Goal of this course: What determines the abundance of different elements in the ocean? How does their distribution depend on physical circulation and biological activity? How does the chemical environment of the ocean influence marine life? How do these factors combine to create the global ocean carbon cycle? Use chemical distributions to understand how the ocean works

Why Chemical Oceanography? 1) Integrating diverse processes Chemical composition of ocean Fundamental processes in Earth Science are recorded in the chemical composition of the ocean. (We’ll study only a subset!) Physical Geological Chemical Biological River discharge Photosynthesis + respiration Circulation + Mixing Acid-base reactions Sediment formation Hydrothermal vents Scavenging Gas exchange

Tools of Chemical Oceanography 1) Observations our focus Enormous information potential, BUT…

The “biological pump” For our favorite elements, we had to learn about: Photosynthesis Sinking particles (“export”) Respiration (“remineralization”) Circulation Gas exchange We’re confronted with a bewildering variety of processes…

Tools of Chemical Oceanography 2) Models But simple models can yield important insights! Radioactive Carbon Biological export = Physical transport ~1000 years!~5 GtC/year!

Wind-driven circulation Surface winds set surface currents into motion (in surprising ways!)…

Upwelling/downwelling … that induce upward and downward motion…

Sea Surface Height H L H …and influence the pressure in deeper layers, causing “gyres”

The Ocean Conveyor Areas of deep winter mixing initiate global deep currents.

Introducing Horizontal Variations Ocean circulation explains areas of high surface nutrients. But it can’t be the whole story, we need biology too! [mmol/m 3 ] 0-50 m

CO 2 C org Phytopl. Zoopl. Bacteria Respiration (Heterotrophic) Photosynthesis “Net Primary Production (NPP)” RhRh NPP Export Production EP Measures of Productivity Photosynthesis converts simple molecules to complex organic matter - some of it sinks, some gets recycled by complex food webs.

Autotrophs: Phytoplankton Innumerable species contribute to the total photosynthesis of the ocean. They’re all similar in many ways (Redfield’s proportions) but differ in many ways too (chemical function, size).

Net Primary Production We can detect the presence of phytoplankton chlorophyll from space and use it to map biological productivity. But what determines this pattern?

To what degree does Temperature appear to limit productivity? Are there warm (cold) waters with low (high) productivity? NPP vs SST

Seasonal light limitation Where should light never be limiting? Where is it limiting in winter? And why (what combination of two factors?). What about summer?

Observed surface nitrate Nitrogen limits photosynthesis throughout most of the surface ocean…

Iron supply by dust Mahowald et al. [2006] Atmospheric Dust Flux (simulated) But iron also plays a role in limiting plankton growth.

Export Production Low export Small+warm High export Big + cold

More soluble gas or colder water. Less soluble gas or warmer water. pApA pBpB CBCB CACA Solubility and Equilibrium air water Gases like O 2 and CO 2 can be held in solution in seawater, but their solubility depends on strongly on temperature.

or Warming Supersaturation Cooling or Undersaturation Temperature Gas Concentration S A (T)*p A Sources of Disequilibrium Production Consumption The ability of gases to go between ocean/atmosphere always pushes them toward exquilibrium. But it is never perfectly reached because its constantly disrupted by heating/cooling or production/consumption of a gas.

O 2 Disequilibrium ( ∆ O 2 ) For O 2, gas exchange is very effective, but not perfect. Supersaturation is a signature of productivity (export), Undersaturation of low-O 2 water upwelling -> respiration!!

Respiration can be shallow, keeping nutrients close to the photic zone, and CO 2 close to the atmosphere. Shallow Respiration

Deep Remineralization …or deep, storing nutrients and CO 2 in the abyss, far from plankton and the atmosphere.

Apparent Oxygen Utilization AOU proves that most of the respiration occurs shallow. But this leads to anoxic conditions in some places!

Oceanic Oxygen [mmol/m 3 ] 400 m How is respiration carried out when oxygen is all gone?

Nutrient Limitation: NO 3 or PO 4 ? Release Uptake Nutrient Uptake and Release N:P = 16:1 By removing nitrate from the ocean, which is why nitrogen is limiting!

Why Chemical Oceanography? 3) Impact of human activities Carbon dioxide is increasing in the atmosphere and oceans. What are the consequences of this? Anthropogenic CO 2 in the ocean

The Global Carbon Cycle What if the ocean stopped absorbing so much CO 2 ?

Influences on pCO 2 From Henry’s Law: Temperature Salinity Total DIC Alkalinity (or pH) Biology and Gas exchange Biology only Heating/cooling Precip/Evap Depends on

pCO 2 vs Temperature, Salinity Temperature Salinity Low pCO 2 High pCO 2 Pre-industrial (280 ppm) The pCO 2 of seawater increases by ~4% for each degree of warming, and by ~1% for each 1% increase in salinity. HeatingCooling

Carbon pumps and pCO 2 Effect of Organic Carbon Pump is to reduce pCO 2 of surface ocean, and thus the atmosphere. Influence of Carbonate Pump counteracts some of that lower pCO 2 by removing CO Net effect of biological pumps is to lower atmospheric CO 2 – by a lot!

Anthropogenic CO 2 in the Ocean ATL PAC IND The “preindustrial” background of DIC can be estimated from a suite of simultaneous physical and chemical measurements, and the anthropogenic CO 2 increase can be inferred by difference from observed DIC. The total inventory of anthropogenic CO 2 (~120 Gt) is nearly half of the historical fossil fuel burning source. Sabine et al. [2004]

Ocean acidification Source: IPCC AR4 The most obvious direct consequence of increasing CO 2 is the reduction of pH. The biological consequences are largely unknown, but could be profound. Eastern North Atlantic Western North Atlantic Central North Pacific pCO 2 pH

Final exam: Just like the midterm: Testing Concepts, not calculations Cumulative Short answer questions Except: You have a lot more time (3 hours) Exam is only a little longer (1.5 hours)