Margaret Lois Delaney

Phosphorus geochemistry of equatorial Pacific sediments

Understanding phosphorus (P) geochemistry and burial in oceanic sediments is important because of the role of P for modulating oceanic productivity on long timescales. We investigated P geochemistry in seven equatorial Pacific sites over the last 53 m.y., using a sequential extraction technique to elucidate sedimentary P composition and P diagenesis within the sediments. The dominant P-bearing component in these sediments is authigenic P (61–86% of total P), followed in order of relative dominance by iron-bound P (7–17%), organic P (3–12%), adsorbed P (2–9%), and detrital P (0– 1 %). Clear temporal trends in P component composition exist. Organic P decreases rapidly in younger sediments in the eastern Pacific (the only sites with high sample resolution in the younger intervals), from a mean concentration of 2.3 μmol P/g sediment in the 0–1 Ma interval to 0.4 μmol/g in the 5– 6 Ma interval. Over this same time interval, decreases are also observed for iron-bound P (from 2.1 to 1.1 μmol P/g) and adsorbed P (from 1.5 to 0.7 μmol P/g). These decreases are in contrast to increases in authigenic P (from 6.0–9.6 μmol P/g) and no significant changes in detrital P (0.1 μmol P/g) and total P (12 μmol P/g). These temporal trends in P geochemistry suggest that (1) organic matter, the principal shuttle of P to the seafloor, is regenerated in sediments and releases associated P to interstitial waters, (2) P associated with iron-rich oxyhydroxides is released to interstitial waters upon microbial iron reduction, (3) the decrease in adsorbed P with age and depth probably indicates a similar decrease in interstitial water P concentrations, and (4) carbonate fluorapatite (CFA), or another authigenic P-bearing phase, precipitates due to the release of P from organic matter and iron oxyhydroxides and becomes an increasingly significant P sink with age and depth. The reorganization of P between various sedimentary pools, and its eventual incorporation in CFA, has been recognized in a variety of continental margin environments, but this is the first time these processes have been revealed in deep-sea sediments. Phosphorus accumulation rate data from this study and others indicates that the global pre-anthropogenic input rate of P to the ocean (20 × 1010 mol P/yr) is about a factor of four times higher than previously thought, supporting recent suggestions that the residence time of P in the oceans may be as short as 10,000–20,000 years.

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research

Thirteen laboratories from the USA and Europe participated in an intercomparison study of Mg/Ca and Sr/Ca measurements in foraminifera. The study included five planktonic species from surface sediments from different geographical regions and water depths. Each of the laboratories followed their own cleaning and analytical procedures and had no specific information about the samples. Analysis of solutions of known Mg/Ca and Sr/Ca ratios showed that the intralaboratory instrumental precision is better than 0.5% for both Mg/Ca and Sr/Ca measurements, regardless whether ICP-OES or ICP-MS is used. The interlaboratory precision on the analysis of standard solutions was about 1.5% and 0.9% for Mg/Ca and Sr/Ca measurements, respectively. These are equivalent to Mg/Ca-based temperature repeatability and reproducibility on the analysis of solutions of ±0.2°C and ±0.5°C, respectively. The analysis of foraminifera suggests an interlaboratory variance of about ±8% (%RSD) for Mg/Ca measurements, which translates to reproducibility of about ±2–3°C. The relatively large range in the reproducibility of foraminiferal analysis is primarily due to relatively poor intralaboratory repeatability (about ±1–2°C) and a bias (about 1°C) due to the application of different cleaning methods by different laboratories. Improving the consistency of cleaning methods among laboratories will, therefore, likely lead to better reproducibility. Even more importantly, the results of this study highlight the need for standards calibration among laboratories as a first step toward improving interlaboratory compatibility.

RECONSTRUCTIONS OF UPWELLING, PRODUCTIVITY, AND PHOTIC ZONE DEPTH IN THE EASTERN EQUATORIAL PACIFIC OCEAN USING PLANKTONIC FORAMINIFERAL STABLE ISOTOPES AND ABUNDANCES

In the hydrographically complex eastern equatorial Pacific Ocean (EEP), the distinction between changes in productivity and changes in upwelling is important to the study of the causes and implications of changes in paleoproductivity during the Last Glacial Maximum (LGM). We studied seven EEP coretops representing a gradient of increasing primary productivity from west to east. Comparison of the coretop data indicates calcification depth and temperature for each planktonic foraminiferal species may change depending on the vertical position of hydrographic features such as the degree of stratification of the water column, as well as associated biological parameters such as the depths of the photic zone and the chlorophyll maximum. Because these biological parameters are related to primary productivity, calcification depth and temperature patterns for each species are somewhat different for high and low productivity regions in the EEP. We use the relationship between modern surface hydrography and coretop planktonic foraminiferal abundances and isotopic composition to interpret upwelling and productivity changes in the EEP over the last 20,000 years. While data indicate higher primary productivity and lower SSTs, they do not indicate that there was greater upwelling at the location of our site during the LGM relative to present.

The oceanic phosphorus cycle and continental weathering during the Neogene

Little is known about the history of dissolved phosphorus (P) input to the ocean during the Cenozoic, an important factor in reconstructing global change because of the role of P in controlling net oceanic productivity and organic carbon burial. We present P accumulation rates from the eastern (Neogene) and western (Cenozoic) equatorial Pacific, a region chosen to reflect oceanic P input trends because of its importance in global models of new production and biogenic sedimentation. P accumulation rates range from 5 to 60 µmol P cm−2 kyr−1 and exhibit a positive correlation with mass sediment accumulation rates, calcium carbonate being the major sediment component. The influences of surface productivity patterns, site migration through time relative to equatorial upwelling, and water depth are observed in the P accumulation rate records. These site-specific effects are relatively minor, however, compared to synchronous, significant trends in P accumulation rates in these equatorial Pacific sites. The most notable event occurred in the late Miocene, when mean P accumulation rates exhibited a strong peak at 5–6 Ma, to the highest mean value of the Neogene (37 µmol P cm−2 kyr−1), followed by a sharp decrease to 1–2 Ma (14 µmol P cm−2 kyr−1), after which values increased slightly to the present. These changes in P accumulation rates in the equatorial Pacific translate into substantial changes in P burial; the decrease from 5–6 to 1–2 Ma signified a 62% decrease in P burial in the equatorial Pacific, and the difference is equivalent to 14% of the estimated modern P input rate to the oceans. Some of the late Neogene change in the equatorial Pacific P accumulation rate record may have been due to redistribution of P burial to high-latitude regions. However, on the basis of P mass balance considerations, redistribution alone cannot account for the bulk of the change in P accumulation and burial in the equatorial Pacific. The P accumulation rate record is markedly different from the unidirectional increases in continental weathering rates over the last 40 m.y. interpreted from the Sr isotope record, suggesting a decoupling of nutrient input fluxes from input fluxes of other dissolved constituents.

Trace metals in contemporary and seventeenth-century Galapagos coral: Records of seasonal and annual variations

We report trace element/calcium ratios for modern (CuCa, MnCa, CdCa, and PbCa) and seventeenth-century (CuCa, MnCa, and PbCa) specimens of Pavona clavus collected in the Galapagos Islands. These data include the first reliable measurements of CuCa ratios in coralline aragonite. We estimate that the ratio of CuCa in the lattice to that in seawater (i.e., the effective distribution coefficient) is ~0.3, lower than the value of 1 observed for several other divalent elements; we estimate the effective distribution coefficient for Mn is ~ 1. Distribution coefficients in coral aragonite for 8 divalent metals are near unity despite different ionic radii and chemical speciation in seawater. Investigation of where these substituents are incorporated in the aragonite lattice is clearly warranted. In a modern Hood Island coral, quarter-annually sampled from 1964–73, CuCa ratios decrease twofold from the late 1960s to early 1970s. MnCa and CdCa ratios vary seasonally with upwelling and the generic El Nino. The El Nino Southern Oscillation (ENSO) events of 1965, 1969, and 1972 are marked by suppression of CdCa ratios by about 2.5 nmol/mol, while the MnCa ratio is highest during the strong 1972 ENSO. PbCa ratios were relatively constant throughout this period. From the CuCa record of a seventeenth-century Urvina Bay coral annually sampled from 1600–1725 and the estimated Cu distribution coefficient, surface seawater Cu concentrations at Galapagos during the seventeenth century were similar to present day at 0.7–1.4 nmol/kg. Estimated Pb concentrations were lower at 5–20 pmol/kg, and Mn concentrations were slightly higher at 1.6–2.8 nmol/kg.

Lithium in foraminiferal shells: implications for high-temperature hydrothermal circulation fluxes and oceanic crustal generation rates

The lithium content of planktonic foraminiferal calcite has been determined to evaluate temporal variability of seawater Li concentrations over the past 116 m.y. Mean foraminiferal calcite lithium/calcium in each time interval is no more than 16% greater nor 25% less than the mean Li/Ca of all samples. Li/Ca minima are observed for samples from 50–60 m.y. and 80–90 m.y., with Li/Ca about 25% lower than in adjacent time intervals. At no time during the past 40 m.y does mean Li/Ca appear to be higher than that at present. Subject to the limitations imposed by sample coverage and diagenesis, a similar conclusion holds for the past 116 m.y. Coupled with an oceanic mass balance model for Li, these data suggest that: (1) oceanic Li concentrations and, therefore, high-temperature hydrothermal circulation fluxes during the past 40 m.y. (and perhaps the past 100 m.y.) have not been more than perhaps 30–40% greater than at present for intervals any longer than a million years at most, and (2) these fluxes werenot a factor of two higher 100 m.y. ago. By inference, variations in oceanic crustal generation rates over these time periods are similarly limited. Decreases in hydrothermal circulation fluxes and crustal generation rates or fluctuations up to 20% in these rates of a few million years duration arenot necessarily ruled out by the Li/Ca data. The lack of variability in Li/Ca over time is not unequivocal evidence that hydrothermal fluxes have not varied because the rates of removal processes may be linked to changes in input fluxes.

An apparent contradiction in the role of phosphorus in Cenozoic chemical mass balances for the World Ocean

Little is known about the fluxes to and from the ocean during the Cenozoic of phosphorus (P), a limiting nutrient for oceanic primary productivity and organic carbon burial on geologic timescales. Previous studies have concluded that dissolved river fluxes increased worldwide during the Cenozoic and that organic carbon burial decreased relative to calcium carbonate burial and perhaps in absolute terms as well. To examine the apparent contradiction between increased river fluxes of P (assuming P fluxes behave like the others) expected to drive increased organic carbon burial and observations indicating decreased organic carbon burial, we determined P accumulation rates for equatorial Pacific sediments from Ocean Drilling Program leg 138 sites in the eastern equatorial Pacific and leg 130 sites on the Ontong Java Plateau in the western equatorial Pacific. Although there are site specific and depth dependent effects on P accumulation rates, there are important features common to the records at all sites. P accumulation rates declined from 50 to 20 Ma, showed some variability from 20 to 10 Ma, and had a substantial peak from 9 to 3 Ma centered at 5–6 Ma. These changes in P accumulation rates for the equatorial Pacific are equivalent to substantial changes in the P mass balance. However, the pattern resembles neither that of weathering flux indicators (87Sr/86Sr and Ge/Si ratios) nor that of the carbon isotope record reflecting changes in organic carbon burial rates. Although these P accumulation rate patterns need confirmation from other regions with sediment burial significant in global mass balances (e.g., the North Pacific and Southern Ocean), it appears that P weathering inputs to the ocean are decoupled from those of other elements and that further exploration is needed of the relationship between P burial and net organic carbon burial.

Two-stepping into the icehouse: East Antarctic weathering during progressive ice-sheet expansion at the Eocene-Oligocene transition

In conjunction with increasing benthic foraminiferal δ 18 O values at the Eocene–Oligocene transition (EOT; ca. 34 Ma), coarse-grained ice-rafted debris (IRD; >425 μm) appears abruptly alongside fossil fish teeth with continentally derived neodymium (Nd) isotope ratios (e Nd ) in Kerguelen Plateau (Southern Ocean) sediments. Increased Antarctic weathering flux, as inferred from two steps to less radiogenic e Nd values, coincides with two steps in benthic foraminiferal δ 18 O values. These results indicate that two distinct surges of weathering were generated by East Antarctic ice growth during the EOT. Weathering by ice sheets during a precursor glaciation at 33.9 Ma did not produce significant IRD accumulation during the first e Nd shift. Glacial weathering was sustained during a terrace interval between the two steps, probably by small high-elevation ice sheets. A large increase in weathering signals the rapid coalescence of small ice sheets into an ice sheet of continental proportions ca. 33.7 Ma. Rapid ice sheet expansion resulted in a suppression of weathering due to less exposed area and colder conditions. Parallel changes in Antarctic weathering flux and deep-sea carbonate accumulation suggest that ice-sheet expansion during the EOT had a direct impact on the global carbon cycle; possible mechanisms include associated changes in silicate weathering on the East Antarctic craton and enhanced fertilization of Southern Ocean waters, both of which warrant further investigation.

Paleoredox changes across the Paleocene‐Eocene thermal maximum, Walvis Ridge (ODP Sites 1262, 1263, and 1266): Evidence from Mn and U enrichment factors

An understanding of sediment redox conditions across the Paleocene?Eocene thermal maximum (PETM) (?55 Ma) is essential for evaluating changes in processes that control deep?sea oxygenation, as well as identifying the mechanisms responsible for driving the benthic foraminifera extinction. Sites cored on the flanks of Walvis Ridge (Ocean Drilling Program Leg 208, Sites 1262, 1266, and 1263) allow us to examine changes in bottom and pore water redox conditions across a ?2 km depth transect of deep?sea sediments of PETM age recovered from the South Atlantic. Here we present measurements of the concentrations of redox?sensitive trace metals manganese (Mn) and uranium (U) in bulk sediment as proxies for redox chemistry at the sediment?water interface and below. All three Walvis Ridge sites exhibit bulk Mn enrichment factors (EF) ranging between 4 and 12 prior to the warming, values at crustal averages (Mn EF = 1) during the warming interval, and a return to pre?event values during the recovery period. U enrichment factors across the PETM remains at crustal averages (U EF = 1) at Site 1262 (deep) and Site 1266 (intermediate depth). U enrichment factors at Site 1263 (shallow) peaked at 5 immediately prior to the PETM and dropped to values near crustal averages during and after the event. All sites were lower in dissolved oxygen content during the PETM. Before and after the PETM, the deep and intermediate sites were oxygenated, while the shallow site was suboxic. Our geochemical results indicate that oxygen concentrations did indeed drop during the PETM but not sufficiently to cause massive extinction of benthic foraminifera.