Theme 1: Changes in ocean chemistry and biogeography
What do we do? - Science

 

Theme leader: Jelle Bijma jbijma (at) awi-bremerhaven.de  AWI

 

WP 2 - Past variability of ocean chemistry (paleo-reconstruction)

WP 3 - Present-day observations of ocean chemistry and biogeography

 

Ocean pH has varied over geological time scales and has been both lower and higher than today. The main controls of the system are well known, but for non-specialists it may be less than obvious, sometimes even counter-intuitive. On time scales of a few million years, pH control may be viewed as an enormous titration experiment where continental weathering supplies Ca2+, CO32- and HCO3- (among other cations and anions) and where the oceanic biology “neutralises” part of these titrants by producing organic carbon and carbonate skeletons. The balance between these two processes determines the position of the “carbonate lysocline”, which is the snowline-like boundary between the upper area of the ocean where calcium carbonate accumulates on the sea floor and the deeper ocean where it dissolves. On longer time scales still, plate tectonics and volcanism also have to be taken into account.

In most cases, the CO2 concentration of the atmosphere is slave to the ocean simply because the oceanic carbon reservoir is so much larger than that of the atmosphere. This is a direct consequence of the fact that CO2, unlike O2 or N2, is a reactive gas when dissolved in water and forms the ionic species bicarbonate and carbonate, such that the total dissolved inorganic carbon controls atmospheric CO2. Hence, we can learn from the gas bubbles enclosed in ice cores that the ocean has been more alkaline during glacials and more acidic during interglacials during the last ca. 800 ky (e.g. EPICA Community Members, 2006). The ca. 190 to 280 ppmv change in atmospheric CO2 translates to a whole ocean pH change of ca. -0.2 units.

During some catastrophic events in the history of the Earth, the ocean released enormous amounts of methane from clathrates fixed by bacteria on oceanic shelves (e.g. Zachos  et al., 2005). Since methane converts quickly to CO2 (its lifetime is ca. 12 years), this situation can be compared to the present-day where the atmospheric CO2 content drives the ocean to take up CO2 and acidify. These events are termed paleo-analogs.

In order to understand how anthropogenic CO2 uptake will affect the future oceanic ecosystem, we need to look into the paleo-analogs as well as into the “natural” cycles of ocean pH change such as the glacial-interglacial cycles and tie these to high resolution, sub-recent archives that have experienced rising atmospheric CO2 levels. The use of paleo-reconstructions allows us to look at changes in ocean chemistry and its biological consequences on different scales of space and time. The paleo-reconstructions can then be compared and calibrated with present day observations of ocean chemistry and geographical distribution of plants and animals (biogeography).

Studies of past ocean chemistry are thus important to understand long-term change in the ocean-climate system. An understanding of climate change in the geological past allows us to test modern climate models and thus to refine predictions of future climate change. Paleoceanographic proxies are required to trace important variables such as pH (δ11B), CO32- (B/Ca), temperature (Mg/Ca, δ18O), salinity, nutrients (δ13C, Cd/Ca) and ocean circulation in order to enable reconstruction of the past oceanic environment. The overarching questions of Theme 1 are (Fig. 1.):

What is the past and recent variability of ocean carbonate chemistry (including nutrients and trace metals) and geographical distribution of marine organisms? Among other methods, paleo-reconstructions will be used to investigate the response of (mainly) calcifying organisms to past changes in ocean acidification.

Within WP2 (Past variability of ocean chemistry), we determine the evolution of ocean carbonate chemistry over a range of time scales going from past systems of natural variability (glacial-interglacial (G/IG) and the Holocene) to high resolution archives that have experienced anthropogenic impact (the “industrial era”).

For the G/IG time scales we build on geochemical analyses of established and new proxies (δ11B, B/Ca, size normalized weight (SNW), Mg/Ca, Cd/Ca, δ13C, δ18O) of cores that have been used in previous studies for identifying natural changes in the global ocean carbonate chemistry (Fig. 2.). In addition to the description of G/IG changes in the carbonate chemistry, we use an experimental approach to investigate sedimentary dissolution kinetics and important rain-ratio issues, both of which are currently a major challenge to understand the dynamics of a natural “titration” of the ocean on G/IG time scales. The results are exploited to advance numerical models in order to better understand the natural system and to produce simulated archives that, by comparison with real cores, provides a new impetus for our system understanding.

Fig. 2. Examples of proxies of ocean carbonate system. A and B: size normalised shell weight of planktonic foraminifer G. bulloides as a measure of changing surface ocean [CO32-] between last Glacial Maximum and today (Barker & Elderfield, 2002) C and D: Boron/Calcium ratio of benthic foraminifera as a measure of deep ocean [CO32-] saturation state and difference in [CO32-] water depth profile in N Atlantic Ocean between the last glacial maximum and today (Yu & Elderfield, 2007).

 

High resolution records from drift sediments are used to study the impact of ocean acidification on the geochemistry and the size normalized weight of planktonic and benthic foraminifera over the last ca. 200 years (e.g. Bijma  et al., 2002). Drift sediments occur where the interaction of deep sea currents with the underlying sea-floor topography results in sediment focusing and enhanced sedimentation rates. For example, a core collected from the Gardar drift in the sub-polar North Atlantic shows a sedimentation rate in excess of 2.2 mm/year, equivalent to resolution of about 6 y cm-1. Sediment trap samples going back to the 1970s are used to determine the impact of ocean acidification on the calcification potential (size normalized weight) as well as on geochemical proxies of planktonic foraminifera. In addition, we investigate the geochemical signatures of organisms that produce high resolution records of ocean chemistry changes such as deep sea corals. The combination of these archives allows us to document changes in ocean (carbonate) chemistry and its consequences for biology over all relevant time scales with highest possible resolution.

Close collaboration with Theme 2 (WPs 4, 5 and 7) is envisioned in order to refine proxies. Within those WPs, experiments are planned to determine the impact of ocean acidification on the physiology and its implications at the community level. The impact of these experimental conditions on carbonate biominerals which exist as fossil remains in geological archives will be used to verify existing proxy-relationships and to develop new ones. Experiments are carried out with benthic and planktonic foraminifera, cold water corals, coccolithophorids and bivalves (see Theme 2) thereby establishing robust links between modern observations and paleo-records.

In addition to the above, Theme 1 investigates the most likely scenario and boundary conditions of a paleo-analog for modern ocean acidification. Since a substantial research is already devoted to the PETM (Paleocene-Eocene Thermal Maximum; see e.g. Zachos  et al., 2005), we focus on the less well understood ocean acidification event of the K/T boundary. The mass extinction at the K/T was particularly severe on marine calcifiers taking out all species of ammonites and rudists, more than 90% of the coccolithophores, and all but three species of planktonic foraminifera. Lesser proportions of siliceous species went extinct. It is thought that this selective extinction was most likely caused by ocean acidification after the impact vaporised gypsum-rich rocks (high in sulphate) on the Yucatan peninsula. This would have been only one of many consequences of the comet’s impact. The goal of this aspect is to attempt a preliminary quantification of the K/T ocean acidification, using a biogeochemical model of the ocean carbon cycle in conjunction with careful scrutiny and comparison of paleo data.

Theme 1 activities set the stage for Themes 2 and 3 in which the impact of ocean acidification at the levels of organisms and ecosystems are investigated. Knowledge of the nature and amplitude of past fluctuations in the past are necessary to assess the stability of modern subsystems and their potential range of variations in the future. Future predictions of ocean chemistry variations are part of Theme 3 (WPs 10, 11 and 12). Models used in Theme 3 are also used in Theme 1 to analyse the past for improved predictability of the future.

Within WP3 (Present-day observations of ocean chemistry and biogeography), we make in situ observations of the natural variability of the modern ocean, in terms of spatial (geographical) distributions and seasonal cycles and ranges. Both chemical and biological variables (including the Continuous Plankton Recorder, CPR, data; Fig. 3.) are collected, with particular interest on carbonate chemistry and calcifying organisms. This information informes modelling in the project (EPOCA's WPs 9, 10, 11 and 12), for example by providing regional data to inform regional models. By comparing the spatial distributions of organisms to the chemical conditions in which they are observed to live, we gain information towards thresholds and tipping points (WP13). Our geographical focus is on the Arctic Ocean and the Nordic Seas, predicted to be among the first regions to become undersaturated, the European shelf seas and the Atlantic Ocean.

Figure 3. Distribution of samples obtained by SAHFOS using the Continuous Plankton Recorder Survey (Richardson  et al., 2006).

From direct measurements of the real ocean over recent years and decades (at time-series sites), we calculate the rates of change in the very recent past of seawater chemistry (pH and carbonate system) and calcareous organisms. These in situ measurements are compared to the latter parts of the industrial era records calculated from proxies (WP2). Through data synthesis and through its incorporation into models, we identify “hotspots”: geographical areas where (1) seawater pH is naturally very low, (2) seawater is already approaching CaCO3 undersaturation, and/or (3) seawater pH or saturation state are rapidly declining (risk information contributing to WP13).

 

References

Barker S. & Elderfield H., 2002. Foraminiferal calcification response to glacial-interglacial changes in atmospheric CO2. Science 297:833-836.

Bijma J., Hönisch B. & Zeebe R. E., 2002. Impact of the ocean carbonate chemistry on living foraminiferal shell weight: Comment on "Carbonate ion concentration in glacial-age deep waters of the Caribbean Sea" by W. S. Broecker and E. Clark. Geochemistry, Geophysics, Geosystems 3(1064), 1064. doi:10.1029/2002GC000388. Richardson A. J. A. et al., 2006. Using continuous plankton recorder data. Progress in Oceanography 68:27-74.

EPICA Community Members, 2006. One-to-one coupling of glacial climate variability in Greenland and Antarctica. Nature 444:195-198.

Yu, J. & H. Elderfield (2007) Benthic foraminiferal B/Ca ratios reflect deep water carbonate saturation state. Earth Planetary Science Letters 258, 73–86.

Zachos J. C. et al., 2005. Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum. Science 308:1611-1615.

 

 This web site is hosted by Observatoire Océanologique de Villefranche sur Mer