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The Importance of Ocean Temperature to Global Biogeochemistry

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The Importance of Ocean Temperature to Global Biogeochemistry Earth and Planetary Science Letters 222 (2004) 333–348 www.elsevier.com/locate/epsl Frontiers The importance of ocean temperature to global biogeochemistry David Archera,*, Pamela Martina, Bruce Buffetta, Victor Brovkinb, Stefan Rahmstorf b, Andrey Ganopolskib a Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA b Potsdam Institute for Climate Impact Research, P.O. Box 601203, 14412 Potsdam, Germany Received 3 December 2003; received in revised form 5 March 2004; accepted 10 March 2004 Abstract Variations in the mean temperature of the ocean, on time scales from millennial to millions of years, in the past and projected for the future, are large enough to impact the geochemistry of the carbon, oxygen, and methane geochemical systems. In each system, the time scale of the temperature perturbation is key. On time frames of 1–100 ky, atmospheric CO2 is controlled by the ocean. CO2 temperature-dependent solubility and greenhouse forcing combine to create an amplifying feedback with ocean temperature; the CaCO3 cycle increases this effect somewhat on time scales longer than f 5–10 ky. The CO2/T feedback can be seen in the climate record from Vostok, and a model including the temperature feedback predicts that 10% of the fossil fuel CO2 will reside in the atmosphere for longer than 100 ky. Timing is important for oxygen, as well; the atmosphere controls the ocean on short time scales, but ocean anoxia controls atmospheric pO2 on million-year time scales and longer. Warming the ocean to Cretaceous temperatures might eventually increase pO2 by approximately 25%, in the absence of other perturbations. The response of methane clathrate to climate change in the coming century will probably be small, but on longer time scales of 1–10 ky, there may be a positive feedback with ocean temperature, amplifying the long-term climate impact of anthropogenic CO2 release. D 2004 Elsevier B.V. All rights reserved. Keywords: carbon cycle; climate; ocean temperature 1. Introduction the ocean and atmosphere. In this paper, we consider the implications of the temperature sensitivity of The mean temperature of the ocean has varied by dissolved gases O2 and CO2 and methane in clathrate more than 10jC over the past 65 My. The mean deposits below the seafloor within the context of ocean temperature in a coupled climate model is Cenozoic climate change. The mechanism and extent roughly as sensitive to pCO2 as is the mean surface of the interaction between these systems and tem- temperature of the earth [1]. Such large changes in perature depends strongly on the duration of the temperature must impact biogeochemical cycling in change in ocean temperature. The largest temperature changes inferred from the geologic record took place * Corresponding author. Tel.: +1-773-702-0823; fax: +1-773- on the 10-My time scale, which is long enough to 702-9505. allow weathering feedbacks to imprint their signa- E-mail address: [email protected] (D. Archer). tures. The glacial cycles are faster than silicate or 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.03.011 334 D. Archer et al. / Earth and Planetary Science Letters 222 (2004) 333–348 organic carbon weathering time scales but do interact 2. The record of deep-ocean temperature with the CaCO3 cycle. The response time for chang- ing deep-ocean temperature is about a millennium, 2.1. Methods so we will not be concerned with time scales shorter than that. 2.1.1. d18O The primary tool for reconstructing deep ocean temperature is the stable oxygen isotopic compo- sition (d18O) of the calcite shells of foraminifera Terminology [2–4]. The temperature dependence varies some- what for different species and across the full range DT : The temperature change resulting from a 2x of ocean temperature, but for benthic foraminifera, doubling of the CO concentration of the 2 the response is f 0.25x per jC, relative to a atmosphere. This term is used in an atypical precision in measurement of fF0.02x and a way in this paper, to describe the mean ocean change in d18Oof f 5.5xover the past 65 My. temperature, rather than the mean surface Reconstruction of temperatures from d18O is com- temperature. plicated by a correlation between local salinity and AABW: Antarctic bottom water. A source of sur- d18O in seawater, and by whole-ocean shifts in face water to the deep, originating in the d18O reflecting storage in isotopically light conti- Southern Ocean. nental ice sheets. The earth was probably ice-free Clathrate: Water frozen into a cage structure tra- before the late Eocene, 40 My, so this issue pping a gas molecule. Ocean margin sediments affects the more recent part of the record. How- contain huge amounts of methane trapped in ever, the older part of the record is more affected clathrate deposits. These are also known as by calcite recrystallization, which biases the recon- hydrates. structed temperature toward the colder pore waters Foraminifera: Single-celled CaCO -secreting hete- 3 [5,6]. rotrophic protista. The chemistry of their shells provides information about the chemical and 2.1.2. Mg/Ca ratio in CaCO physical conditions in which they grew. Plank- 3 The Mg/Ca ratio of biogenic calcites increases tontic foramifera lived near the sea surface; exponentially with temperature. While the relation- benthic foraminifera lived on the seafloor. ship probably has thermodynamic underpinnings, bio- Gton: 109 metric tons, used here exclusively to genic calcite carries a species-dependent overprint measure carbon (not CO ). 2 [7,8] which must be determined by empirical calibra- LGM: Last Glacial Maximum, 21–18 ky. tion [7,9]. Most low-Mg foraminifera used in paleo- NADW: North Atlantic deep water. A source of ceanographic studies have a Mg/Ca response of f 9– water carrying the chemical imprint of the 10% per jC, relative to an analytic precision of better surface ocean into the deep. than 2% and variation in Mg/Ca of more than 50% PETM: Paleocene Eocene Thermal Maximum, an over the Cenozoic [10,11]. excursion of d13C and d18O that is generally The concentrations of Mg and Ca ought to have interpreted as an abrupt warming and release of been relatively stable over several million years, but isotopically light methane from ocean clathrate on the 40-million-year time scale of the Cenozoic, deposits. ocean chemistry is less certain, and the calibration of Radiative equilibrium: The balance between influx extinct species of foraminifera becomes more diffi- and output of energy from the planet. With an cult [10–12]. Calcite solubility increases with Mg increase in greenhouse trapping of outgoing content, so that partial dissolution tends to deplete infrared light from the surface, the surface Mg, biasing the reconstructed temperature cold [13]. temperature must increase to maintain radiative In spite of these difficulties, Mg paleothermometry is equilibrium. useful for separating ice volume effect in the d18O record. D. Archer et al. / Earth and Planetary Science Letters 222 (2004) 333–348 335 2.2. My time scale changes deep-sea records also show fluctuations on the order of 2jC associated with the higher frequency Milankovitch The temperature of the deep sea has changed con- cycles (41 and 23 ky) [13,19].Atanevenhigher siderably over the last 65 million years (see Fig. 1). The resolution, Mg/Ca and d18O records from moderate lack of evidence for ice sheets prior to the end of the sedimentation rate deep-sea cores imply short warming Eocene suggests that d18O primarily reflects tempera- events of 0.5–1.5jC on the order of several thousand ture during that time. Deep-ocean temperature appears years during the last glacial, consistent with compari- to have been around 8jC at the start of the Paleocene, son of d18O records and high-resolution records of sea warming to a Cenozoic maximum of nearly 12jC in the level change reconstructed from coral reefs [20]. early Eocene ( f 50 Ma). Both Mg/Ca and d18O indicate a steady cooling throughout the Eocene of f 5–7jC, culminating in an additional abrupt cooling 3. Mechanisms of ocean temperature change to f 4jC coincident with the development of Antarc- tic Ice Sheets that persisted during most of the Oligo- 3.1. My time scale cene [10,14]. The d18O records imply a warming with the temporary waning of ‘permanent’ Antarctic Ice On the longest time scales, the mechanism by which Sheets in the late Oligocene [15]. Mg/Ca, however, the deep-ocean temperature achieves the warmest ob- suggests that the cold temperatures persisted through served levels is something of a mystery. The central the middle to late Miocene [10,11] and possibly until issue is whether the ocean flips from its present the middle Pliocene [11], with temperature fluctuations dominantly temperature-driven to a salinity-driven of around 1–2jC persisting on time scales of millions overturning circulation, or alternatively if tempera- of years. Most of the records suggest a rapid, steady ture-driven circulation can generate the observed cooling to the present mean ocean temperature of warming. Salt-driven circulation would lead naturally 1.5jC over the last 5–10 million years. Both d18O to warmer deep-ocean temperatures, as the sites of and Mg/Ca show a f million-year warming event convection move to lower latitudes. The coupled at- around 10 Ma, the Mid-Miocene Climatic Optimum. mosphere/ocean simulations of the Eocene from Huber There are also several f 100,000-year spikes in the and Sloan [21] were described as ‘‘quasi halothermal’’ Cenozoic deep temperature record, the most well- circulation. Deep-water formation took place in a cool defined spike, a warming event called the Paleocene/ ( f 9jC) but salty subpolar North Atlantic.
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