Michael J. S. Tevesz

The Effects of Benthos on Physical Properties of Freshwater Sediments

In this chapter we review the effects of freshwater benthos on the physical (as opposed to chemical) properties of the bottom. Specifically, we will focus our discussion on the effects of macrobenthos (adult length >1 mm) on fine-grained bottoms (sediments that contain approximately 50% by weight silt—clay-sized particles) of lakes and slow-flowing rivers. There are three reasons for this approach. The first is that there is simply too little known about freshwater meio- and microbenthos to merit a review of their effects on sediment properties. More importantly, the macrobenthos are probably the most potent modifiers of sediment properties by virtue of their size relative to sediment grains, their population density, their ability to move through a relatively large volume of sediment, and their feeding and respiratory habits. Finally, we have restricted ourselves to fine-grained bottoms because these comprise the bulk of freshwater lake sediments and because the structure of this sediment is more easily altered than is that of sediments with larger grain size. Some information from slow-flowing river bottoms is included with lakes because their sediments and faunas are very much alike (Hynes, 1970).

Thermal potentiation and mineralogical evolution in the Bivalvia (Mollusca)

The most important factor controlling the timing of Phanerozoic mineralogical evolution in the Bivalvia appears to be thermal potentiation of calcite deposition in colder marine and estuarine environments. Cold temperature has promoted mineralogical evolution in the Bivalvia by kinetically facilitating (potentiating) initially weak biological controls for calcite, thereby exposing their genetic basis to natural selection. Calcite has evolved in bivalve shells for a variety of selective advantages, including resistance to dissolution; resistance to chemical boring by algae and gastropods; reduced shell density in swimming and soft-bottom reclining species; enhanced flexibility in simple prismatic shell layers; and fracture localization and economy of secretion in association with certain foliated structures. Endogenous calcite in bivalve shells varies from biologically induced to weakly and strongly biologically controlled. Biologically controlled calcite generally first appears in bivalve shells as an impersistent component of the outer shell layer, only later, in some groups, expanding to include the entire outer and then part or all of the middle and inner shell layers. The initial stages of mineralogical evolution are shown by certain modern Mytilidae, Veneridae and Petricolidae. In the latter two families, the calcite occurs as conellae in the outer part of the outer shell layer. Calcitic conellae in the inner shell layer of Pliocene Mercenaria are not barnacle plates, as previously indicated, but endogenous calcite comparable in origin to other venerid conellae. Their occurrence in Mercenaria may reflect thermal potentiation of weak biological controls for calcite, as well as local detachment of the secretory mantle epithelium near the pallial and adductor musculature.

The Effects of Size-Selective Feeding by Oligochaetes on the Physical Properties of River Sediments

ABSTRACT Tubificid oligochaetes selectively ingest silt- to clay-sized particles at depth within the substratum, transport them vertically upward through their gut, and deposit them as feces at the sediment-water interface. These activities form three distinct sedimentary layers. The sediment-water interface becomes covered with sand-sized fecal pellets. A silt-clay layer forms directly below this. The third layer is a sandy concentrate that represents the zone of tubificid feeding. The upper, pelletized layer is enriched in water content and organic carbon. The high water content of this layer, its irregular surface, and the low density of the constituent pellets destabilize the sediment surface and increase its erodability. In addition, the coarse-to-fine layered structure of the deposits fo ms distinctive biogenic graded bedding that is a potentially useful indicator of low current velocities and low rates of inorganic sediment accumulation in ancient fluvial environments.

δ18O in mollusk shells from Pliocene Lake Hadar and modern Ethiopian lakes: Implications for history of the Ethiopian monsoon

Abstract Two of the five lacustrine intervals in the largely fluvial Hadar Formation, Afar, Ethiopia, occur in the Sidi Hakoma Member deposited 3.4–3.2 Ma. In a perspective of the δ18O of 11 modern Ethiopian lakes and their shells, the δ18O of the Hadar fossil shells provide a snapshot of the nature of ancient Lake Hadar and Ethiopia’s climate in the Pliocene. Ethiopia’s modern lakes both in the Rift and on the Western Plateau are fed by drainage of Plateau rain with its well established barely negative δ18OSMOW of −1.3‰. Except for the man-made Lake Koka reservoir, all other Ethiopian lakes are isotopically quite positive ranging from +5.4 to +16.0‰, indicating how significant evaporation is in their water budget. Shells from lakes with extant mollusk populations are mostly in isotopic equilibrium with the δ18O and temperature of their lake water. The upper transgressive interval in the Sidi Hakoma Member is the largest one in the Formation beginning at its base with the ‘Gastropod Beds’ beach deposits. Mollusks from shell beds other than the ‘Gastropod Beds’ show more positive and more variable δ18O between shells, with internal variations within shells as much as 7‰. At these times the site must have been underlain by a shallow partially isolated embayment of Lake Hadar which underwent rapid expansions and then contractions by evaporation, within the few year lifetimes of the individual mollusks. The results from the ‘Gastropod Beds’ are of most significance for interpreting the overall paleoclimate at Hadar. Their uniformly negative δ18OPDB shell values that average −6.7‰ represent a much less evaporated stage of Lake Hadar when its δ18OSMOW was 8‰ lower than the spectrum of modern lakes in Ethiopia, and indeed even 3‰ or more lower than average modern Plateau rain. To explain such negative values we hypothesize that the Atlantic-derived air mass component to the Ethiopian monsoon was persistently strengthened during Pliocene summers, which intensified the amount and the negative isotopic character of rainfall onto both the Afar and the Ethiopian Plateaus that drained to Lake Hadar. A similar phenomenon characterized the brief periodic pluvial episodes of the Quaternary, including the latest in the early Holocene, known as the African Humid Period. In contrast to the hot semi-desert steppe conditions of today’s western Afar, the diverse abundant terrestrial fossil fauna at Hadar, including the early hominid Australopithecus afarensis, is explained by the wetter, and probably cooler, summers that persisted throughout the Late Pliocene.

Soft-Bottom Succession and the Fossil Record

It is perhaps ironic that the editors of this book would emphasize in their own contribution that certain patterns of biotic interactions found in modern soft-bottom communities are not likely to be preserved in the fossil record and that the interpretation of some preserved patterns is problematical. But this is our conclusion with respect to successions on soft bottoms. After examining both live and dead shelled faunas of nearshore clastic facies, we also conclude that the areal distribution of fossil species cannot be used to establish with certainty the dominant controls of the distribution of living fauna. But these conclusions are provisional, and more subtle and clever analysis may eventually vitiate our pessimism and better explain the causes for some distributional patterns.

Particle Mixing Rates of Freshwater Bivalves: Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae)

Abstract Sediment mixing by freshwater suspension feeding bivalves Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae) was studied by adding illite clay particles with adsorbed I37 Cs as a submillimeter thick layer to the surface of silt clay sediments contained in clear rectangular cells maintained in a temperature regulated aquarium. A Nal gamma detector scanned the sediment column in each cell at 0.2 cm intervals five times over 22 days and recorded changes in I37 Cs activity over time with depth in cells containing 3 A. grandis , 4 S. striatinum , and a control cell containing no bivalves. Sediment mixing by these organisms was diffusional. The diffusion coefficient in the control cell was 0.02 cm 2 /yr, consistent with molecular diffusion of 137 Cs tracer. Whole cell biodiffusion coefficients (D b )for A. grandis and S. striatinum were 0.81–2.11 cm 2 /yr and 0.53 cm 2 /yr, respectively. Adjusting to equal population densities, the 11-27× higher sediment mixing rate of A. grandis was likely due primarily to its larger size. When D b for similar sized organisms was compared, S. striatinum was found to mix sediments at about the same rate as the marine bivalve Nucula proxima but at a 5× lower rate than the freshwater amphipod Diporeia sp. A. grandis mixes sediments 5–14× more slowly than the similar sized conveyor belt deposit feeding marine bivalve, Yoldia limatula .. While deposit feeding organisms are the dominant sediment mixers in the Great Lakes, suspension feeding bivalves can be locally significant.

Oxygen and carbon isotopic composition and shell microstructure of the bivalve Laternula elliptica from Antarctica

Oxygen and carbon isotopic measurements were obtained from microsamples of an adult Laternula elliptica, an infaunal, aragonitic, Antarctic bivalve. The δ 18 O values of samples from the exterior surface are, on average (∼ 4.5‰), similar to calculated values inferred to represent precipitation in equilibrium with ambient environmental conditions. This indicates that bulk samples as well as many microsamples from the exterior surface would provide reliable isotopic estimates of paleotemperatures and paleosalinities. Nevertheless, both oxygen and carbon isotopic values from discrete shell areas may be influenced by vital effects

Geological Significance of Aquatic Nonmarine Trace Fossils

In Chapters 3 and 4 it was shown that benthic invertebrates interact with modern fluvial and lacustrine sediments and, through these interactions, alter the physical, chemical, and biological properties of sediments. In this chapter, the literature documenting invertebrate-sediment interactions in ancient fluvial, lacustrine, and associated terrestrial environments is reviewed in order to show that invertebrate activities influenced benthic processes and properties in these environments in the past. The literature concerning invertebrate traces of nonmarine origin is scant and scattered and has not been the subject of recent review. Curran (1980) recently emphasized the need for a review of this kind. We will attempt to show that aquatic nonmarine trace fossils are abundant and widely distributed (Table I) and are useful for a variety of geological purposes. Further research on these traces will probably improve the resolution of paleoecological and paleoenvironmental reconstructions involving non-marine rocks and increase the understanding of the origin and history of freshwater life.

Primitive life habits and adaptive significance of the pelecypod form

The typical pelecypod form, long thought to be primitively adaptive to burrowing, is likely to have been originally adaptive to a suspension feeding, epifaunal, possibly crawling mode of life. At small body size (< 1 cm), pelecypods possessing typical burrowing features can function as epifaunal crawlers. Pelecypods arose at small body sizes and are part of a molluscan evolutionary sequence in which relative size of the mantle cavity increased to ac- commodate a few large gills specialized for suspension feeding. Acquisition of a bivalved shell by ancestral epifaunal suspension feeders may have offered protection from sediment clogging on soft bottoms, additional control over the direction, volume, and rate of water flow through the mantle cavity, more effective protection from predators, and better short-term control of the internal environment. Consideration of in- vertebrate groups analogous to the pelecypods (Branchiopoda, Cladocera, Ostracoda, Phyl- locarida) support the view that the bivalve condition is primarily an adaptation for suspension feeding and predator avoidance in benthic environments. The earliest known pelecypod, Fordilla troyensis Barrande, was not necessarily infaunal just because it had features similar to much larger, Recent burrowers. The size, shell mor- phology and environment of preservation of F. troyensis all suggest that it is reasonable to envision Cambrian pelecypods as epifaunal suspension feeders, possibly crawling on sedi- mentary bottoms. Moreover, the sudden Ordovician expansion of pelecypods and increase in individual size may be explained as a result of invasion of the infaunal adaptive zone.

Stable Carbon and Oxygen Isotope Records From Lake Erie Sediment Cores: Mollusc Aragonite 4600 BP–200 BP

We present the first δ18O and δ13C data from mollusc aragonite from Lake Erie for the 4.6–0.2 ka (4600 BP to 200 BP 14C yrs) time interval and describe single and composite species isotope trends. Composite species δ18O data show an almost 2.0%o increase from 3.3 to 3.0 ka followed by a nearly 2.5%o decrease at 2.8 ka. Oxygen isotope values then fluctuate by < l‰ until 0.2 ka. This trend in oxygen values is also evident in single species analyses of Sphaerium striatinurn. The most dramatic changes in isotope values, which occur from 3.3–2.8 ka, may reflect a pattern of water level changes in Lake Erie which occurred during the Nipissing flood and its lower water aftermath. Carbon isotope data show progressively more 13C enriched values from 4.6 ka (averaging–6.5‰ PDB) to the present (–0.57‰). This trend may reflect the dilution of isotopically light CO2 from the oxidation of organic matter due to rising lake levels. The short-term increase in δ18O values is coupled with a corresponding decrease in δ13C values. A similar pattern for Lake Erie prior to 10.5 ka was also associated with a lowering of water levels in the lake.