Sedimentary Lysocline Research Articles

Carbonate dissolution revealed by silt grain‐size distribution: comparison of Holocene and Last Glacial Maximum sediments from the pelagic South Atlantic

AbstractThe current issue of global warming and the role of the ocean in global exchange of CO2 increases the interest in solid budgets of marine carbonate production and dissolution. The present study utilizes grain‐size composition of pelagic sediments in order to trace spatial and temporal variability of carbonate sedimentation in the South Atlantic for the Holocene and Last Glacial Maximum (LGM, 19–23 cal kyr BP). A decrease in grain size (e.g. sand content, mean grain size of coarse carbonate silt) indicates increased carbonate dissolution as a result of increased fragmentation of calcareous microfossils. The spatial grain‐size pattern suggests a threshold water depth below which a gradual grain‐size decrease becomes increasingly rapid. This water depth is considered as the sedimentary lysocline. For the Holocene time slice, a constant, gradual decrease of foraminifer carbonate of about 5–10% per 1000 m water depth above the lysocline gives evidence for supra‐lysoclinal dissolution. The water depth of the lysocline for the Holocene is tied to the interface of North Atlantic Deep Water and Antarctic Bottom Water (AABW) (ca 4100 m). Submarine ridges which restrict intrusion of AABW into the Angola Basin cause an asymmetry in carbonate preservation across the Mid‐Atlantic Ridge. The lysocline was reconstructed at ca 3100 m for the LGM. These data suggest that the ca 1000 m rise of the lysocline eradicated the Holocene east–west asymmetry.

Carbonate dissolution and planktonic foraminiferal assemblages observed in three piston cores collected above the lysocline in the western equatorial Pacific

Planktonic foraminiferal assemblages were analyzed in three piston cores recovered from depths above the recent sedimentary lysocline (3.5 km) in the western Pacific Ocean for the purposes of (1) evaluating the effects of carbonate dissolution on fossil assemblages and (2) the impact of dissolution on paleoceanographic reconstructions for the past 300 kyr. We used the perfect test ratio (PTR) of Globorotalia menardii as an indicator of carbonate dissolution. Down-core variations of the PTR indicate that significant dissolution occurred during reglaciation steps represented by isotope stages 7–6, 5–4, and 3–2. The results of principal component analysis (PCA) based on the relative abundances of dominant species of planktonic foraminifera indicate that down-core changes in factor 1 loadings correlate with variations in PTR and with variations in the percentage of coarse fraction (>63 μm) present in the sediments. These correlations indicate that foraminiferal assemblages were apparently altered by dissolution events despite the fact that they were deposited at water depths shallower than the modern lysocline. We also estimated variations in paleo-sea surface temperature (paleo-SST) using the transfer function FP-12E. A negative correlation was found in communality as calculated from PCA and factor 1 loadings; this trend is distinct for the case of communality lower than 0.9. Thus, we conclude that estimated paleo-SST values based upon analysis of planktonic foraminifera are biased by carbonate dissolution.

Carbonate dissolution in the South Atlantic Ocean: evidence from ultrastructure breakdown in Globigerina bulloides

Ultrastructure dissolution susceptibility of the planktic foraminifer Globigerina bulloides, carbonate ion content of the water column, calcium carbonate content of the sediment surface, and carbonate/carbon weight percentage ratio derived from sediment surface samples were investigated in order to reconstruct the position of the calcite saturation horizon, the sedimentary calcite lysocline, and the calcium carbonate compensation depth (CCD) in the modern South Atlantic Ocean. Carbonate ion data from the water column refer to the GEOSECS locations 48, 103, and 109 and calcium carbonate data come from 19 GeoB sediment surface samples of 4 transects into the Brazil, the Guinea, and the Cape Basins. We present a new (paleo-) oceanographic tool, namely the Globigerina bulloides dissolution index (BDX). Further, we give evidence (a) for progressive G. bulloides ultrastructural breakdown with increasing carbonate dissolution even above the lysocline; (b) for a sharp BDX increase at the sedimentary lysocline; and (c) for the total absence of this species at the CCD. BDX puts us in the position to distinguish the upper open ocean and the upwelling influenced continental margin above from the deep ocean below the sedimentary lysocline. Carbonate ion data from water column samples, calcite weight percentage data from surface sediment samples, and carbonate/carbon weight percentage ratio appear to be good proxies to confirm BDX. As shown by BDX both the calcite saturation horizon (in the water column) and the sedimentary lysocline (at the sediment–water interface) mark the boundary between the carbonate ion undersaturated and highly corrosive Antarctic Bottom Water and the carbonate ion saturated North Atlantic Deep Water (NADW) of the modern South Atlantic.

Modeling Cenozoic sedimentation in the central equatorial Pacific and implications for true polar wander

High productivity along the central Pacific equator has left thick deposits of calcareous and siliceous ooze. West of 102°W, these have deposited on the Pacific plate which has been drifting northwards and therefore the thickest deposits are displaced north of the equator. This study attempts to simulate the sediment distribution using information from Deep Sea Drilling Project (DSDP) cores and models for Pacific plate motion. The DSDP core data are used to account for accumulation rate variations with time, paleolatitude and paleodepth, accounting for variable carbonate dissolution. The model predicts the distribution on 35–50 Ma seafloor relatively well and demonstrates that the sedimentary bulge is distorted by variable dissolution over a regional bathymetry gradient due to the northern edge of the South Pacific Superswell. A westerly deepening of the carbonate compensation depth and sedimentary lysocline evolution are proposed, which improve the model for seafloor 30 Ma and older. The degree of match between model and data provides a test of the amount of northward drift of the Pacific plate and, assuming a model for Pacific hotspots motion, of true polar wander (TPW, motion of the hotspot frame relative to the Earth's spin axis). Assuming the Duncan and Clague [1985] model for Pacific hotspots motion, the sedimentary modeling favors TPW models which have little or no effect on the central Pacific paleoequator over the past 30 Ma. We also explore implications of interhotspot motion and the suggestion that the bend in the Hawaiian volcanic chain represents an abrupt change in Pacific hotspot motion within the last 5 Ma.

Variations in Calcite Dissolution Fluxes in the Equatorial Atlantic and Pacific and Evaluation of Carbonate Dissolution Due to Oxidation of Corg

To explain the glacial drop of atmospheric CO 2 it is necessary to understand the conditions for CaCO3 dissolution in the oceans. We measured high resolution profiles of 23~ normalized carbonate in pairs of cores from the Ceara Rise in the Western equatorial Atlantic and from the Ontong Java Plateau in the Western equatorial Pacific. In each case one core (GeoB 1523-1 in the Atlantic and RNDB 74P in the Pacific) is located at a depth above the lysocline and the other core (EW9209 1JPC in the Atlantic and ERDC 127P in the Pacific) at a depth below the lysocline. We then reconstructed (i) the dissolution of CaCO3 due to variations of the depth of the lysocline and (ii) the depth of the sedimentary lysocline during the last 250 kyrs. In the Atlantic we deduced an increase of the lysocline-induced carbonate dissolution during the glacials and even during some of the cooler interglacial substages. The correlation to the Southern Ocean deep water carbonate preservation index (SOPI) (Howard and Prell, 1994) suggests that during the glacials and during the glacial substages of isotope marine isotope stage five (MIS 5) a reduction in the production of NADW and/or an enhanced northern penetration of AABW occurred. On the Ontong Java Plateau we found a dissolution peak at the onset of MIS 6, as well as enhanced lysoclineinduced dissolution in MIS 5 and at the end of the MIS 4. 'Excess' carbonate dissolution takes place at all times on the seafloor even at depths lying above the calcite saturation horizon (Emerson and Bender, 1981). Dissolution has at least two causes. The first one arises from the geochemical environment (e.g. the CO 2concentration in the water column) bathing the sediments. The other one is due to the oxidation of organic matter in the sediments, as proposed by Archer and Maier-Reimer (1994). We evaluated total excess carbonate dissolution in the same way as done by Broecker and Sanyal (1997). In the Atlantic total excess CaCO3 dissolu-

Pacific carbonate dissolution revisited

In order to understand the global CO 2 cycle, it is important to be able to infer past ocean chemistry from the sedimentary carbonate record. Using a simple box model, we show that the modern sedimentary lysocline in the equatorial Pacific appears to be out of steady-state equilibrium with respect to water chemistry, as first suggested by Kier (1984). We attribute large excursions in lysocline depth to changes in carbonate chemistry and/or productivity fluxes. If carbonate ion changes in the atmospheric pCO 2 change; about a quarter of the glacial-interglacial amplitude. We attribute the relative invariance of the depth of the CCD over the past 800 Ka to slow accumulation rates accompanying high dissolution fluxes. Relaxation times of the sediment mixed layer in these highly dissolved sediments are considerably longer than the timescale of glacial-interglacial cycles. A radiocarbon dilemma is manifested by an increase in the age of coretop calcite with water depth (and thus also with dissolution flux) below 4500 m in the equatorial Pacific. We present radiocarbon data from the Atlantic that suggest that this phenomenon is not produced by chemical erosion as has been previously suggested (Broecker et al., 1991). A fraction of the dissolution flux must occur at the sediment-water interface or by an equivalent mechanism.

Climatic change and CaCO3 preservation: An 800,000 year bathymetric Reconstruction from the central equatorial Pacific Ocean

Sixteen deep‐sea cores from the central equatorial Pacific are used to reconstruct a continuous 800,000‐year (800‐kyr) record of bathymetric variations in carbonate preservation as measured by calcium carbonate (CaCO3) content. The depth of the sedimentary lysocline has fluctuated markedly in conjunction with late Pleistocene climate cycles while the carbonate critical depth (CCrD), which is the water depth where the sediments contain 10% CaCO3, has remained relatively constant. As a result, the depth of the lysocline controls the bathymetric position and thickness of the CaCO3 transition zone, defined as the depth from the lysocline to the CCrD. Modern and interglacial‐aged sediments show poor CaCO3 preservation and thick CaCO3 transition zones. Glacial‐aged sediments show good preservation and deep, thin zones due to the deepening of the lysocline. Detailed comparison of the CaCO3 preservation and oxygen isotope records from the central equatorial Pacific confirms the observation that preservation maxima and minima tend to occur during the latter half of glacial and interglacial stages and on climate transitions rather than during the middle of climatic stages. During the nine major glacial stages of the last 800 kyr, the lysocline deepened by at least 400 to 800 m. This deepening indicates an increase in the abyssal carbonate ion concentration ([CO3=]) and a depression of the calcite saturation horizon best explained by the deeper presence of a more carbonate‐saturated water mass. The bottom of the transition zone has remained at a relatively constant depth during the Brunhes Chron, indicating a balance between CaCO3 sedimentation and dissolution in the deepest waters of the central equatorial Pacific.

Carbonate Dissolution on the Muir Seamount (Western North Atlantic): Interglacial/Glacial Changes

ABSTRACT Although the history of carbonate dissolution has been studied for many years, few records of dissolution through an entire glacial cycle are available for the western North Atlantic (North American Basin). Using 14 cores from the Muir and Siboney Seamounts, located about 240 km northeast of Bermuda, changes in carbonate dissolution were examined over the last 125,000 years. The core depth range spans more than 3000 m, 1780 m-4980 m, insuring that major changes in dissolution will be recorded. Weight percent carbonate was plotted as a function of water depth for five stratigraphically defined levels to determine temporal changes in the sedimentary lysocline and calcite compensation depth (CCD). During the Holocene, the lysocline is located between 4050 m and 4350 m; the CCD is not pre ent even in the deepest sample (4980 m). The late glacial ( 18,000 years B.P.) lysocline may have shoaled to about 2500 m, whereas during early glacial time ( 72,000 years B.P) it deepened to about 3400 m. The mid-glacial carbonate maximum ( 60,000 years B.P; interstadial?) is marked by a deepening of the lysocline to about 3800 m. During the entire glacial episode, the CCD aappears to have remained fixed at 4650 m. The last interglacial ( 125,000 years B.P) was a time favorable to carbonate preservation; the lysocline was deeper than 4650 m, the deepest core containing sediments of this age Increased glacial dissolution appears to result from changes in deep circulation. The establishment of the CCD during glacial time is interpreted as indicating increased Antarctic Bottom Water flow. The lysocline, on the other hand, may reflect the degree of stagnation of North Atlantic Deep Water.

Calcite Dissolution: An in situ Study in the Panama Basin

The results of an in situ study of calcite dissolution in the Panama Basin indicate that the rate of dissolution in the water column increases suddenly below a water depth of about 2800 meters. This coincides with the depth at which the calcium carbonate content of surface sediments begins to decrease rapidly or the sedimentary lysocline. Since this level of increased dissolution both in the water column and on the sea floor does not appear to be related to the transition from supersaturation to undersaturation with respect to carbonate, there may be a kinetic origin for the lysocline in this region.

Sedimentation of planktonic foraminifera

Fossil assemblages of planktonic Foraminifera contain many valuable clues to paleoclimate and paleo-oceanography. Unfortunately, our understanding of production, dissolution, redeposition, and other processes of foraminiferal sedimentation is but rudimentary. Lacking direct observations, information largely rests on comparisons between abundance and composition patterns of life-, death-, and sediment-assemblages. Standing stock and production of life assemblages vary greatly, depending on the fertility of the water. Fertility is greatest near continents and along the equator. The shell supply in these fertile regions is several times greater than in the sterile subtropical areas. The distribution of living and empty shells in the water column suggests that virtually all production takes place within the mixed layer, possibly near its base. Size distributions suggest that most formainiferal production becomes food for predators and is not available for sedimentation. Morphology patterns of living and empty shells confirm this suggestion. Thus, very rapid turnover of populations is required to produce the sedimentation rates observed. A residence time of about one week for living Foraminifera larger than 150 μ is proposed for fertile regions, implying life spans of no more than about 2 weeks. Most of the foraminiferal shells reaching the ocean floor are redissolved. Dissolution is complete below the calcite compensation depth (CCD), but much solution takes place above this level also. This fact has long escaped notice, because percentages of calcite are poor indicators of solution patterns. Any reasonable solution profile, even a linear one, would produce a “sudden” decrease of calcite content at the level where the amount of calcite becomes comparable to the amount of non-calcareous dilutants. This is evident from discussion of the formula: L=100(1− Ro R ) where L is the loss of sediment necessary to increase the insoluble fraction R 0 to R percent. (For example, set R 0 = 5%; R = 10%, yielding L = 50%; i.e., 50% of the sediment, or more than half of the carbonate, was lost while the carbonate content went from 95 to 90%!). The same formula yields minimum dissolution in formainiferal assemblages, if resistant planktonic species (or alternatively, benthonic forms) are assumed insoluble. Resulting solution estimates improve understanding of the sedimentary lysocline, a depth level that separates well-preserved from poorly preserved assemblages. Calcite saturation levels inferred from thermodynamic calculations are conceptually and actually different from both lysocline and CCD. While the depth (i.e., pressure) control of solution patterns is obvious, circulation and fertility patterns also influence preservation of foraminiferal sediments. In general, fertile areas tend to have poorly preserved calcareous assemblages, even at shallow depths, because much CO 2 is developed in the sediments. The associated CCD may be quite deep, however, because of high sedimentation rates of calcium carbonate. The ultimate reason for dissolution of shells is that organisms supply more CaCO 3 to the ocean floor than is being introduced from external sources into the oceanic system. This excess supply leads to undersaturation. Excess supply and dissolution rates must balance for geochemical steady state to prevail. Undersaturation, therefore, proceeds to a value where dissolution rates are just right to provide this balance.