# Presentation: 464841

## Interdisciplinary studies of CDOM in the global ocean

**Norman B. Nelson****1****, Chantal M. Swan****1****, David A. Siegel****1****, Craig A. Carlson****1,2**
-    *1**Institute for Computational Earth System Science,  **2**Dept. Ecology, Evolution and Marine Biology**, **University of California, Santa Barbara*

| Sample Site | Latitude | Region | Depth (m) | T (°C) | Sal. (ppt) | Chl-a
(ug/l) | Initial aCDOM 
(m-1) at 325nm | Initial aCDOM
 (m-1) at 440nm | (325nm,325nm)
(m2 mol photons-1) | (440nm,440nm)
(m2 mol photons-1)
 | Total irradiation time (days) |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| PB189S5 | 34°N | Coastal Pacific
(So. Cal. Bight) | 0 | 16 | 33.1 | 0.600 | 0.20 | 0.02 | -0.08 | -0.004 | 2.02 |
| BATS | 32°N | Subtropical
N. Atlantic | 80 | 23 | 36.6 | 0.200 | 0.06 | 0.001 | -0.05 | -0.001 | 3.02 |
| P16NS51 | 29°N | Subtropical
N. Pacific | 140 | 19 | 35.2 | 0.078 | 0.07 | 0.003 | -0.09 | -0.002 | 2.97 |
| P16NS51 | 29°N | Subtropical 
N. Pacific | 40 | 20 | 35.3 | 0.096 | 0.05 | 0.001 | -0.05 | 0 | 3.06 |
| P16NS76 | 55°N | Subarctic
Pacific | 200 | 4 | 33.9 | 0.004 | 0.17 | 0.02 | -0.07 | 0.004 | 2.00 |
| P16NS76 | 55°N | Subarctic
Pacific | 80 | 3 | 33.0 | 0.019 | 0.14 | 0.01 | -0.03 | 0.009 | 2.00 |
| P16NS19 | 0.5°N | Equatorial
Pacific | 100 | 22 | 35.6 | 0.060 | 0.06 | 0.005 | -0.05 | 0.015 | 2.95 |
| P16SS61 | 46°S | Subantarctic Pac. Frontal Zone | 80 | 10 | 34.4 | 0.242 | 0.07 | 0.01 | 0 | 0.012 | 2.06 |
| P16SS61 | 46°S | Subantarctic Pac.
Frontal Zone | 0 | 13 | 34.4 | 0.132 | 0.05 | 0.01 | 0 | 0.008 | 3.01 |

***Table:*** Summary of associated hydrographic data and calculated  values (325nm and 440nm, shaded blue). Sites and values marked in red denote where (440nm;440nm) is positive, corresponding to observed photoproduction of CDOM during irradiation.

***Photobleaching / photoproduction quantum yields***

**Schematic of inversion terms using Subtropical N. Atlantic (BATS) example:**

-  Loss of CDOM absorption in the UV due to full-spectrum solar irradiation occurs in nearly all regions sampled. *We can constrain environmental range of open ocean ****photolysis*
-  50% of regions sampled show a very *unexpected *concurrent phenomenon at longer wavebands: *formation of CDOM due to irradiation*

***Quantum yield (************) of CDOM photolysis -- the major sink of CDOM***
***Chantal Swan, NASA Earth System Science Fellowship***

-  Photolysis moderates global surface distribution of CDOM
-   (and photolysis rate) can be used in concert with upper ocean vertical mixing rate to deduce microbial production rate of CDOM (a term otherwise hard to measure)
-  Constraining a mixed-layer budget of CDOM will permit its use as the first remotely-sensed tracer of upper ocean circulation

***Key Findings:***

- Completed (full measurement set including CDOM, microbes, optics)

- Completed (limited measurement set, CDOM and hydrography)

- Future (in planning)

***Global distribution and dynamics of CDOM in the surface and deep ocean***
***Norm Nelson, Dave Siegel, Craig Carlson***

- In these flow charts, the straight yellow arrows represent advective fluxes of CDOM (including horizontal transport, upwelling, and downwelling), curled arrows represent local production of CDOM, and the red arrows represent photobleaching. The top row of boxes represent surface waters, and the second row represents the main thermocline down to 1 km. The color corresponds to CDOM absorption coefficient (as in the section plot to the left).

***Bermuda Bio-Optics Project: Decadal scale observations***
***Norm Nelson, Dave Siegel***

***Overview***

- We are currently engaged in several research efforts concerned with the distribution, dynamics, and characterization of chromophoric dissolved organic matter (CDOM) in the global ocean. These include:
- Ocean color algorithm development for retrieving CDOM absorption as well as chlorophyll and particulate backscatter in surface waters  (with Stephane Maritorena). This has led us to new insight concerning the influence of CDOM on retrieval of chlorophyll as well as pointing toward research on the nature of CDOM cycling.
- Time-series study of apparent and inherent optical properties at the BATS site southeast of Bermuda in the subtropical North Atlantic. Our results have revealed a seasonal cycle in CDOM distribution that indicated photolysis was the main sink, and secondary production the main source.
- Photochemistry of CDOM: Measuring quantum yields for bleaching and photoproduction of CDOM on samples collected from the field in low CDOM areas where these rates are difficult to measure.
- Our results are leading to new insights concerning the nature and cycling of CDOM in the global ocean. Ongoing and future efforts include characterization of CDOM along gradients of ventilation age, mixing, and photolysis using optical and chemical methods, and incorporation of CDOM terms into mixed layer and general circulation models.

- Remote sensing of CDOM distributions in surface waters over the global ocean highlight a superficial correlation with chlorophyll and productivity, but with some significant differences. Significant CDOM in upwelling zones raised the question of whether CDOM was present in the ocean interior, if so what controlled its abundance in the deep sea, and how are the surface and interior coupled. We are in the process of conducting a global field survey of CDOM distribution and characteristics relative to hydrography, optics, and selected biological parameters, as an ancillary project on the U.S. CO2/CLIVAR Repeat Hydrography surveys. Since 2003 we have collected data on meridional sections  covering the full range of surface CDOM in every open ocean basin.
- Our results have highlighted the importance of thermohaline circulation and remineralization in determining the abundance of CDOM in the deep ocean. In the Atlantic, rapid meridional overturning mixes CDOM more homogenously, rapidly transmitting surface CDOM concentrations to the interior. In the North Pacific and Indian Ocean, slow mixing allows accumulation of CDOM formed as a result of remineralization. In the Southern Ocean, low production at the surface and rapid ventilation transmit low CDOM signals to the interior, creating the interhemispheric imbalance reflected in remotely sensed data.

- acdom (325 nm, m-1)

- CDOM Dynamics: Pacific / Indian

**CDOM Cycling Box Models**

- In surface waters, the distribution of CDOM is easily explained by a balance between production and photolysis. In subtropical waters persistent stratification and net downwelling leads to low CDOM concentrations.  Formation of subtropical mode water in regions with seasonal mixed layers carries low CDOM water to the ocean interior, where a low CDOM signature is easily observed in the mode waters of the North and South Atlantic. Meridional sections across the Equator clearly shows the transport of CDOM to the surface where it is bleached.
- An outstanding question: is the high CDOM observed in the North Atlantic a residual of terrestrial CDOM from the Arctic? We hope to resolve this with ongoing research and a repeat of the 2003 North Atlantic sections in 2011-2012.

- The contribution of CDOM to ocean color variability not related to chlorophyll abundance was first suggested by Siegel and coworkers studying the first year (1992) of in situ radiometry data from the Bermuda Bio-Optics Project, a time-series study of optical properties in the water column piggybacking on the successful Bermuda Atlantic Time-series Study cruises.
- Since 1994 the real contribution of CDOM to ocean color has been assessed using spectrophotometric measurements of CDOM absorption spectra, as part of an integrated study of component absorption.
- Sustained observations of CDOM are being considered in the context of climate-related changes in the factors controlling CDOM abundance.

- At BATS, seasonal mixing homogenizes the CDOM profile from a characteristic summer pattern that includes a surface minimum, a local maximum in the 60-150m range, and a local minimum in the mode water. But this mean pattern summarizes much variability. We are examining interannual patterns in CDOM distribution in the upper water column to assess the relative contribution of local effects (production, mixing) versus remote effects (mode water formation, overall irradiance leading to photolysis). An intriguing hint of CDOM-climate teleconnections can be seen in a correlation between the North Atlantic Oscillation and CDOM abundance at 160 m.

**Acknowledgments**:
**NASA Ocean Biology and Biogeochemistry **
**NSF Chemical Oceanography**
**U.S. CLIVAR/CO2 Repeat Hydrography Project**(Jim Swift, Lynne Talley, Dick Feely, Rik Wanninkhof, Rana Fine)
**UCSB Field Teams**: Dave Menzies, Jon Klamberg, Meredith Meyers, Ellie Wallner, Meg Murphy, Natasha McDonald
**Hansell Group**: Dennis Hansell, Charlie Farmer, Wenhao Chen
- Bill Landing (FSU) and Chris Measures (UHI) (Water samples)
- Bill Smethie & Samar Khatiwala, LDEO (CFC ages)
- Ru Morrison & Mike Lesser, UNH (MAA analysis)
- Wilf Gardner and Team, TAMU (C-Star transmissometer)
- Mike Behrenfeld and Team, OSU (Equatorial BOX project)
- Erica Key and Team, U Miami (AMMA-RB 2006)
- Jim Murray and Team, UW (EUCFe 2006)
- R/Vs Brown, Knorr, Revelle, Melville, Thompson, Ka’I, Kilo Moana