Ten cores contain successions of planktonic foraminiferal assemblages and sedimentary components that record late Quaternary climatic changes in the western equatorial Atlantic Ocean. The climatic cooling that began at the end of the last interglaciation (X zone) and continued throughout the last glaciation (Y zone) led to the sequential disappearance of tropical Globorotalia menardii flexuosa , Globoquadrina hexagona , and Pulleniatina obliquiloculata and the incursion of cool-water Globoquadrina dutertrei, Globorotalia truncatulinoides, G. inflata, and Globigerina bulloides . Despite a twofold increase in cool equatorial species, a tropical climate prevailed throughout the last glaciation. Paleotemperature estimates derived by factor analysis and regression techniques indicate only a small (0.1° to 3.6°C) difference between glacial and postglacial winter temperatures. The coldest sea-surface temperatures occurred at about 73,000 B.P. Concurrently, calcareous remains underwent extensive dissolution, which is reflected in the cores by CaCO 3 and coarse-fraction minimums, excessive fragmentation of planktonic foraminiferal shells, absence of pteropods, and an increase in the ratio of benthic to planktonic foraminifera. Faster water-mass circulation during the early part of the last glaciation as compared to the Holocene Epoch is postulated to account for the increased dissolution. Sea-level lowering (approximately 100 m) during the last glaciation and most of the last interglaciation allowed South American rivers to discharge large quantities of terrigenous sediment, which was continuously transported to the continental rise and abyssal plains by gravity-controlled sediment flows. Sea-level rise during the Holocene Epoch shut off the terrigenous-sediment supply to the deep sea, and the continental rise and abyssal plains became a regime of pelagic sedimentation. Prior to the Holocene Epoch, the supply of terrigenous sediment was shut off only for a brief period at the beginning of the last interglaciation. This indicates that a warm (interglacial) period similar to the Holocene Epoch occurred at the beginning of the last glacial-interglacial cycle. Comparison of the timing of calcium-carbonate fluctuations of the cores with the timing of stadial-interstadial periods inferred from the continental stratigraphy of the eastern Great Lakes region reveals an excellent correlation. Interstadial (warm) periods correlate with carbonate maximums, whereas stadial (cold) periods correlate with carbonate minimums.

Intensification of the North and South Equatorial Current systems and trade winds occurred during glacial periods, according to a comparison of late Holocene (interglacial), 18,000 B.P. (glacial), and late Quaternary (0 to 180,000 B.P.) faunal assemblages and sea-surface temperature estimates from the equatorial Atlantic and Caribbean regions. Faster circulation of the North Equatorial Current system in glacial Northern Hemisphere winters (February) is indicated by increased upwelling of cool (15°C) water off northwest Africa and slightly cooler conditions across the northern tropical Atlantic and Caribbean. Intensification of the South Equatorial Current occurred along the Equator during the Southern Hemisphere winter (August). This interpretation is based on the dominance of a cool-equatorial assemblage, which indicated that waters of 16° to 18°C replaced the tropical assemblage that lives today in 24° to 26°C water in this region. The cool influence of the glacial (August) Benguela-South Equatorial Current decreased rapidly westward along the equatorial belt so that the fauna was dominated by the tropical assemblage in the Caribbean. Sea-surface temperatures increased rapidly from east to west in the equatorial belt, so that at long 35°W, the 16°C water had reached ambient temperatures of 24° to 26°C. Both faunal assemblages and temperature estimates of eight late Quaternary Atlantic and Caribbean sediment cores show that the equatorial region experienced three maximum incursions of cool Benguela Current water during the past 150,000 yr—at approximately 135,000 B.P., 73,000 B.P., and 18,000 B.P. Differences of glacial to interglacial sea-surface temperatures range from 5° to 10°C in the eastern equatorial Atlantic to 2° to 3°C in the western Atlantic and the Caribbean. During this time, only two periods with similar faunas and surface temperatures occurred—today and 125,000 B.P. Seasonal temperature contrast (August to February) is three to four times greater in all cores for glacial conditions than for interglacial conditions. The winter temperatures (February to the north of the thermal equator and August south of it) show the greatest changes, and they control the overall temperature pattern. Identical temperature patterns for cores affected by the North and the South Equatorial Currents suggest that the Northern and Southern Hemispheres are generally in phase and that more severe winters control the glacial temperature pattern.

ABSTRACT The physicochemical parameters needed for the calculation of the degree of saturation of calcium carbonate in deep sea from the chemical data measured at ambient barometric pressures are reviewed. When the alkalinity and total carbon dioxide concentrations are used to calculate total CO 3 = species, the result is not sensitively affected by the choice of the available sets of the apparent dissociation constants for carbonic and boric acids in sea water determined by Lyman (1957), Hansson (1973a), and Mehrbach and others (1974). The apparent solubility products of calcite in seawater determined by Ingle and others (1973) at 1 bar total pressure are preferred over those determined by McIntyre (1965), because of the possible disequilibrium conditions existing in McIntyre’s experiments. An inconsistency of an order of 10 cm 3 /mole for the partial molal volumes of Ca ++ and/or CO 3 = are present among the available data. This would cause an uncertainty of about 15 percent in the degree of saturation calculated for a pressure of 500 bars (or a 5,000 meter depth). The degree of saturation of calcite in the Atlantic and Pacific Oceans has been calculated from about 4,000 sets of the alkalinity and total CO 2 measurements obtained during GEOSECS. It has been found that the calcite compensation depths obtained by Berger and Winterer (1974) coincide with the level of 75 and 65 percent undersaturation respectively in the Atlantic and the Pacific Oceans with an exception of the southern extreme of those oceans. In the southern extreme (~ 60° S), the calcite compensation depths become shallower coinciding with a level of 90 percent undersaturation. On the basis of the rates of dissolution of calcite in seawater determined by Morse and Berner (1972) and the calculated pH values at in situ pressure and temperature conditions, the dissolution rates for calcite at the calcite compensation depths have been estimated. General agreement between the dissolution rates thus obtained and the carbonate productivity in the surface water suggests that the calcite compensation depth in the oceans is controlled by the biological productivity of carbonates in the surface water and the degree of undersaturation of calcite in seawater which, in turn, regulates the rate of dissolution. Specific areas of studies needed to be investigated are summarized.

ABSTRACT Progressive structural breakdown of planktonic foraminiferal shells was obtained by three types of laboratory dissolution experiments. First, individual specimens of Globorotalia truncatulinoides and Orbulina universa were dissolved in sea water baths at a pH of 6.5. Percent dissolution, determined by weight loss, ranged from a few percent to 83 percent. Second, a single specimen of G. truncatulinoides and a suite of specimens of Pulleniatina obliquiloculata were subjected to repeated exposures in an acetic acid bath at a pH of 6. Third, four aliquots of a death assemblage of planktonic foraminifera were dissolved to varying degrees in the pH-stat of Morse (1974) at a pH of 7.53 at 5°C. Changes in the species composition were noted with increasing dissolution. The sequential breakdown of shell ultrastructures in each experiment was observed in a scanning electron microscope. For G. truncatulinoides, this breakdown was similar in each experimental method and closely resembles those occurring in nature. Tests were carried out to determine specimen orientation during laboratory dissolution. It was observed that G. truncatulinoides assumed a preferred spiral-side resting position after being stirred in a cylindrical container. In some of our experiments this preferred orientation determined the final shell morphology by selectively exposing certain parts of the shell to dissolution. Natural species assemblages from four surface sediment samples of various depths in the west-central North Atlantic reflected changes in species composition caused by natural dissolution. However, the trends of relative resistance to dissolution of individual species in these natural assemblages showed only minor agreement with the trends of species dissolved in the laboratory. The effects of natural dissolution was studied for Pulleniatina obliquiloculata, because this species is highly dissolution-resistant and possesses a relatively homogeneous surface texture (cortex) in the adult stage. This species is potentially useful as an indicator of the degree of dissolution of planktonic foraminiferal assemblages on the sea floor.

ABSTRACT Experimental dissolution of Globigerinoides ruber , Globigerinoides trilobus , Globorotalia menardii , and Orbulina universa from core top sediments in the Caribbean Sea are being conducted by reaction with a sodium acetate-acetic buffer solution of pH 6.1 at atmospheric pressure and room temperature. Preliminary results suggest that the foraminifera tests are attacked by the solution primarily from the outside surface such that layers of calcite are successively destroyed. SEM micrographs of successively “stripped” layers indicate that a finely crystalline, smooth layer forms the outer shell and that a more coarsely crystalline layer underlies this surface layer. Ca/Mg analysis of the buffer solutions after successive intervals of dissolution indicate that the calcite dissolving first is richer in Mg than the calcite which dissolves last. δO 18 concentrations of undissolved and dissolved tests are not significantly different. Dissolution of foraminifera in size fractions greater and less than 250 microns indicates that the small-size fraction dissolves significantly faster than the coarser one. These laboratory results are compatible with observations of the selective dissolution of foraminifera in Caribbean core P6304-8.

ABSTRACT The problem of whether planktonic forams and associated microfossils dissolve during settling or on the sea floor was investigated in the eastern tropical Pacific Ocean. Net tow samples, from 3,000 meters depth and deeper, showed excellent preservation of most microfossils. Relatively rapid-settling forams (> 150 μ ) were well preserved showing only minor etching, whereas effects of dissolution were noticeable in the finer fraction. Whole pteropod tests were common, in part showing considerable etching. Several tows collected abundant large centric diatom frustules, the valves being generally intact and the preservation excellent. Box cores recovered assemblages of foraminifera at the sediment-sea water interface at depths well below the regional calcite compensation depth. The assemblages contained a wide range of preservation states, that is, delicately spined individuals having experienced no dissolution as well as keel fragments of Globorotalia species being the final stages of solution of some of the most resistant species. Such mixed assemblages are expected if dissolution occurs mainly on the sea floor. The results from deep net tow and box core samples indicate that larger forams and pteropods experience little solution during settling: solution occurs mainly at the sediment-sea water interface. Estimates of the average residence time of foram tests on the sea floor before disintegration yield a value on the order of one month for the area studied.

ABSTRACT The foraminiferal lysocline, defined as the boundary zone between well-preserved and poorly preserved foraminiferal assemblages on the sea floor, is not by necessity a level of accelerated dissolution or “lysocline” in a general sense, such as found by Peterson (1966). The foram lysocline has the properties of a compensation depth, in that it can exist independently of the particular shape of a dissolution profile as long as rates increase with depth. There is considerable evidence for pronounced dissolution above the foram lysocline in the area just south of Peterson’s experiment. Detailed stratigraphic work and quantitative modeling of progressive dissolution of foram assemblages is necessary to establish to what degree the hydrographic lysocline (Peterson’s level) leaves a trace on the sea floor, in the form of a sedimentary lysocline; that is, a marked decrease in the rate of sedimentation.

ABSTRACT The distribution of coccoliths in surface sediments of the Pacific is greatly influenced by dissolution processes. Etching, fragmentation, and differential removal are obvious from about 3 km depth downward, and increase rapidly below about 4 km depth. Overgrowth is observed on some placoliths in samples at intermediate stages of dissolution. Cluster analysis defines groups of varying preservation aspects in tropical waters, in the central gyre, and at high latitudes. Dissolution rankings for tropical and extratropical regions are established using pairing analysis. The coccolith lysocline is difficult to define, but can be recognized near 4,000 m depth as a considerable drop in diversity of assemblages with respect to the solution resistance of their members. A comparison of dissolution aspects of coccoliths and forams shows that coccolith dissolution indices are sensitive above the lysocline and foram dissolution indices are sensitive below the lysocline.

ABSTRACT Suspended coccoliths are abundantly distributed throughout the water column down to 5 km along a meridional aphotic water profile of the central North Pacific Ocean. Their state of preservation varies from unaltered to slightly etched at all depths. This observation contradicts the expectation that coccoliths falling from the photic layer should dissolve and disappear in the calcite-undersaturated part of the water column. The species composition of coccolithophore assemblages in the overlying productive layer was reflected throughout the aphotic water column with no significant change. Electron microscopic study of fecal pellets from grazers collected at depth by a sediment trap as well as laboratory experiments feeding cultured coccolithophores to small zooplankton suggest that fecal transport is an efficient process removing the coccoliths produced in surface waters directly to the deep-sea floor. The suspended coccoliths can be classified into two categories; the free-falling coccoliths from the surface productive layer which are the result of shedding from host coccolithophores while they are living, and the suspended coccoliths replenished at any depth by spilling out from the host fecal pellets while they descend. The type 1 coccoliths probably will be dissolved as soon as they pass the calcite saturation depth. The majority of the type 2 coccoliths may eventually dissolve at under-saturated depths before descending very far, but are widely distributed throughout the deep-sea column at least temporarily. Thus the suspended coccoliths may play a very small role but the direct and rapid transport of coccoliths via fecal pellets appears to be the main channel in carbonate ooze sedimentation on the deep sea floor.

ABSTRACT Deep-sea sediments from the eastern equatorial Atlantic indicate that dissolution of carbonate was greater during the late Pleistocene glacial stages than at present or during the last interglacial stage. Surface (Holocene) carbonate distribution shows a correlation with the 1.9°C potential temperature isotherm which represents the top of Antarctic Bottom Water. A comparison of the surface versus glacial Pleistocene (18,000 YBP) carbonate distribution indicates that the effects of Antarctic Bottom Water were felt 200 to 700 meters shallower during glacial then interglacial conditions. Records of carbonate composition for the past 200,000 years show increases in both carbonate dissolution and terrigenous noncarbonate dilution during glacial stages. Likewise, microscopic examination of the planktonic foraminiferal faunas indicates much greater test fragmentation and destruction and higher percentages of benthonic forams per total foram populations during glacial conditions. The available evidence suggests that Antarctic Bottom Water was the mechanism that produced the increased carbonate dissolution. Therefore, probably either an increase in the production and circulation of Antarctic Bottom Water during glacial stages or the production of a glacial North Atlantic Bottom Water occurred. Regardless of the model used to explain the observations, all of these data suggest that the eastern equatorial Atlantic experienced greater carbonate dissolution during late Pleistocene glacial stages.

ABSTRACT Application of various micropaleontological techniques and δ 18 O stratigraphy to cores from the tropical east Pacific reveals the record of temporal variations in CaCO 3 solution intensity in the area. In general, the solution intensity increased approximately 115,000–65,000 years BP, but according to the location with respect to the lysocline the information registered in the cores varies. Below the lysocline the magnitude of the fluctuations in the solution intensity is large. Near the lysocline the changes are relatively small. The increased solution intensity results in higher relative abundance of resistant planktonic foraminifera, but does not have an apparent effect on the percent CaCO 3 . Above the lysocline effects of increased solution are evident only at the end of the high solution period, and as in the previous case, the magnitude of the fluctuations is small and the percent of CaCO 3 is not affected. Comparison of the solution record of the equatorial Pacific with the record of CaCO 3 accumulation in high latitudes reveals that in general the solution intensity increases when more carbonate is deposited. There are some discrepancies however. The major changes in the solution occur several thousand years after the major changes in accumulation. It is theorized that the solution changes are driven by the variations in accumulation in the high latitudes, produced by climatic change. Since the amount of carbonate available for deposition in the ocean is limited, fluctuations in carbonate accumulation in the high latitudes are compensated by changes in the solution intensity. The timing discrepancy between the cause, climatic change, and the resulting solution may be due to the slow response time of the oceanic carbonate system.