Andrea Lini

Correlation of Early Cretaceous carbon isotope stratigraphy and platform drowning events: a possible link?

The Early Cretaceous carbonate carbon isotope record is marked by three positive high-amplitude (> 1.5‰) excursions each covering time spans of more than 106 years. They are of late Valanginian-Hauterivian and early and late Aptian age. In a case study along a transect across the western Tethys Ocean we identified a coincidence between δ13C excursions, black shale formation, and widespread carbonate platform drowning events. We conclude that the δ13C excursions reflect a change in partitioning of carbon between the organic and carbonate carbon sinks which was triggered by climate induced ecological changes in Cretaceous pelagic and neritic environments. Episodes of intensified greenhouse climate conditions led to an increase in weathering, erosion and runoff rates and to elevated nutrient transfer rates from continents into oceans. The resulting increase in oceanic nutrient levels favoured marine phytoplankton production and black shale deposition while conditions for carbonate producing biotas became unfavourable. Partial choking of carbonate production along river influenced coasts resulted in widespread carbonate platform drowning during times of sea-level rise in the Valanginian and Aptian. Widespread contemporaneous black shale deposits and drowned carbonate platforms therefore reflect the contrasting response of the marine organic and carbonate carbon pumps to nutrient-enhanced phytoplankton productivity. The change in marine carbon partitioning is mirrored in a shift of the δ13C record towards more positive values. The transition to the δ13C peak values lasted up to several hundred thousand years. The peaks of the excursions, also covering a time span of up to several hundred thousand years, reflect a new stabilisation of the carbon partitioning between carbonate and organic carbon sinks. A renewed intensification of carbonate sedimentation under mesotrophic conditions was facilitated by stabilisation of the sea-level rise at a high level. Decreasing δ13C values record increasing carbonate carbon burial rates at constant or decreasing organic carbon accumulation rates. These changes contributed to the stabilisation of the marine carbon budget and the global carbon cycle up to millions of years after its initial perturbation.

Millennial-scale storminess variability in the northeastern United States during the Holocene epoch

For the purpose of detecting the effects of human activities on climate change, it is important to document natural change in past climate. In this context, it has proved particularly difficult to study the variability in the occurrence of extreme climate events, such as storms with exceptional rainfall. Previous investigations have established storm chronologies using sediment cores from single lakes, but such studies can be susceptible to local environmental bias. Here we date terrigenous inwash layers in cores from 13 lakes, which show that the frequency of storm-related floods in the northeastern United States has varied in regular cycles during the past 13,000 years (13 kyr), with a characteristic period of about 3 kyr. Our data show four peaks in storminess during the past 14 kyr, approximately 2.6, 5.8, 9.1 and 11.9 kyr ago. This pattern is consistent with long-term changes in the average sign of the Arctic Oscillation, suggesting that modulation of this dominant atmospheric mode may account for a significant fraction of Holocene climate variability in North America and Europe.

10 000 yr record of extreme hydrologic events

Well-dated lacustrine sediments provide a hydrologic record indicating that the frequency and magnitude of runoff events, and by inference, storms, have varied over the past 10 k.y. in northern New England. We used five sediment cores and radiocarbon dating to develop a chronology of Holocene hydrologic events for the Ritterbush Pond basin, northern Vermont. Chemical and physical analyses allow us to identify 52 distinct layers of predominately inorganic sediment that represent terrestrially derived material delivered to the pond by runoff events. The thickness of some layers suggests hydrologic events at least equal in size to, and probably much larger than, any storm or flood recorded during nearly 300 yr of written regional history. Layer thickness and frequency and, by analogy, storm size and recurrence, change through the Holocene. The largest events occurred 2620, 6840, and 9440 calibrated 14C years before present (cal 14C yr B.P.). The most frequent hydrologic events occurred in three periods: 1750 to 2620, 6330 to 6840, and >8600 cal yr B.P. The recurrence interval of layer deposition during stormy periods averages 130 ± 100 cal yr, whereas the recurrence interval during less stormy periods is longer, 270 ±170 cal yr. The Ritterbush Pond event record illustrates the potential of inorganic lacustrine sediment to serve as a proxy record for estimating paleoflood frequency and deciphering climate change.

Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet

Deep Freeze Geologists usually consider glaciers and ice sheets to be gigantic abrasives, scouring the ground beneath them and carving out relief on the underlying landscapes. Bierman et al. (p. 402, published online 17 April) show that this is not always the case. They found that the silt at the very bottom of the Greenland Ice Sheet Project 2 core contained significant amounts of beryllium-10, an isotope produced in the atmosphere by cosmic rays and which adheres to soils when it is deposited on them. Hence, the dust at the bottom of the ice sheet indicates the persistence of a landscape under 3000 meters of glacial ice that is millions of years old. Soil has been frozen to the central part of the bed of the Greenland Ice Sheet for at least 2.7 million years. Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been nonerosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets.

Nonlinearities in Phosphogenesis and Phosphorus-Carbon Coupling and Their Implications for Global Change

The exponential increase in mining of igneous and sedimentary phosphates, and their utilization in agriculture, industry, and the household in the last few decades have lead to a progressive mobilization of phosphorus, which affects the global phosphorus cycle to an increasing degree (Sheldon, 1969, 1982; Stumm, 1973; Lerman et al., 1975). The present-day anthropogenic share in the transfer of reactive phosphorus from sedimentary and igneous reservoirs into the marine and terrestrial biosphere amounts to an estimated 0.4x1012 g P/yr. This number approximates 35% of total phosphorus influx rates into the oceans and, according to Mackenzie et al. (this volume), may balance 10% of the yearly increase in atmospheric CO2 from manmade sources, assuming an average atomic C/P ratio of 250:1, and a complete and permanent storage of the biologically produced carbon (Figure 1; cf. Mackenzie et al.; Meybeck, both this volume).

Reconstructing lake and drainage basin history using terrestrial sediment layers: analysis of cores from a post-glacial lake in New England, USA

Four sediment cores and twenty-five 14C ages from Ritterbush Pond in northern Vermont provide a detailed and continuous temporal record of Holocene lake and watershed dynamics. Using visual logs, carbon content, magnetic susceptibility, stable isotope signatures, and X-radiography, all measured at 1-cm scale, we identify and date discrete layers of terrestrially-derived sediment in the organic-rich, lacustrine gyttja. These inorganic layers range in thickness from <1 mm to >10 cm and range in grain size and sorting from homogeneous silt to graded sand. AMS radiocarbon ages both from macrofossils within the thickest layers, and gyttja bracketing these layers, provide the basis for correlation among the cores, the dating of 52 basin-wide sedimentation events, and the development of a detailed sedimentation chronology for the Holocene.Physical, chemical, and isotopic analyses suggest the inorganic layers are terrestrially derived and result from hydrologic events large enough to erode and transport sediment from the watershed into the pond. The temporal and spatial distribution of the inorganic layers suggests changing basin-wide sedimentation and thus erosion dynamics since deglaciation over 12,000 years ago. Specifically, for intervals lasting 400 to 1000 years, during the early (>8600 cal yBP), middle (6400 to 6800 cal yBP) and late Holocene (1800 to 2600 cal yBP), the Ritterbush Pond watershed eroded more rapidly than at other times and terrestrially derived material poured into the pond. Analysis of Ritterbush Pond sediments demonstrates the potential for North American lakes to preserve a record of drainage basin dynamics.

THE DENDROCLIMATOLOGICAL POTENTIAL OF AN ALPINE SHRUB, CASSIOPE MERTENSIANA, FROM MOUNT RAINIER, WA, USA

Abstract The application of dendrochronological techniques to shrubs found in arctic and alpine plant communities is opening previously untapped regions to the exploration of plant‐climate ecological relationships and climate reconstruction. In this pilot study, we present growth (1963–2004), reproduction (1963–2004), and stable carbon isotope ratio (1975–2004) chronologies for Cassiope mertensiana from a subalpine site in Mount Rainier National Park, Washington, USA. Based on simple linear correlation analysis, positive correlations characterize plant growth and previous year mean maximum temperature in April and June, suggesting the influence of temperature on snowpack and, in turn, on growing season length, plant and soil insulation, and nutrient and moisture availability. Plant growth and reproduction are significantly correlated with current year July mean maximum temperature and total precipitation, indicating the importance of a warm and extended growing season for optimal plant development. Using step‐wise multiple linear regression analysis, we developed a preliminary calibration model for July mean maximum temperature (R=0.63), extending over the 1974–2004 time period. This archive has the potential to elucidate multi‐scale, spatially‐explicit, ecological and climatic information for alpine ecosystems situated along a north‐south transect from the southern Yukon to the Pacific Northwest of the United States.

Multiple Climate Signals Characterize Cassiope Mertensiana Chronologies for a Site on Mount Rainier, Washington, USA

This study investigates the nature and strength of the climate signals that characterize an alpine dwarf-shrub, Cassiope mertensiana (mountain bell heather). For the first time, we present six new C. mertensiana chronologies (two growth, two reproduction, 1963-2004; δ13C, δ18O, 1975-2005) based on 14 plants from a subalpine meadow site on Mount Rainier, Washington, USA. Correlation, factor, and multivariate regression analyses revealed that multiple and different climate factors control the chronologies. Annual growth and leaf production are controlled by previous- and current- year growing season and previous-year February temperature. Warm growing season temperatures positively influence soil nutrient uptake rates, while winter temperatures may temporally alter moisture distribution resulting in reduced plant growth. Annual flower bud production is controlled by previous and current-year mean summer, growing season, and annual dewpoint temperature. Annual flower production is controlled by current-year mean spring, summer, growing-season, and annual dewpoint temperature. Increased atmospheric moisture facilitates open stomata, higher photosynthetic activity, and increased reproduction. Previous-year growing season and annual dew-point temperature, fall relative humidity, and temperature control δ18O values, indicating the influence of stomatal conductance on evaporative enrichment. Previous-year spring dewpoint and annual maximum temperature control δ13C values, suggesting the values are a product of photosynthetic rate controlled by temperature or photon flux.