Benthic nepheloid layer dynamics and potential role in carbon cycling on continental margins

Cindy PilskalnUniversity of Massachusetts

School for Marine Science and TechnologyNew Bedford, MA

O C b d Bi h i t W k hOcean Carbon and Biogeochemistry WorkshopJuly 2010

Scripps Inst of OceanographyScripps Inst. of Oceanography

Schematic diagram of major fluxeswithin the Benthic Boundary Layer:

Interface between pelagic and benthic environments1. Exchange of BBL-interior

water by turbulent eddies and advection.

p g

2. Particle sinking and resuspension; migrating

zooplankton.

**Common occurrence of physically**Common occurrence of physically

Particleresuspensionlayer/b thi h l id l

Top of BBLCommon occurrence of physically Common occurrence of physically

maintained, intermittent or maintained, intermittent or permanent particle resuspension permanent particle resuspension layer = Benthic Nepheloid Layer layer = Benthic Nepheloid Layer

(BNL) (BNL) 6.6.benthic nepheloid layer

3. Exchange of dissolved, colloidal, and suspended particle matter

across the sediment-water interface.interface.

4 & 5. Bioturbated sediment mixing zone : injection and modification of particulate, colloidal and dissolved

materialmaterial.

6. Lateral advection input of material.

Modified from CBED Rept., 2004

Benthic Nepheloid Layers in the Ocean:

For many years interest and focus on deep BNLs @>2 km (e.g., McCave, Biscaye, Eittrem, Spinrad, Gorsline, Ewing).

Layer was primarily defined and recognized by substantial increase in suspended particles indicated by significant increase in light attenuation and

extending to sediment-water interface.

Found along base of deep slopes and in abyssal basins strongly influenced by western boundary currents and deep penetrating eddies thus driven by

deep energetic flowsdeep energetic flows.

Composition assumed and found to be largely lithogenic—fine clays C p g y g yparticles (2-5 m) responsible for optical signature of high turbidity.

Close correlation between suspended particle mass (SPM) and attenuation indicates suspended matter in deep BNLs is refractory, high scattering

mineral particles.

Increased examination of continental margin BNLs, biological rate studies and new optical and particle collection technologies reveal a far moreand new optical and particle collection technologies reveal a far more

geochemically, biologically, and physically dynamic BNL with a variable and diverse particle population .

Optical and pump studies: BNL contains mm-size flocs, light scattering AND absorbing particles lack of strong correspondence between cp and SPM;

strong quantitative relationship between cp and POC; variable PSD with depth g q p p pin BNL impacts cp; aggregation and disaggregation of organic detrital particles

(Boss, Biscaye, Bishop, Hill, Pilskaln).

Geochemical and optical studies: organic matter in BNL labile POM such as chlorophyll; tightly bound aged organic carbon on clays; seasonal variation in delivery of POC and biogenic minerals to BNL; pulsed inputs to BNL of N-

d f di t i bl CDOM t ti l t d t icompounds from sediments; variable CDOM concentrations.; elevated protein, POC, PON vs. immediately overlying clear water (Ransom, Pilskaln, Townsend,

Christensen, Mayer).

CBED Rept.

Biological and chemical studies: HIGH zooplankton biomass, diversity and grazing/respiration rates in BNL; attached and free-living bacterial communities distinguishable from surface water communities; elevated

protozoan biomass relative to overlying particle-free water (Smith, Wainright, Wishner, Puig, Townsend, Rooney-Varga)

Physical flow/optical/chemical studies: significant role of tidally generated internal waves in forming and maintaining margin BNLs;

seasonal storms and inflow of slope waters onto shelf increased turbidityseasonal storms and inflow of slope waters onto shelf increased turbidity and nutrient injection to BNL (Fanning, Drake, Cacchione, Grant).

Diverse data sets indicate that BNL is a distinct environment within which biologically and physically driven particle basedwithin which biologically and physically-driven, particle-based chemical transformations occur and represent early diagenetic reactions, thus impacting the balance between remineralization and the benthic delivery of PON and POC over variable timeand the benthic delivery of PON and POC over variable time

scales.

Common occurrence of BNLs on the continental margins where POC production, export, benthic remineralization, and

accumulation are maximal argues for better understanding of biogeochemical processes occurring in environment through which all particle matter must pass prior to sediment/water

interface delivery.

I th i t h ft b d b t di t d POC d liIs the mismatch often observed between predicted POC delivery rates and carbon required to fuel benthic oxygen consumption due

in any measure to POC modification in the BNL?

Gulf of Maine: Semi-enclosed shelf sea with several deep (~-300 m) basins, shallow ledges and extensive glacially-depositedsand bank (Georges Bank).

Black arrows: Surface flow, <75 mRed arrows: Deep flow, >75 m

Brooks 1985; Beardsley et al. 1997; Lynch et al. 1997; McGillicudy et al. 1998; Pettigrew et al1998; Pettigrew et al. 1998

Strong tidal mixing, localized upwelling and periodic inflow of upwe g a d pe od c ow owarm, nutrient-rich Warm Slope Water or cold, low salinity Labrador Slope Water.

Townsend et al, 2010

Temporal variability in net community production (NCP = autotrophy –respiration integrated to 1% light level) from high-rate O2 data (UNH Buoy)

40005000

Western Gulf of Maine

‐10000

1000200030004000

mg C/m^2/day)

‐5000‐4000‐3000‐20001000

6/7/2009 7/7/2009 8/6/2009 9/5/2009 10/5/2009 11/4/2009 12/4/2009

NCP

 (m

/ / / / / / / / / / / / / /

Date

Mass Balance Model:

NCP = Zd O2

+ FMean Winter NCP =  ‐ 601 mg C/m2/d

NCP = Z dt

+ Fs Mean Summer NCP =  485 mg C/m2/d

Salisbury and Vandemark, unpublished

Thick BNLs in basins: First documented with CTD/beam attenuation profiles in 1986 (Spinrad).CTD/beam attenuation profiles in 1986 (Spinrad).

Found to be persistent and consistent feature of 10-30 m thickness throughout Gulf (numerous

i ti t 1986 t)investigators, 1986-present).

WilkinsonBasin

Jordan Basin

JBJB

WB

Pilskaln et al., 1998; 2007, 2008

Examples of 2004 Gulf Portsmouthacross Stellwagen p

of Maine beam attenuation profile

transects.Mass Bayto Wilkinson Basin (A)

to Wilkinson Basin (B)

SPM (mg/l on GFF):<0.5 blue-purple>4.0 orange-redBeam Attenuation (m-1)0 25 1 0

250 m250 m

*Evidence of contour-aligned BNLs

0.25 1.0

Deep coastal current and Gulf of Maine cyclonic circulation

Muscongus Bay toWestern Jordan Basin (E)

Penobscot Bayto Jordan Basin (G)

producing contour-focusing of BNL (?)

250 m

200 m

Mt. Desert to Jordan

*Beam attenuation due to

ti l ( )Basin (H) NorthwesternJordan Basin (J)

particles (cp) vs. SPM relationship

problematic in GoM:

250m 200m

Beam Attenuation (m-1)0 25 1 0

*Low r2 value of 0.46

Grand Manan Is. Bay of

Beam Attenuation (m 1)0.25 1.0

Reflects particle diversity

=to Bay of FundyChannel (M)

Fundy (N) A mix of light absorbing & scattering,

inorganic andinorganic and organic particles

Not just 220m 125m suspended

refractoryclay minerals!

Time-series sediment traps: Substantially higher deep-water resuspension fluxes

Clear evidence of active benthic nepheloid layers.

**Significantly higher resuspension fluxes in east (Jordan Basin).

*Seasonal peaks in mass export commonly reflected in deep BNL p p y presuspension fluxes.

1995-’97 Jordan Basin: 150 m

1995 ’97 J d B i 2501995-’97 Jordan Basin: 250 m

Translation of seasonal CO2 uptake and 10 production to 100 m in Western Gulf of

Maine/Wilkinson Basin:

High rates of remineralization leads to <1% POC produced delivered to 100 m.

2009: Summer phytoplankton bloom--becoming more common in past few

years included a very large bloom ofyears--included a very large bloom of dinoflagellate Alexandrium sp.

Salisbury and Vandemark, unpubl.; Pilskaln, unpubl.

Examples of POC and biogenicExamples of POC and biogenic mineral resuspension fluxes

measured in Gulf of Maine BNL.

Seasonal peaks in biogenic component fluxes in upper trapscomponent fluxes in upper traps

well above BNL typically reflected in BNL resuspension fluxes.

0.6

0.8

1

1.2

Bea

m c

(m-1

)

Beam cBNL particle properties:

from transmissometer,

4 3

4.4

0.2

0.4

10/27/05 11/10/05 11/24/05 12/8/05 12/22/05 1/5/06 1/19/06 2/2/06 2/16/06 3/2/06 3/16/06 3/30/06 4/13/06

B

Di l d O

,backscattering sensor & chl a fluorometer

Oct 2005-Apr 2006

3 8

3.9

4.0

4.1

4.2

4.3

DO

(ml/L

)

Dissolved O2Oct 2005 Apr 2006

time-seriesJordan Basin, 254 m

*Increase in light3.7

3.8

10/27/05 11/10/05 11/24/05 12/8/05 12/22/05 1/5/06 1/19/06 2/2/06 2/16/06 3/2/06 3/16/06 3/30/06 4/13/06

Date

Chl a

Increase in light attenuation coincident

with decreasing DO and increase in

particles of relativelyparticles of relatively higher organic

content & higher [chl a]

Higher inorganic content

Higher organic content

Backscatter/Beam attenuation

(Based on Twardowski et al., 2001)

Temperature (0C)Jordan Basin:

BNL organic carbon age and lability:

21 ‰

Basin:April2006

Refractory/Labile?

*Average BNL (and surface di t) ti l t i

14Cvalues

Depth

sediment) particulate organic carbon content = 3%

*BNL resuspension flux:

valuesin blue

DepthpRelatively low C/N ratio of 8

Pump-sampling and 14C of Jordan Basin suspended POC

-91 ‰

-171 ‰

Jordan Basin suspended POC April 2006:

Mixed POC sources of aged

Beam attenuation (m-1)

carbon from bottom sediments and younger planktonic carbon

originating in upper water column (Hwang & Eglinton unpubl )

Sediment sample 14C= -214 ‰

column (Hwang & Eglinton, unpubl.)

2009-2010 Wilkinson Basin BNL CDOM Time-Series: 200 m

March 2010 input of fresh water to BNL reflected in large CDOM spike from typically low values of higher salinity water.

10/02/09 1/21 1/10 3/01 4/19/10

No BNLDNA/PCR (polymerase chain reaction) analyses for microbial

it iti (8community composition (8 stations, 41 samples). OTU =Operational taxonomic unit.

Near surfaceNear bottom

Free-living and attached bacteria in BNL (no surprise).

Strong BNLNear surfaceNear bottom

Intriguing results:

Where BNL absent, surface and near-bottom bacterial communities similar.bottom bacterial communities similar.

Where BNL present, surface and near-bottom communities distinctly

diff tdifferent.

?More denitrifiers when BNL present?

In the deeper Gulf of Maine basins decrease in dissolved oxygen associated with benthic nepheloid layer.

Macrozooplankton pfrom low O2 BNL in

Eastern Gulf of Maine

Biological O2 consumption in BNL and oxidation of resuspended sedimentary reduced compounds: Supported by Christensen benthic fluxes/Packard respiratory ETS/Townsend

et al., 1992 BNL biological data.

Cyst size: 50 x 100 m (from Kirn et al., 2005)

Abundant biological component in Gulf BNL: toxic dinoflagellate Alexandrium cysts.

Deep Western Gulf of MaineDeep Eastern Gulf of Maine

Wilkinson Basin 2008-09:200 m

# c

ysts

/m2/d

#

Surface sediment (0-1 cm) Alex. cyst counts, Oct. 2009

Location of 2008-2010 Wilkinson basin

Anderson and Keafer, unpubl.

Location of 2008 2010 Wilkinson basin subsurface time-series traps &extremely high

cyst resuspension fluxes

Gulf of Maine ongoing work:

Gulf BNL offers natural laboratory to examine numerous biogeochem. and physical i i h lf b d BNL i lprocesses occurring in shelf-based, BNLs in general.

Mooring/ buoy-based data collection continuing thru –integrated ~2year data sets forthcoming.g

Modeling of Gulf-wide BNL distribution and movement and address question of offshore transport to adjacent deep slope.

Conclusions:

Lateral movement of BNL indicated by comparison of trap resuspension fluxes, surface sediment cyst abundance and seasonal delivery from overlying water column.sediment cyst abundance and seasonal delivery from overlying water column.

BNL shows strong compositional relationships to seasonal upper water column biogenic fluxes and impacts of various fresh and saltwater inflows to Gulf.

50% of POC in BNL resuspension fluxes: 50% POC, 25% opal, less than 10% CaCO3originates in upper water column.

So check out your local BNL—may surprise you!

Th k t NOAA RMRP d ECOHAB d t tThanks to NOAA RMRP and ECOHAB programs and to many past and present Gulf of Maine-iac colleagues.