Thomas F. Pedersen

Geochemistry of Recent oxic and anoxic marine sediments: Implications for the geological record

Abstract The distributions of certain minor and trace elements in marine sediments should potentially provide forensic tools for determining the redox conditions of the bottom waters at the time of deposition. The ability to identify such conditions in the geological past is important because (1) current models of the conditions of formation of organic-rich rocks require reexamination, (2) a method to determine whether the areal extent of anoxic waters expanded or retracted in response to palaeoceanographic changes is required, and (3) the effects of such environmental changes on the geochemical balance of these elements in the ocean need to be understood. Recent research has suggested that some minor and trace elements are precipitated where free dissolved sulphide is present (Cu, Cd, Ni, Zn) without undergoing a valency change, whereas others undergo a change in valency and are either more efficiently adsorbed onto solid surfaces under oxic (I) or anoxic (V) conditions or are precipitated under anoxic conditions (Cr, Mn, Mo, Re, U, V). Hence, the enrichment of these minor and trace elements relative to their crustal abundances indicates that the host sediments accumulated under anoxic conditions, although not necessarily under anoxic bottom waters. Examination of the chemical composition of the sediments of anoxic basins, continental margin sediments and oxidized deepsea sediments shows that I and Mn enrichments are reliable indicators of bottom water oxygenation, whereas enrichments of the remaining elements reflect either bottom water anoxia or element uptake by subsurface anoxic sediments below a relatively thin surficial oxic veneer. Hence, the absence of metal enrichment in these cases can be taken as firm evidence that the bottom waters of a basin of sedimentation were not anoxic. These behaviours may be used to propose, for example, that the Holocene sapropel in the Black Sea accumulated under oxic bottom waters, whereas the modern facies reflects its formation under the prevailing intensely anoxic conditions, and that the Panama Basin bottom waters were not anoxic during the Last Glacial Maximum when the rate of accumulation of organic carbon increased. Likewise, the enrichment of Mn as a mixed carbonate phase in some ancient black shales strongly suggests that they formed under oxic bottom waters rather than anoxic conditions as is commonly assumed.

Impacts of Atmospheric Anthropogenic Nitrogen on the Open Ocean

Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the oceans external (nonrecycled) nitrogen supply and up to ∼3% of the annual new marine biological production, ∼0.3 petagram of carbon per year. This input could account for the production of up to ∼1.6 teragrams of nitrous oxide (N2O) per year. Although ∼10% of the oceans drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in N2O emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.

Rhenium and molybdenum enrichments in sediments as indicators of oxic, suboxic and sulfidic conditions of deposition

Abstract The trace elements Re and Mo both behave conservatively in seawater yet are strongly enriched in reducing sediments. Their potential for authigenic enrichment above crustal concentrations is greater than for many other elements, due to the high ratio of [metal] sw /[metal] crust . We present sedimentary Re and Mo data from box- and multi-cores spanning a range of redox conditions, from well-oxygenated sites to locations with substantial sulfide concentrations. At the oxic sites, Re and Mo, as expected, accumulate at concentrations close to their crustal abundances. Re shows substantial enrichment in suboxic (absence of O 2 and H 2 S) sediments of the Sea of Japan, in sediments within the oxygen minimum of the Pakistan margin, as well as in sediments underlying the sulfide-bearing waters of the Black Sea and Saanich Inlet. Re enrichment occurs in these cores just below the depths of Fe and U reduction. Only in the sediments underlying the sulfidic waters of the Black Sea and Saanich Inlet is there substantial authigenic Mo accumulation. Absence of Re and Mo enrichments in sediment trap samples from the sulfide-bearing basins suggests that the addition of both metals occurs at or below the sediment-water interface. The Re and Mo concentration profiles are modeled to hindcast the removal depth of Re and Mo in the sediments which, along with the Re/Mo ratio, determine whether oxic, suboxic or sulfidic conditions were present in the water column or in the sediments in the past. Using this approach, historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals.

Glacial‐interglacial variability in denitrification in the World's Oceans: Causes and consequences

The late Quaternary history of water-column denitrifcation of the eastern Pacific margins and the Arabian Sea is reconstructed using sedimentary δ15N measurements. The δ15N values in six piston cores raised from these regions show remarkably similar cyclic variations, being heavy (9–10.5‰) during the interglacials and 2–3‰ lighter during the glacials. This implies that denitrification in these regions decreased substantially during the glacial periods. The glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone. Since water-column denitrification in these areas accounts for about half of the fixed-nitrogen loss in the modern ocean, the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods. Because nitrate is a limiting nutrient, oceanic productivity and attendant changes in CO2 may therefore have been modulated on glacial-interglacial timescales by variations in the oceanic NO3 content.

Increased productivity in the eastern equatorial Pacific during the last glacial maximum (19,000 to 14,000 yr B.P)

A widespread pronounced enrichment in the organic carbon content occurs at shallow depths in eastern equatorial Pacific sediments. The C org peak is shown by radiocarbon dating to be temporally coincident with the last (Stage 2 or Wisconsin) glacial maximum. In contrast, CaCO 3 profiles show less overall variation, which appears to be partly a function of the location of the sediments relative to the lysocline. Consideration of the iodine:C org ratio in the sediments rules out enhanced preservation as a cause of the Stage 2 organic carbon enrichment. The peak, instead, must have resulted from a significant increase in productivity during the glacial maximum.

Carbon Cycling in the Glacial Ocean: Constraints on the Ocean’s Role in Global Change

This is a comprehensive progress report on the multidisciplinary field of ocean and climate change research. It compiles introductory background papers and leading scientific results on the ocean-atmosphere carbon cycle with emphasis on the oceans carbon inventory and the various components involved. The relationship between plankton productivity, carbon fixation, oceanic PCO2 and climate change is investigated from the viewpoint of long-term climatic change during the late Quaternary cycles of Ice Ages and Warm Ages. The various approaches range from micropalaeontology over organic and trace element geochemistry to molecular isotope geochemistry.

Onset of permanent stratification in the subarctic Pacific Ocean

The surface waters of the modern subarctic Pacific Ocean are isolated from the nutrient-rich waters below by a steep vertical gradient in salinity (halocline), a feature which is a dominant control on upper-ocean stratification in polar environments. The physical processes which maintain the halocline and, in turn, its physical, biological, and geochemical effects have long been subjects of intense inquiry. The stratification of polar surface waters influences the exchange of CO2 between ocean and atmosphere, so the history of the subarctic Pacific halocline may have played a role in past changes in atmospheric CO2 concentration. Here we report opal accumulation rates and nitrogen-isotope data from sediments in this region which indicate that the subarctic Pacific halocline developed abruptly 2.73 million years ago, coincident with the onset of extensive Northern Hemisphere glaciation. The halocline would have reduced the transport of nutrient-rich deep water into the euphotic zone, leading to a decrease in biological production but an increase in the fraction of nutrient stocks utilized. This increase in the efficiency of the ‘biological pump’ would have lowered the rate of CO2 evasion from ocean to atmosphere, potentially reducing atmospheric CO2 concentrations from the suggested higher level of the preceding mid-Pliocene warm interval.

The geochemistry of manganese carbonate in Panama Basin sediments

Abstract The Mn distribution in Panama Basin area sediments and interstitial waters is discussed. Striking surficial Mn enrichments produced by a well-known diagenetic recycling process characterize the sediments of the region. Thermodynamic solubility calculations indicate that in at least one core interstitial waters approach saturation with respect to MnCO3. A mixed carbonate phase of composition (Mn48 Ca47 Mg5)CO3 was recovered from an ash band in the same core. The association of this material with the coarse volcaniclastic debris is thought to result from facile manganous carbonate precipitation in sediment horizons of coarser mean grain size. Since sulphate reduction in the upper two metres of Panama Basin sediments is fairly minor, little increase in alkalinity is observed, and it is postulated that significant production of carbonate alkalinity is not a prerequisite for manganous carbonate generation in hemipelagic sediments. A more important factor appears to be the availability of Mn oxides for solution during early diagenesis. Stable C isotopic analyses indicate that little C of organic origin is used in the precipitation reaction in either Panama Basin or Loch Fyne (Scotland) sediments.

Carbon dioxide release from the North Pacific abyss during the last deglaciation.

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods, but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal. Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that—together with previously published results—show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000–15,000 years ago), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition ∼14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration. We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic, which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir, but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.

Glacial-Holocene Biogenic Sedimentation Patterns in the South China Sea: Productivity Variations and Surface Water pCO2

A bathymetric transect of cores in the South China Sea extending from 4200-m to less than 1000-m water depth has been examined for glacial-interglacial changes in carbonate and organic carbon sedimentation. Typical “Pacific carbonate cycles” (high carbonate content during glacials and low carbonate content during interglacials) characterize cores from water depths deeper than 3500 m. In contrast, “Atlantic carbonate cycles” (low carbonate during glacials and high carbonate during interglacials) are observed in cores from depths shallower than 3000 m as a result of increased dilution of carbonate by terrigenous material during glacial low stands of sea level. Glacial-interglacial changes in the carbonate chemistry of South China Sea intermediate and deep waters resulted in significant changes in the positions of the carbonate compensation depth (CCD) and the aragonite compensation depth (ACD). During the last glacial the CCD and ACD were at least 400 and 1200 m deeper, respectively, than at present. Organic carbon accumulation rates in the South China Sea were approximately 2 times higher during the last glacial than the Holocene. Carbon isotopic analyses and C/N ratios of the organic matter indicate that only a small fraction of the increase in glacial organic carbon accumulation can be attributed to input of terrestrial carbon. On the basis of this we conclude that surface water productivity in the South China Sea was approximately 2 times higher during the last glacial maximum. This is consistent with previous studies which have demonstrated that glacial productivity was higher in low- to mid-latitude regions of the Atlantic and eastern Pacific. The deglacial decrease in organic carbon accumulation is accompanied by a decrease in δ13Corg. Using the relationship between δ13Corg and [CO2](aq) developed by Popp et al. [1989], we estimate that surface water pCO2 values in the South China Sea during the last 25,000 years were very similar to atmospheric CO2 concentrations.