1,721,017 research outputs found
FIG. 3 in Modern Rhodolith-dominated carbonates at Punta Chivato, Mexico
No full textFIG. 3. — Percent carbonate content of samples (all size fractions combined).Published as part of Halfar, Jochen, Eisele, Markus, Riegl, Bernhard, Hetzinger, Steffen & Godinez-Orta, Lucio, 2012, Geodiversitas 34 (1) on pages 99-113, DOI: 10.5252/g2012n1a6, http://zenodo.org/record/459742
Facies architecture and sedimentology of a meandering fluvial system: a Palaeogene example from the Weisselster Basin, Germany
No full textThe Schleenhain open pit coal mine, located 30 km south of Leipzig, Germany, exposes Upper Eocene and Oligocene non-marine strata representing fluvial deposition in the centre of the Weisselster Basin. Active mining and successive cuts provided the rare opportunity to obtain a three-dimensional perspective of laterally extensive surface outcrops. These were used to construct a detailed fence diagram, which provided the basis for recognition of key architectural elements in the weakly consolidated meandering stream deposits. In addition to the eight basic architectural elements of Miall (1985), the element SL (shallow lake deposits) was newly defined and the element CH (channel) was subdivided into CHg (palaeo-river system) and CHk (small channel). The profiles contain parts of two fining-upward cycles, which are separated by an unconformity spanning the Early Oligocene. Deposits of the first cycle begin with transverse sand bars (downstream accretion deposits-DA) and point bars (lateral accretion deposits-LA). The upper part of the cycle is represented by overbank fines (OF) and the element SL, which consists of laterally discontinuous lenses of dark, plant-bearing, kaolinite-rich clays, that were deposited in shallow lakes adjacent to the active channel. Coal seams interlayered with palaeosols are the main constituents of element OF. Sheetlike bodies of medium to fine gravels (gravel bars and bedforms-GB) on an erosive coal surface mark the beginning of the second cycle. Dissolution of underlying Permian salts and sulphates prior to, during, and after the deposition of the Palaeogene strata caused the development of two synclines within the outcrop. Coal seams and clay horizons which thicken and dip towards the centre of the synclines, provide evidence for their chronological development
Growth and high‐resolution paleoenvironmental signals of rhodoliths (coralline red algae): A new biogenic archive
No full textWe investigated rhodoliths (coralline red algae) from a subtropical locality in the Gulf of California (Lithothamnium crassiusculum) and a subarctic locality in Newfoundland (Lithothamnium glaciale) for their potential as paleoenvironmental archives using microanalytical geochemical techniques to measure variations in delta(18)O, Mg, and Ca. Rhodoliths are potentially well suited as recorders of shallow water paleoenvironmental signals because they (1) have worldwide distribution from the tropics to polar regions, (2) are long lived from decades to centuries, and (3) display well-developed growth bands. Our results indicate that rhodolith growth bands preserve ultrahigh-resolution records of paleoceanographic-paleoclimatic change and likely constitute an important new archive for reconstructing the paleoenvironmental history of littoral-neritic areas in which these algae are found. The delta(18)O content of individually sampled rhodolith growth bands ranges from -2.4 to -4.6 parts per thousand in L. crassiusculum and from -3.2 to -0.3 parts per thousand in L, glaciale. In both cases, the range of delta(18)O values suggests a slightly lower amplitude of variation in sea surface temperature than that actually measured in the ocean at the two study sites. Both L, crassiusculum ann L, glaciale show a negative offset from isotopic equilibrium. Electron microprobe analysis of magnesium and calcium in growth bands reveals cyclic variations with values ranging between 7.7-18.5 mol % MgCO3 in L. glaciale and 13.2-22.5 mol % MgCO3 in L. crassiusculum. In addition, electron microprobe element maps highlight individual growth bands, provide a powerful approach to study rhodolith formation, and indicate that the specimens we analyzed have Vertical growth rates of 250-450 mu m/yr
Coralline red algae as high-resolution climate recorders
No full textMost high-resolution, proxy-based paleoclimate research has concentrated on tropical oceans, while mid- and high-latitude marine regions have received less attention, despite their importance in the global climate system. At present, sclerochronological analyses of bivalve mollusks supply the bulk of annual- to subannual-resolution extratropical marine climate data, even though interpretation is complicated by a slowdown of growth with increasing shell age. Hence, in order to address the need for additional high-resolution proxy climate data from extratropical regions, we conducted the first year-long in situ field calibration of the coralline red alga Clathromorphum compactum in the Gulf of Maine, United States. Coralline red algae are widely distributed in coastal regions worldwide, and individual calcified plants can live continuously for several centuries in temperate and subarctic oceans. Stable oxygen isotopes extracted at subannual resolution from growth increments of monitored specimens of C. compactum relate well to in situ-measured sea-surface temperatures during the May to December calcification period, highlighting the suitability of coralline red algae as an extratropical climate archive. Furthermore, there is a strong correlation between a 30 yr delta(18)O record of C. compactum and an instrumental sea-surface temperature record (r = -0.58,p = 0.0008) and a proxy reconstruction derived from the bivalve Arctica islandica collected in the central Gulf of Maine (r = 0.54, p = 0.002)
Amplification of the Atlantic Multidecadal Oscillation associated with the onset of the industrial-era warming
No full textNorth Atlantic sea surface temperatures experience variability with a periodicity of 60-80 years that is known as the Atlantic Multidecadal Oscillation (AMO). It has a profound imprint on the global climate system that results in a number of high value societal impacts. However the industrial period, i.e. the middle of the 19th century onwards, contains only two full cycles of the AMO making it difficult to fully characterize this oscillation and its impact on the climate system. As a result, there is a clear need to identify paleoclimate records extending into the pre-industrial period that contain an expression of the AMO. This is especially true for extratropical marine paleoclimate proxies where such expressions are currently unavailable. Here we present an annually resolved coralline algal time series from the northwest Atlantic Ocean that exhibits multidecadal variability extending back six centuries. The time series contains a statistically significant trend towards higher values, i.e. warmer conditions, beginning in the 19th century that coincided with an increase in the time series' multidecadal power. We argue that these changes are associated with a regional climate reorganization involving an amplification of the AMO that coincided with onset of the industrial-era warming
Effects of light and temperature on Mg uptake, growth, and calcification in the proxy climate archive <i>Clathromorphum</i> <i>compactum</i>
No full textThe shallow-marine benthic coralline alga Clathromorphum compactum
is an important annual- to sub-annual-resolution archive of Arctic and
subarctic environmental conditions, allowing reconstructions going back
> 600 years. Both Mg content, in the high-Mg calcitic cell walls,
and annual algal growth increments have been used as a proxy for past
temperatures and sea ice conditions. The process of calcification in
coralline algae has been debated widely, with no definitive conclusion about
the role of light and photosynthesis in growth and calcification. Light
received by algal specimens can vary with latitude, water depth, sea ice
conditions, water turbidity, and shading. Furthermore, field calibration
studies of Clathromorphum sp. have yielded geographically disparate
correlations between MgCO3 and sea surface temperature. The
influence of other environmental controls, such as light, on Mg uptake and
calcification has received little attention. We present results from an
11-month mesocosm experiment in which 123 wild-collected C.
compactum specimens were grown in conditions simulating their natural
habitat. Specimens grown for periods of 1 and 2 months in complete darkness
show that the typical complex of anatomy and cell wall calcification develops
in new tissue without the presence of light, demonstrating that calcification
is metabolically driven and not a side effect of photosynthesis. Also, we
show that both light and temperature significantly affect MgCO3 in
C. compactum cell walls. For specimens grown at low temperature
(2 °C), the effects of light are smaller, with a 1.4 mol %
MgCO3 increase from low-light (mean  =  17 lx) to high-light
conditions (mean  =  450 lx). At higher (10 °C) temperature there
was a 1.8 mol % MgCO3 increase from low to high light. It is
therefore concluded that site- and possibly specimen-specific temperature
calibrations must be applied, to account for effects of light when generating
Clathromorphum-derived temperature calibrations.</p
Comunidades de moluscos asociados a ambientes de carbonatos modernos en el Golfo de California
Full text linkImpreso y PDFEl objetivo del presente estudio es determinar los procesos ecológicos y oceanográficos que inciden en las comunidades de moluscos asociados a sedimentos de carbonatos en la costa occidental del Golfo de California. Para la obtención de los datos se seleccionaron cuatro sitios: Cabo Pulmo (23° N), Isla San José (25º N), Punta Chivato (27° N) y Bahía de Los Ángeles (29º N), en donde se recolectaron 64 muestras de sedimento y se utilizaron como indicadores del ambiente a la temperatura, profundidad, clorofila a, transparencia,
salinidad y tipo de grano del sedimento. Se analizó la abundancia y diversidad de la comunidad bentónica malacológica como descriptores ecológicos y se estimó el crecimiento de las especies de mayor valor relativo como indicador de producción de carbonato de calcio. Las comunidades de moluscos en los cuatro sitios de estudio mostraron ser diferentes entre si en cuanto a su composición específica, su distribución se limitó a los fondos someros de arenas gruesas entre los 2 a 30 m de profundidad. El grupo exhibe una variabilidad en el índice de condición tafonómica en los cuatro sitios de estudio, predominando conchas de moluscos en buen estado de conservación. El máximo valor del índice de diversidad de Shannon-Wiener se obtuvo en Punta Chivato con 3.8 bits/individuo y Cabo Pulmo tuvo los menores valores registrados (1.1 y 1.2 bits/individuo). La mayor riqueza específica y
biomasa relativa de los macromoluscos se observó las zonas norte y centro que presentan condiciones eutróficas y mesotróficas en términos geológicos; menor riqueza y biomasa en el sur, donde la mayor riqueza está representada por micromoluscos en ambiente oligo-mesotrófico. En Punta Chivato y en la Isla San José se encontraron los valores más altos de diversidad y presentan similitud en sus condiciones mesotróficas. Los moluscos, a través de sus conchas depositan CaCO3 a los sedimentos en proporción a su densidad; se estimó que individuos de una cohorte de Megapitaria squalida, a una talla máxima aporta 10 g por año de CaCO3 a los sedimentos, mientras que Chione californiensis, 5 g/ año.Instituto Politécnico Nacional. Centro Interdisciplinario de Ciencias MarinasDoctorado en Ciencias Marinasxvi,149 h
Comunidades de moluscos asociados a ambientes de carbonatos modernos en el Golfo de California
No full textImpreso y PD
Arctic sea-ice decline archived by multicentury annual-resolution record from crustose coralline algal proxy
No full textNorthern Hemisphere sea ice has been declining sharply over the past decades and 2012 exhibited the lowest Arctic summer sea-ice cover in historic times. Whereas ongoing changes are closely monitored through satellite observations, we have only limited data of past Arctic sea-ice cover derived from short historical records, indirect terrestrial proxies, and low-resolution marine sediment cores. A multicentury time series from extremely long-lived annual increment-forming crustose coralline algal buildups now provides the first high-resolution in situ marine proxy for sea-ice cover. Growth and Mg/Ca ratios of these Arctic-wide occurring calcified algae are sensitive to changes in both temperature and solar radiation. Growth sharply declines with increasing sea-ice blockage of light from the benthic algal habitat. The 646-y multisite record from the Canadian Arctic indicates that during the Little Ice Age, sea ice was extensive but highly variable on subdecadal time scales and coincided with an expansion of ice-dependent Thule/Labrador Inuit sea mammal hunters in the region. The past 150 y instead have been characterized by sea ice exhibiting multidecadal variability with a long-term decline distinctly steeper than at any time since the 14th century
- …
