I. EXECUTIVE SUMMARY

OCTET (Ocean Carbon Transport, Exchanges and Transformations) is a planning initiative to promote research on the global carbon cycle, specifically to understand carbon dynamics in the ocean. The near-term focus of OCTET is to develop the improved conceptual framework and tools required for estimating basin-scale patterns, seasonal cycles, and interannual variability of the sea surface pCO₂ distribution. These distributions will lead to better quantitative measures for the net anthropogenic CO₂ uptake into the different ocean basins and provide important constraints on atmospheric and terrestrial CO₂ fluxes. A longer term focus of OCTET is to determine the role of the ocean carbon cycle in amplifying or ameliorating natural and anthropogenic variation in atmospheric CO₂, and thus climate change. OCTET will study the ocean's response to climate as well as climate feedbacks. Over decade to century time scales, this response of ocean ecology and biogeochemistry to climate change (e.g., global warming) may be as significant as the impact of ocean biogeochemistry on atmospheric CO₂. This report summarizes the proceedings of the OCTET Workshop held at Airlie House, Warrenton, VA, in March, 2000.

The OCTET Workshop was organized to identify and define problems in ocean carbon cycle research, establish priorities, and suggest a framework for implementing solutions. We organized working groups around eight central themes:

• Science Themes: At least two interdependent carbon pumps deplete the ocean surface of total CO₂ relative to deep water and dominate regional and vertical partitioning of carbon in the ocean. The SOLUBILITY PUMP transfers CO₂ to the deep sea as the formation of cold deep waters at high latitudes acts as a temperature-dependent sink for atmospheric CO₂ (or as a source of CO₂ to the atmosphere in upwelling areas where cold, CO₂-enriched waters warm and degas). The BIOLOGICAL PUMP removes carbon from surface waters by gravitational settling, diffusion, and active biotransport of organic and inorganic carbon derived from biological production.
• Regional Themes: The NORTH ATLANTIC, NORTH PACIFIC and SOUTHERN OCEAN are critical regions in the Earth's carbon cycle, and although each is characterized by a set of specific processes, some fundamental processes are common to all regions.
• Approach and Technique Themes: State-of-the-art numerical MODELING studies will be an integral part of the design, implementation and synthesis phases of OCTET. Studies of PALEOCLIMATE VARIABILITY will place OCTET research in the context of the evolution of the Earth system. Modern autonomous and continuous OBSERVING SYSTEMS are necessary to extend the temporal and spatial scales of our observations which is critical for our understanding of the ocean carbon cycle.

These topics are explored below.

OCTET builds on the legacy of oceanographic research that has developed in the past 15 years (e.g., WOCE, JGOFS, GLOBEC, IRONEX, CLIVAR, etc.), which includes a strong emphasis on multidisciplinary approaches and collaboration. These programs have developed ideas that are central to the design of OCTET, and leave behind an immense new knowledge, an unprecedented open data system that encourages cross-discipline analysis, and a new generation of scientists in the evolving field of biogeochemistry. Moreover, individual scientists working independently have been instrumental in developing many of the ideas. OCTET seeks to continue using both types of approaches within a common planning framework.

Building on past global ocean research programs, OCTET will focus on those areas that have been identified as critical to improving our ability to predict the future course of ocean carbon cycling and processes. We no longer can afford to pursue every topic of interest, but rather we must identify those areas with the greatest uncertainty in order to increase confidence in our predictions. Attaining these goals will require adaptive and flexible implementation strategies and a close integration of both modeling and observations.

We are currently (Fall, 2000) in a period of intense international planning with regard to global carbon cycle research. In this OCTET Workshop Report, we hope to provide a scientific framework that identifies and defines major research topics of immediate concern. To this end, we make a series of recommendations revised and extracted from the main body of the report. The rationale for these recommendations, and further details can be found in the introduction and working group reports.

1. Planning, Infrastructure and Preparation:

1.1. Seek to integrate OCTET, EDOCC and SOLAS in a coordinated and directed program of ocean biogeochemical and ecological research.

1.2. Build on the current JGOFS and WOCE synthesis and modeling programs to initiate basin-scale and other process modeling studies to aid in designing OCTET field programs.

1.3. Initiate strong linkages with studies of physical processes, including basin, meso- and large-scale studies of ocean-atmosphere processes such as CLIVAR.

1.4. Continue development of new and improved methods for measuring important biogeochemical properties (e.g., pCO₂ and TCO₂, nutrients, iron, optical properties, etc). Continue development of new instrumentation for continuous or autonomous sensing of these properties. (See Modern Autonomous Observing Systems Working Group Report).

1.5. Initiate strong linkages with studies of the distribution of CO₂ and related tracers in the atmosphere. In common with OCTET, such studies seek to document anthropogenic and biogeochemical CO₂ fluxes on the basin scale, and determine the magnitude and interannual variability in these rates.

2. Field Studies:

2.1. Ocean Basins: In keeping with the Carbon Cycle Science Plan and to understand and constrain estimates of carbon storage in North America, initiate a large-scale and long-term series of oceanographic field programs in the North Atlantic and North Pacific Oceans. Initiate a program to understand the response of the Southern Ocean to climate change, and to constrain the size of the carbon sink in the Southern Ocean exploiting ships of opportunity (e.g., the regular transit of research vessels across the Drake Passage en route to the Antarctic Peninsula and those crossing the SW Pacific sector to the Ross Sea).

2.1.1 North Atlantic Ocean. (See North Atlantic Working Group report).

• Continue time series observations at the BATS site, solidify its core support and augment the routine measurement suite.
• Initiate (in conjunction with European institutions) a high-latitude time series site and a network of autonomous, moored time series nodes.
• Design and implement a new basin-scale survey program optimized to improve estimates of the size and interannual variability of the carbon sink in the N. Atlantic.
• Design and implement new process studies to estimate the contribution and interannual variability of the spring phytoplankton bloom to annual carbon storage, and to determine the importance of active nutrient transport, nitrogen fixation, eddy dynamics and other processes in the supply of nutrients to the oligotrophic euphotic zone.
• Use new studies of the North Atlantic Oscillation to understand the potential for climate change-induced modifications in the carbon cycle of the N. Atlantic basin.

2.1.2. North Pacific Ocean (See North Pacific Working Group report).

• Continue time series observations at the HOT site, solidify its core support and augment the routine measurement suite.
• Initiate (in conjunction with North Pacific-basin institutions) a high-latitude time series site and a network of autonomous, moored time series nodes.
• Design and implement a new basin-scale survey program optimized to improve estimates of the size and interannual variability of upper ocean carbon sources and sinks in the N. Pacific.
• Design and implement a new program of surveys and process studies focussed on improving estimates of the magnitude and interannual variability of nitrogen fixation and denitrification rates, and to determine the importance of active nutrient transport and nitrogen fixation in the supply of nutrients to the euphotic zone.
• Initiate a new program of cruise-based observations and moored sensor deployments to determine how carbon fluxes and ecosystem structure respond to physical variability on ENSO and PDO time scales, and build on these results to improve predictions of climate-induced modifications of the carbon cycle in the N. Pacific basin.

2.1.3. Southern Ocean (See Southern Ocean Working Group report).

• Plan, design and implement a program to evaluate spatial and interannual variability in ecosystem structure and carbon fluxes, emphasizing development of time-series observations and exploiting ships of opportunity.
• Establish mechanistic relationships between ecosystem structure, carbon fluxes, physical forcing and environmental boundary conditions, incorporating evolving hypotheses concerning linkages to the state of the tropical oceans, as steps toward understanding the response of Southern Ocean biogeochemical systems to climate change.
• Explore collaboration with nascent initiatives in France and Australia as a mechanism of implementing early time-series observations of seasonal and interannual variability on which to build later process studies.

2.2. Ocean Margins: Configure a series of ocean margin studies designed to resolve the contribution of continental margin processes to basin scale carbon dynamics.

2.3. Focussed Process Studies: Design and implement smaller, independent studies that focus on poorly understood ocean processes, regimes and subsystems:

2.3.1. Manipulative mesocosm and other whole-ecosystem-unit field studies.

• To quantitatively define the role of iron and other trace metals in modulating N vs. P limitation, species succession, and community structure ocean response to climate change in three ocean basins (Atlantic, Pacific and Southern).

2.3.2. Biological pump studies.

• To improve understanding of the current operation, geographical and temporal (especially interannual) variability of biological pump mechanisms, and determine its responses to global warming, increased stratification, and changes in nutrient limitation regimes. (See Biological Pump Working Group report).

2.3.3. Solubility pump studies.

• To better quantify and constrain the mechanisms regulating air-sea CO₂ exchange and solubility-driven storage of CO₂, instrument VOS with pCO₂, temperature, salinity, nutrients, and chlorophyll sensors (in collaboration with CLIVAR), deploy moorings and drifters in key oceanographic provinces with atmospheric and ocean pCO₂, temperature, salinity, nutrients, chlorophyll, mixed layer depth sensors. (See Solubility Pump Working Group report).

2.3.4. Intensive studies of biogeochemical processes and ecology in the "Twilight Zone".

• To determine the mechanistic basis of water column remineralization and variations in its efficiency and length scales in the mesopelagic region of net organic matter remineralization from roughly 100 &endash; 1000 m in key oceanic provinces. (See Biological Pump Working Group report).

3. Historical and Paleoclimate Variability Studies: Synthesize existing data sets and design new, targeted field studies to evaluate the ocean carbon cycle response and feedbacks to climate variability. (See Paleoclimate Variability Working Group report).

4. Modeling: Initiate a broad-based program of biogeochemical modeling, including training, as a parallel and fully equal element of the OCTET program. (See Modeling Working Group report).

5. Data Management and Archiving: Maintain and expand the data management efforts begun in JGOFS and WOCE, to provide both active project data management for OCTET PI's, and long term archiving and access to existing data. Improve and expand distributed, online access to biogeochemical data sets worldwide.