea-level in Guanabara Bay, Brazil, over the last 6000 years” by Alberto G. Figueiredo Jr., Mauro B. de Toledo, Renato C. Cordeiro, José M.O. Godoy, Fabiano T. da Silva, Sérgio C. Vasconcelos, Ricardo A. dos Santos [Palaeo 415 (2014) 83–90

Comment on: “Linked variations in sediment accumulation rates and sea-level in Guanabara Bay, Brazil, over the last 6000 years” by Alberto G. Figueiredo Jr., Mauro B. de Toledo, Renato C. Cordeiro, José M.O. Godoy, Fabiano T. da Silva, Sérgio C. Vasconcelos, Ricardo A. dos Santos [Palaeo 415 (2014) 83–90

Palaeogeography and evolution of the Godavari delta, east coast of India during the Holocene: An example of wave-dominated and fan-delta settings

We investigated the Holocene evolution of the Godavari delta along the east coast of India, by analyses of landforms and 11 borehole cores collected from the delta plain during 2010–2013 with 113 14C dates. The Godavari delta covering an area of 5820km2 is one of the world's largest wave-dominated deltas fringed by a ~30km wide beach-ridge plain. Our analysis of sediment facies and their ages indicated that the Holocene marine sequence thickens seaward from ~10m to more than 50m and that the local relative sea level rapidly rose from 43.5m to 7.5m below the present sea level during 10.9–8.0calkyBP followed by a slow rise up to its current level around 5calkyBP. Spatio-temporal variations in sediment accumulation rates and palaeo-shorelines for the last 6ky revealed successive changes of the main depocentre (delta lobe). We recognised six stages of delta evolution with distinct cyclic shifts of delta lobes controlled by avulsions. A linear beach ridge located 12–14km inland from the present shoreline, which separates the first three lobes from the last three, represents the effect of coastal erosion of past cuspate delta lobes, as does the current linear shoreline of the central part of the delta. Our study documents continuous net progradation during the Holocene on millennial to centennial time scales. However, a considerable decrease of sediment discharge due to dam construction and water diversion since the 1970s has exacerbated coastal erosion, resulting in shrinking of the Godavari delta plain and tipping it into a persistent destruction phase.

Magnetostratigraphy, biostratigraphy, and chemostratigraphy of the Pignola-Abriola section: New constraints for the Norian-Rhaetian boundary

A detailed magnetostratigraphic investigation of the Pignola-Abriola section of Norian to Rhaetian age permits the identification of 22 magnetic polarity reversals grouped in 10 magnetozones. We correlate the magnetostratigraphy of the Pignola-Abriola section with the Newark astrochronological polarity time scale (APTS). In total, 19 correlation options were tested, and only one (option 7) yielded a statistically significant correlation that was consistent with the available information on the stratigraphic age of the Newark APTS. After some adjustments to minimize erratic variations in sediment accumulation rates, a final correlation (option 7.1) was used to generate an age model of sedimentation for the Pignola-Abriola section. The Pignola-Abriola section has been correlated with Rhaetian sections from the literature, notably the current global boundary stratotype section and point candidate for the base of the Rhaetian at Steinbergkogel, Austria, where the Norian-Rhaetian boundary is proposed to be placed at a stratigraphic level containing the first appearance datum (FAD) of conodont Misikella posthernsteini , traced on the Newark APTS to ca. 209–210 Ma. Issues regarding the taxonomy of M. posthernsteini , a species characterized by transitional forms with its ancestor Misikella hernsteini , lead us to propose the alternative option of placing the Norian-Rhaetian boundary at a prominent negative δ 13 C org spike observed in the Pignola-Abriola section at meter 44.5, 50 cm below the level containing the FAD of M. posthernsteini sensu stricto and close to the base of radiolarian Proparvicingula moniliformis zone. This level has been magnetostratigraphically correlated to Newark magnetozone E20r.2r at ca. 205.7 Ma. Assuming an age of ca. 201.3 Ma for the Triassic-Jurassic boundary, the Rhaetian Stage would have a duration of ~4.4 m.y.

Linked variations in sediment accumulation rates and sea-level in Guanabara Bay, Brazil, over the last 6000 years

Variable sediment accumulation rates in cores collected in Guanabara Bay, Rio de Janeiro State, indicate that filling of the bay started before 6000years ago and that it was not uniform through time. Comparison of these variable sediment accumulation rates and sea-level fluctuations over the last 5500years suggests a linear relation between them. The sea-level drop between 5500 and 5000years ago, for example, corresponds to higher accumulation rates between 5130 and 4350calendaryearsBP. At this time, the sea-level change from 5 to 2m above present sea-level caused a drop of wave base level and consequently an increase of sediment supply to deeper parts of Guanabara Bay. Between approximately 4350 and 2000–1800years ago sea level became stable with consequent reduction in the accumulation rate. From this time until approximately 500years ago accumulation rates increased again because of another sea-level drop from 2m to the present sea-level. The rate of the second sea-level drop was not as high as the first one, and neither was the accumulation rate increase. Although radiocarbon dates suggest a dramatic reduction of accumulation rates between ~4000–2000years ago, estimates for the last 150years based on 210Pb indicate a progressive increase in the accumulation rates despite the presumed stability and perhaps even a small elevation of modern sea-level in the last 150years as deduced from tide gauge records. Accumulation rates increase from 0.14cm·yr−1 to 0.49cm·yr−1 for the period of 1922 to 1951 and reach 0.60cm·yr−1 from 1963 to 1993. In the last five years accumulation rates double, increasing from 0.60cm·yr−1 to 1.25cm·yr−1. These higher accumulation rates during stable or rising sea-level are likely due to local increases in deforestation, expansion of farming land, dredging of channels, and increased paving of roadways.

Pragian–Emsian successions in Uzbekistan and Bohemia: magnetic susceptibility logs and their dynamic time warping alignment

High-resolution low-field magnetic susceptibility (MS) logs of the Lower Devonian limestone beds of the Zeravshan–Gissar Mountain Region (Uzbekistan in Central Asia) and Prague Synform (Barrandian area, Bohemia, Czech Republic, Europe) were stratigraphically correlated using dynamic timewarping (DTW) alignment techniques that are sensitive to local variations in sediment accumulation rates. A method of adjustment of raw data before the actual DTW alignment was developed in order to eliminate multiple sources on the MS signal, and a successful MS-DTW correlation was performed between the Zinzilban Gorge section in Uzbekistan and the Pozar-3 section in Bohemia. The results basically corroborate some of the most recent biostratigraphic correlations, especially those suggested by P. Carls and J.I. Valenzuela-Rios, and provide a 10- to 100-fold better resolution. The projection of the basal Emsian Global Boundary Stratigraphic Section and Point (GSSP) in the Zinzilban Gorge (Uzbekistan) to the succession of Bohemian strata points to a level in the lower parts of the Lodenice Limestone, i.e., to the lowermost one-third of the Praha Formation. Hence, more than two-thirds of the original Pragian in Bohemia (~Praha Formation) remain above the basal Emsian GSSP level in Uzbekistan. This implies that the decision about this GSSP ratified by IUGSin August 1996 led to a significant reduction of the extent of the original Pragian to only its oldest strata. Many other stratigraphically correlatable levels were identified within the sequences of MS stratigraphic patterns in these two widely separated sections in spite of the fact that the Pragian–Emsian palaeoclimatic conditions of the northern hemisphere were different from those of the southern hemisphere. The base of the Norbonak Beds at Zinzilban corresponds to a stratigraphic level lying ~3.6m below the base of the Zlichov Limestone in the Pozar-3 section. The Bohemian anoxic Graptolite Event correlates to a level about 3m below the boundary of the Zinzilban and Norbonak beds at Zinzilban.

Arctic late Paleocene–early Eocene paleoenvironments with special emphasis on the Paleocene‐Eocene thermal maximum (Lomonosov Ridge, Integrated Ocean Drilling Program Expedition 302)

We reconstruct the latest Paleocene and early Eocene (∼57–50 Ma) environmental trends in the Arctic Ocean and focus on the Paleocene‐Eocene thermal maximum (PETM) (∼55 Ma), using strata recovered from the Lomonosov Ridge by the Integrated Ocean Drilling Program Expedition 302. The Lomonosov Ridge was still partially subaerial during the latest Paleocene and earliest Eocene and gradually subsided during the early Eocene. Organic dinoflagellate cyst (dinocyst) assemblages point to brackish and productive surface waters throughout the latest Paleocene and early Eocene. Dinocyst assemblages are cosmopolitan during this time interval, suggesting warm conditions, which is corroborated by TEX86′‐reconstructed temperatures of 15°–18°C. Inorganic geochemistry generally reflects reducing conditions within the sediment and euxinic conditions during the upper lower Eocene. Spectral analysis reveals that the cyclicity, recorded in X‐ray fluorescence scanning Fe data from close to Eocene thermal maximum 2 (∼53 Ma, presence confirmed by dinocyst stratigraphy), is related to precession. Within the lower part of the PETM, proxy records indicate enhanced weathering, runoff, anoxia, and productivity along with sea level rise. On the basis of total organic carbon content and variations in sediment accumulation rates, excess organic carbon burial in the Arctic Ocean appears to have contributed significantly to the sequestration of injected carbon during the PETM.

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: IX. Summary of paleorecords of environmental change and catchment deforestation at Lake Tanganyika and impacts on the Lake Tanganyika ecosystem

Paleorecords from multiple indicators of environmental change provide evidence for the interactions between climate, human alteration of watersheds and lake ecosystem processes at Lake Tanganyika, Africa, a lake renowned for its extraordinary biodiversity, endemism and fisheries. This paper synthesizes geochronology, sedimentology, paleoecology, geochemistry and hydrology studies comparing the history of deltaic deposits from watersheds of various sizes and deforestation disturbance levels along the eastern coast of the lake in Tanzania and Burundi. Intersite differences are related to climate change, differences in the histories of forested vs. deforested watersheds, differences related to regional patterns of deforestation, and differences related to interactions of deforestation and climate effects. Climate change is linked to variations in sediment accumulation rates, charcoal accumulation, lake level and water chemistry, especially during the arid-humid fluctuations of the latter part of the Little Ice Age. Differences between forested and deforested watersheds are manifested by major increases in sediment accumulation rates in the latter (outside the range of climatically driven variability and for the last ∼40 years unprecedented in comparison with other records from the lake in the late Holocene), differences in eroded sediment and watershed stream composition, and compositional or diversity trends in lake faunal communities related to sediment inundation. Variability in regional patterns of deforestation is illustrated by the timing of transitions from numerous sedimentologic, paleoecologic and geochemical indicators. These data suggest that extensive watershed deforestation occurred as early as the late-18th to the early-19th centuries in the northern part of the Lake Tanganyika catchment, in the late-19th to early-20th centuries in the northern parts of modern-day Tanzania, and in the mid-20th century in central Tanzania. Rapid increases in sediment and charcoal accumulation rates, palynological and lake faunal changes occurred in the early-1960s. We interpret this to be the result of greatly enhanced flushing of sediments in previously deforested watersheds triggered by extraordinary rainfall in 1961/62. Regional differences in deforestation histories can be understood in light of the very different cultural and demographic histories of the northern and central parts of the lake shoreline. Incursion of slaving and ivory caravans from the Indian Ocean to the central coast of Lake Tanganyika by the early-19th century, with their attendant diseases, reduced human and elephant populations and therefore maintained forest cover in this region through the late-19th to early-20th centuries. In contrast, the northeastern portion of the lakeshore did not experience the effects of the caravan trades and consequently experienced high human population densities and widespread deforestation much earlier. These studies demonstrate the importance of paleolimnological data for making informed risk assessments of the potential effects of watershed deforestation on long-term lake ecosystem response in the Lake Tanganyika catchment. Differences in sediment yield and lake floor distribution of that yield, linked to factors such as watershed size, slope, and sediment retention, must be accounted for in management plans for both human occupation of currently forested watersheds and the development of future underwater reserves.

Comparative geochemistry of cadmium, rhenium, uranium, and molybdenum in continental margin sediments

The concentrations of authigenic phases of Cd, Re, U, and Mo increase with depth in four 45-cm-long sediment box cores collected along the axis of the Laurentian Trough, Gulf of St. Lawrence. Average authigenic accumulation rates, estimated from element inventories, are similar to rates in other continental margin environments. Strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of Cd and Re. This suggests that slow precipitation kinetics controls the accumulation of Cd and Re in these sediments. The accumulation rate of authigenic U is more variable; it may be tied to the kinetics of microbially mediated U reduction and be controlled by the availability of reactive organic matter. Authigenic Mo is distinguished by a sharp subsurface concentration minimum, above which Mo cycles with manganese. Mo released to pore water upon reduction of Mn oxides diffuses downward and enriches the subsurface sediment. Mo accumulates most rapidly in the sediment with the highest sulfide content. Slow conversion of molybdate to thiomolybdate may explain the much slower Mo accumulation rate in the less sulfidic sediments. A component of authigenic Mo accumulates with pyrite in an approximately constant Mo:Fe ratio. The accumulation rate of pyrite and associated Mo is insensitive to AVS abundance. Pyrite formation may be limited by the reactivity of iron oxide minerals.

The use of magnetic susceptibility and trace element geochemistry for the correlation of fine-grained siliciclastic sequences: a Late Llandovery example from northwest England

High-resolution lithological and magnetic susceptibility logs were made of two sections of the Upper Llandovery Browgill Formation in northwest England: Stockdale Beck, the type section of the Browgill Formation, and Spengill. The Browgill Formation is composed of fine-grained deep marine siliciclastics, which can be divided broadly into two facies: a homogeneous grey mudstone, deposited under oxygenated bottom-water conditions, and subordinate beds of laminated, graptolite-bearing black mudstone deposited under low bottom-water oxygen levels. The latter facies is often partially or fully diagenetically altered to chlorite nodules, occasionally with manganese carbonate nucleii. Magnetic susceptibility logs are shown to reflect variations in the illite–chlorite ratio of the clay mineralogy. Chlorite is a paramagnetic mineral, so the bands of chlorite nodules produce magnetic susceptibility highs. Correlation demonstrates that diagenetically altered beds of laminated black mudstone are continuous between the two sections, now situated 32 km apart. This lateral continuity would favour pelagic fallout in preference to gravity flow as a depositional model for both facies. A correlated sequence within the turriculatus Biozone is 3.8 times thicker at Spengill than at Stockdale Beck, probably reflecting variations in sediment accumulation rates. Correlation also identifies significant non-sequences in both sections: at least 47% of the turriculatus Biozone (including maximus Sub-biozone) is missing at Stockdale Beck and at least 77% of the crispus Biozone is missing at Spengill. The identification of non-sequences may be of value for refining graptolite biostratigraphy.

Distribution and Abundance of Macroborers in an Upper Miocene Reef System, Mallorca, Spain: Implications for Reef Development and Framework Destruction

The importance of boring organisms within reef environments, as agents of framework destruction, and thus as controls to rates of reef accretion, has been widely recognised. Relatively little, however, is known about variations in the distribution of macroborers (principally sponges, bivalves and worms), within individual reef systems. The Upper Miocene reefs of Mallorca present a unique opportunity in which to assess such distributional variations. Borings are preserved as casts within the molds of former corals, and these trace fossils typically exhibit highly characteristic morphologies. Based on the distribution of different bore morphologies, it has been possible to assess variations in both the distribution and abundance of individual borers, and the intensity of boring activity across the reef system. Different parts of the reef are characterised by different associations of boring organisms, with the highest diversity of borers, and the most intense boring activity, observed within corals from the lagoonal facies. The principal control on distributional patterns appears to have been the morphology and structure of the reef framebuilders. Massive corals are always more extensively bored, and typically contain a higher diversity of boring organisms than branched or platy corals. On a more local scale, variations in sediment accumulation rates, and degree of substrate encrustation appear to have exerted some degree of control. Rates of reef accretion would have been influenced not only by the abundance of different boring organisms in different parts of the reef, but on the response of different framebuilders to boring activity. Platy and branched corals, whilst typically less intensely bored, would have been more vulnerable to the effects of boring, while massive corals, which were often extensively bored, were more capable of surviving such damage. This preservational bias has important implications for interpreting styles and rates of reef accretion.

Late holocene paleoclimatic and paleobiologic records from sediments of Devils Lake, North Dakota

An 84 cm sediment core collected from the center of Devils Lake, North Dakota, was analyzed at 1-cm intervals for, 21°pb, 137Cs, sediment conductivity, the concentrations of, biogenic silica, total organic carbon, carbon to nitrogen ratio, and the carbon and nitrogen isotopic composition of tbe organic fraction. Variations in Z1°Pb activities in the upper 20 centimeters indicate that sediment accumulations rates in Devils Lake are not constant, and that accumulation rates were highest during periods of high lake level. The mean sedimentation accumulation rate was calculated as 0.24 cm -1 yr. The 137Cs profile is characterized by near-surface maximum concentrations, possibly the result of redistribution of 137Cs during salinity excursions. Biogenic silica is strongly correlated to lake level in Devils Lake. Periods of low lake level (characterized by high sediment conductivity) correspond to low biogenic silica concentrations. The trends in biogenic silica are attributed m variations in diatom productivity in the lake and to variations in sediment accumulation rates. Based on biogenic silica content and the composition of organic matter in the sediment (total organic carbon, carbon:nitrogen ratio and the 613C and dlSN composition of total organic matter), paleobiologic conditions of Devils Lake during low lake stands were characterized by, (1) decreased primary productivity, (2) decreased input of detrital organic matter, and (3) increased nitrogen availability. During the 350 years of sediment accumulation represented by the 84-cm sediment core Devils Lake has experienced two periods of sustained high lake level; one between about 130 and 170 years ago (1820 to 1860 A.D.) and the second between 270 and 310 years ago (1680 to 1720 A.D.). Devils Lake experienced a period of intense drying about 260 years ago (1720 A.D.).

Pliocene and Pleistocene sedimentation in south-central Colorado: Opportunity for climate studies

A well-dated Plio-Pleistocene stratigraphic sequence, ranging in age from 2.6 to 0.68 Ma, has been recovered from the Alamosa Formation in the San Luis Valley, south-central Colorado. The sequence is dated by magnetostratigraphy tied into the Huckleberry Ridge (2.02 Ma) and Bishop ashes (0.74 Ma). Sediments, which were primarily deposited by meandering streams, exhibit intermittent modification by partial soil development. Aggradation was relatively continuous, with only minor variations in sediment accumulation rate. Samples of sediments deposited from 0.9 to 0.7 Ma are from surface outcrops. Classic paleontological techniques have been used to identify evidence of climatic cycles in these sediments. The climatic changes can be correlated with deep-sea core Oxygen Isotope Stages 22 through 18. Older sediments, obtained by coring, have been faunally analyzed for vertebrate fossils, ostracodes, and molluscs, florally analyzed through pollen, and geochemically analyzed through both inorganic and biogenic carbonate stable isotopes. The oldest sediments in the core may have been deposited during Oxygen Isotope Stage 110 (which occurs within the Gauss Chron, ∼2.6 Ma). Paleontological and isotopic evidence may eventually provide a long continental climatic record comparable to those from the deep-sea.

Punctuated anagenesis and the importance of stratigraphy to paleobiology

The depositional history of Upper Miocene through Recent sediments from DSDP Site 214 (Ninetyeast Ridge, Indian Ocean) is reexamined. Samples of the Globorotalia tumida planktic foraminiferal lineage, originally obtained from these sediments by Malmgren et al. (1983), serve as the empirical basis for the recognition of punctuated anagenesis as a distinct mode of phenotypic evolution and have been the subject of numerous additional investigations. However, conclusions reached by previous authors depend strictly on the validity of the original chronostratigraphic interpretation of these sediments. Graphic correlation analysis of first- and last-appearance datum levels for a total of 41 planktic foraminiferal, radiolarian, and calcareous nannoplankton taxa provides evidence for a more complex depositional history at this deep-sea site than originally believed. Based on a conservative model of variation in the pattern of sediment accumulation rates, the lowermost portion of the studied section (6.5-4.3 Ma) represents an interval of temporally condensed sediment accumulation (1.88 cm/1,000 yr) followed by an interval (4.3-2.8 Ma) of temporally expanded sediment accumulation (3.97 cm/1,000 yr). This interval, in turn, is followed by a depositional hiatus or an extremely condensed interval, at least 800,000 yr in duration, which is followed by another relatively condensed (1.36 cm/1,000 yr) interval from 2.0 Ma-Recent. Although this chronostratigraphic reinterpretation deviates substantially from the original, which recognized Site 214 as being both temporally continuous and exhibiting a constant sediment accumulation rate from the Upper Miocene through the Upper Pliocene, it is more consistent with expectations based on Neogene eustatic sea-level fluctuations and global surveys of Neogene hiatus distributions. Age assignments for samples of the Gr. tumida lineage based on the revised chronostratigraphic model reverse some findings of previous investigators with respect to the distinctiveness of phenotypic evolutionary rates characterizing the transition from Gr. plesiotumida to Gr. tumida. Finally, a brief survey of similar marine invertebrate lineage studies shows that changes in the rate of phenotypic evolution often appear to coincide with major physical changes in the paleoceanographic environment. Such correspondences may be due, at least in part, to the effect of these environmental changes on sediment accumulation rates. Paleobiologists who seek to understand patterns of phenotypic change over time must remove the effects of variations in sediment accumulation rates from their data before evolutionary hypothesis testing and remain aware of the limitations imposed on their interpretations by the uncertain nature of chronostratigraphic inference.

Variations in Sediment Accumulation Rates and the Flux of Labile Organic Matter in Eastern Lake Superior Basins

Benthic environments in Lake Superior are variable and dynamic. The complex topography and morphology of eastern Lake Superior results in a variety of depositional environments, from shallow coastal bays to deep profundal basins. Direct observations of bottom types and collection of sediments across the gradient in environments reveal benthic systems that range in sediment accumulation rates from ∼1.7 mm per year to virtually zero net long-term accumulation. Diagenetic modeling of organic carbon and nitrogen under steady state conditions indicate that 15% to 40% of the organic carbon and nitrogen deposited is recycled into the overlying water. This “metabolizable” fraction has a residence time in the sediment column estimated to be 30 to 60 years for organic carbon and 15 to 30 years for nitrogen. Concentration gradients of total dissolved inorganic carbon, ammonium, and oxygen in sediment pore waters yield fluxes of remineralized carbon and nitrogen across the sediment-water interface consistent with diagenetically modeled fluxes. Spatial variability in sediment accumulation patterns, however, is great, particularly in the deep, steep relief basins in the open lake. Here, distances of a few kilometers can radically alter deposition patterns. Evidence of aperiodic bottom currents were observed in the deepest charted basin of the lake as extensive fields of sediment ripples over hard, compacted relict silt and clays. While no currents were observed in these summertime dives, it is assumed that storms during isothermal conditions generate suffficient bottom currents at depth to very effectively scour the bottom. Differences in sediment metabolism appear to be determined by the flux of labile organic matter to the bottom coupled to the physical processes of sedimentation, resuspension, and horizontal transport.