J. Robert Dodd

Paleoecology: Concepts and Applications

Taxonomic Uniformitarianism Biogeochemistry Skeletal Structure Adaptive Functional Morphology Ichnology Taphonomy Populations in Paleoecology Ecosystems and Communities in Paleoecology Paleobiogeography: The Provincial Level Temporal Patterns.

Paleosalinities within a Pliocene Bay, Kettleman Hills, California: A Study of the Resolving Power of Isotopic and Faunal Techniques

Pliocene sediments of the Kettleman Hills, California, were deposited in a marine embayment that occupied part of the San Joaquin Valley. Faunal and sedimentologic data from the upper Pliocene Pecten zone indicate a decreasing salinity trend south and southwestward across the Kettleman Hills. In general, the least diverse faunas and coarsest sediment are in the west-central part of the area, near an apparent small delta and are relatively distant from the entrance of the embayment. Salinity and substrate are probably the main factors controlling the composition of the eight fossil communities recognized by Q-mode cluster analysis. Oxygen and carbon isotopic age determinations were made on specimens of Pecten , Aequipecten , Ostrea , and the fresh-water clam, Anodonta , in order to determine paleosalinities. Assuming that Anodonta grew in water that was representative of the fresh water that flowed into the embayment, that the shells with the heaviest isotopic composition grew in normal marine water, and that the temperature was constant throughout the area at that time, the salinity at which the shells grew can be calculated from their oxygen isotopic composition. The salinities calculated by this procedure range from normal marine (35 ‰) in the north and east parts of the Kettleman Hills to brackish (about 10 to 15 ‰) in the west-central part. These results are in good agreement with those suggested by faunal composition and diversity. A similar but more limited study of the stratigraphically higher upper Mya zone indicates that the salinity during this time was reduced but did not vary in a systematic pattern as during Pecten zone time. Comparison of fossil and modern communities provides the means for detailed paleoenvironmental reconstruction by establishing analogous modern settings. Environmental interpretation, based on isotopic analysis, is similar to interpretations based on faunal analyses but, in addition, provides more quantitative estimates of the paleotemperature and salinity.

Carbon isotopic evidence for photosynthesis in Early Cambrian oceans

Were the first metazoan reefs ecologically similar to modern tropical reefs, enabling them to persist under oligotrophic conditions? We tested the hypothesis of ecological similarity by employing a geochemical approach. Petrography, cathodoluminescence, trace elements, and stable isotope analyses of primary precipitates of the Lower Cambrian Ajax Limestone, South Australia, indicate preservation of original C isotopic composition. All primary carbonate components exhibit C isotopic values similar to the composition of inorganically precipitated fibrous marine cements, suggesting that archaeocyaths and the calcimicrobe Epiphyton precipitated skeletal carbonate in equilibrium with ambient seawater in the absence of vital effects. Such data do not support the contention that archaeocyaths possessed photosymbionts. However, a +0.55‰ shift in δ 13 C occurs in reefs developed under shallower-water conditions relative to deeper reefs. This shift suggests the stratification of primary production in Early Cambrian oceans. The pattern is similar to that seen in the modern ocean, whereby significant photosynthesis modulates the C isotopic composition of the photic zone.

Effect of Late Pleistocene Karst Topography on Holocene Sedimentation and Biota, Lower Florida Keys

Detailed mapping of bedrock topography on Bahia Honda and Big Pine Keys has revealed a buried karst topography not previously documented in the lower Florida Keys. This topography, developed during lowered sea level of the Pleistocene, strongly controls Holocene sediment thickness and present biotic distribution. Circular to oval sinkholes, which are up to 75 m or more in diameter and over 4 m deep, are usually completely filled with peat and carbonate sediment. Sinkholes are well developed on both the Miami Limestone (oolitic facies) and the Key Largo Limestone (both late Pleistocene in age). Thick sediment in buried sinkholes in more than a few inches of water favors the growth of thick patches of turtle grass (Thalassia testudinum). Shallower water and supratidally located sinkholes (that is, those partly or wholly surrounded by subaerially exposed bedrock) are generally marked by thick growths of either red or black mangroves (Rhizophora mangle and Avicennia nitida). These distinct, nearly circular vegetation patterns are extremely abundant in the study area, as shown by aerial photographs which suggest that Bahia Honda and Big Pine Keys are “riddled” with sinkholes.

Diagenetic Stability of Temperature-Sensitive Skeletal Properties in Mytilus from the Pleistocene of California

Skeletal structure, strontium and magnesium content, and mineralogy of the pelecypod Mytilus have been shown to vary with growth temperature and salinity. Based on these skeletal properties, paleotemperatures were determined from fossil specimens of Mytilus from five Pleistocene exposures along the Pacific coast. Paleotemperatures determined by the various methods do not always agree, suggesting that diagenesis has differentially modified some of the skeletal properties. As skeletal structure is not easily destroyed by diagenesis, skeletal-structure paleotemperatures can be used to evaluate the extent of alteration of shell chemistry and mineralogy. Strontium content appears to be altered at only one of the sites studied. A small amount of conversion of aragonite to calcite seems to have occurred. Oxygen isotopic composition was also determined for a sample from each locality. When used in conjunction with paleotemperatures determined by the other methods, oxygen isotopic composition of the samples can be used to determine the δO18 value of the water in which the shell grew, yielding information on paleoclimatic conditions. Paleotemperatures for all late Pleistocene localities are near present-day surface water temperature values for the nearby ocean. The paleotemperature data and oxygen isotope data suggest an interglacial origin for all late Pleistocene localities. The strontium content and δO18 value for the early Pleistocene (upper Merced Formation) locality indicate a lower temperature and slightly reduced salinity.

Paleomagnetic stratigraphy of Pliocene Centerville Beach section, northern California

Abstract Samples from the upper two-thirds of the approximately 1900 m thick Neogene section exposed south of Centerville Beach on the northern California coast have predominantly reversed detrital remanent magnetism. Fossil evidence suggests a lower Pliocene through lower Pleistocene age for the section. The combined paleomagnetic and fossil data indicate that a large part of the section was deposited during the Matayama reversed epoch (2.4-0.7 million years ago). Samples from correlative sections exposed a few kilometers inland from the Centerville Beach section have a predominantly normal polarity and appear to have been remagnetized. The Centerville Beach section is important because it may serve as a standard with which to compare both other on-land Pliocene sections from western North America and nearby Deep Sea Drilling Project cores.

Teichichnus pescaderoensis—New ichnospecies in the Neogene shelf and slope sediments, California

SummaryTeichichnus pescaderoensis is a new ichnospecies from the Neogene of central and northern California. It is approximately an order of magnitude larger than previously describedTeichichnus species. It occurs in Pliocene and Pleistocene siltstone and very fine sandstone that was probably desposited on the upper slope to outer shelf, but ranging into the middle shelf.ZusammenfassungTeichichnus pescaderoensis wird als neue Ichnospezies aus dem Neogen von Zentral-und Nordkalifornien beschrieben. Die Art ist wesentlich größer als die bisher beschriebenen Arten vonTeichichnus. Die neue Art kommt in Silt- und Sandsteinen des Pliozäns und Pleistozäns vor, die vermutlich im oberen Hangbereich des äußeren Schelfs abgelagert wurden, wobei die Sedimentation auch noch in den mittleren Schelfbereich gereicht haben könnte.

Possible Living Analog of the Pleistocene Key Largo Reefs of Florida

A living, linear reef about 3.8 km in length has been discovered 0.8 to 1.2 km off Big Pine Key and the Newfound Harbor Keys in the lower Florida Keys. The reef is approximately 60 m wide, lies at depths from 4.5 to 7 m, and is growing on, and slightly landward of, a slope where Pleistocene bedrock rises with relative abruptness. A deeper, flat area on the seaward side of the reef is, in most places, a Thalassia - Cymodocea (sea grasses) meadow which is usually separated from the reef by a narrow, barren belt of fine-grained sediment. The outer portion of the reef consists of small, scattered coral heads and octocorals separated by a patchy veneer of sediment. The main body of the reef includes large, commonly coalescent coral heads, some standing at least 2.5 m above the sea floor. Next landward is a zone of smaller coral heads and abundant octocorals, followed by an area of large heads of Montastrea annularis . The previously known patch reefs off the Newfound Harbor Keys lie farther landward in shallower water. Although the sediment veneer is thin across much of the linear reef, accumulations of skeletal sand and gravel up to 50 cm thick, are common in areas of greatest coral growth. Geometry and biota of this reef, as well as its location subparallel to the present shoreline, suggest that the reef may be a living analog of at least a part of the Pleistocene Key Largo Limestone.

Depositional history of a Mississippian crinoidal mound on the east flank of the Illinois basin

The middle Mississippian Stobo buildup, Edwardsville Formation (Borden Group), Monroe County, Indiana, was deposited as a broad mound with low relief in an interdistributary area near the Borden delta platform edge. Echinoderm skeletal debris comprises most of the mound, which is similar to several other mounds in the Edwardsville Formation of southern Indiana. Six distinct facies occur in rocks comprising and surrounding the Stobo mound. Echinoderm rudstone and echinoderm packstone were deposited under relatively low-energy conditions with the locally derived skeletal grains. Echinoderm grainstone was deposited under higher energy conditions, perhaps during storms. Dolomitic echinoderm wackestone and mudstone were deposited in low-energy areas on the mound surface. Intertonguing siltstone was deposited when silt deposition temporarily overtook carbonate deposition near the mound margin. Echinoderm-bearing silstone was deposited when silt buried the uppermost echinoderm-bryozoan meadow. Progradation of the Borden delta eventually covered the mound with silt. Stobo mound is part of a spectrum of Mississippian buildups which range from the large, relatively deep-water, mud-rich, classical Waulsortian mounds to smaller, grain-dominated, platform-depth mounds such as Stobo and other Borden mounds.

Oxygen isotopic composition of Neogene molluscan fossils from the Eel River Basin of California

Temporal changes in the oxygen isotopic composition of specimens of fossils (mainly the bivalves Macoma and Pecten ) from four sections of Neogene rocks exposed in the Eel River Basin of northern California resulted from changes in temperature and global glacier volume. The temperature was higher (probably 10–15 °C) during the early history of the basin than later. The temperature was probably 5–10 °C during much of the Plio-Pleistocene and showed no well-defined trends, even though the depth of deposition decreased considerably during that time interval. This uniformity was due to strong upwelling of deep water that kept the shallow-water temperatures low. Samples from shallow-water and relatively inland settings at Holmes and Garberville have lower 18 O/ 16 O ratios than do samples from the other two sections. This is because of either higher temperature of these shallow waters or dilution of the sea water by isotopically light fresh water. The difference in fractionation of oxygen isotopes between calcite ( Pecten ) and aragonite ( Macoma ) fossils potentially provides a means of determining oxygen isotope–based paleotemperatures that is independent of the isotopic composition of the water in which the animals lived. The carbon isotopic composition shows a vital effect and is more variable in the nearshore samples.