050428 Carbonate Solubility Lecture 20 DM 2701

8/9/2019 050428 Carbonate Solubility Lecture 20 DM 2701

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Carbonate Solubility

Solubility

Dissolution mechanisms

Dissolution rate expressions

Saturation state in the oceanT, P, and CO

2release

In situ [CO32 and the saturation hori!on

Sedimentary e"idence o# carbonate dissolution

$inetic and%or thermodynamic controls


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p& scales

'ree

Total

Sea(ater

)eebe and *ol#+ladro(


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p& scale comparisons

p$ comparisons


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Carbonate Solubility

CaCO3-./ Ca02

0 CO32

De#ine satauration state as1

. IP % $sp

*ith IP 4ion acti"ity product5

. [Ca02[CO32 #or the solution,

and (ith the apparent solubility product1

#or a solution in e6uilibrium (ith solid CaCO34T,P,S5

02 +27 3sp

[Ca [CO $

8=


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$9sp depends on mineralo:y 4calcite - ara:onite - ;:+calcite,


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O#ten, (e ta=e [Ca02 as a constant #unction o# salinity,

and express in terms o# the saturation carbonate ion concentration

so, i# [Ca02 constant,

De#ine CO32 as 4[CO

32

in situ+ [CO

32

sat5

02 +2 02 +2

3 in situ 3 in situ

7 02 +2

sp 3 saturation

[Ca [CO [Ca [CO

$ [Ca [CO = =

+2

3 in situ

+2

3 saturation

[CO

[CO


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Controls on saturation state1

Pressureand temperature e##ects on carbonate solubility

Solubility hi:her in deeper and colder (ater

Pressure, temperature, and respiratione##ects on [CO32

in situ

[CO32 lo(er in deeper, colder, and >older? (ater

@roec=er and Pen:


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@en+Aa=o" et alB,

in situ saturometer


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Pro#ile o# saturation

carbonate ion concentration,

based on in situ saturometerE

stron: increase in solubility

(ith increasin: depthB

@roec=er and Pen:


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Sayles, 8FGH

se obser"ed

$9sp in J+8H cm

inter"al o# in situ

pore (ater pro#iles4l= 0 p&, . Ca5

to estimate

solubilityB


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Solubility "s depth #or Sayles9 pore

(ater data and lab solubility studies


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Solubility "s depth

comparisonB

@roec=er and Pen:


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*hat is the relationship bet(een de:ree o# undersaturation and

dissolution rateK

CaCO3-./ Ca020 CO

32

De#ine the mass+normali!ed dissolution rate1

>=L? is the reaction rate 4M%day5

>n? is the reaction order

>L? to account #or M CaCO3in bul= sediments, and #or sur#ace area

Can express dissolution rate in terms o# carbonate ion concentration

N . =L 4[CO32

sat+ [CO

32

in situ5n

*here 4[CO32

sat is the saturation carbonate ion concentration 4T,P5

+8 LN 4M d 5 48 5nk=


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n unresol"ed pu!!le1

aboratory dissolution studies 4$eir, 8FGJ5 and

theoretical ar:uments 4;orse5 su::est that

=L 8JJJ % day

and

n . BH

stron:ly non+linear

#ast dissolution

really #ast at lo(

@ut all pore(ater dissolution studies :i"e

=L 8 + 8J % day48J2to 8J3slo(er5

and

n . 3, or e"en 8

4&ales and Qmerson, 8FF5


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;orse and r"idson + &i:h order

=inetics consistent (ith controlo# dissolution by one o# the

sur#ace processes

4adsorption, mi:ration, reaction,

mi:ration, desorption5

&ales and Qmerson, Sayles and ;artin

Shallo( depth at (hich pore(aters

reach saturation implies lo(+order

=ineticsE other(ise #inal approach tosaturation (ould ta=e much

lon:er 4deeper5B


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&ales and Qmerson

Necalculate (ith ne(

constants1 n . 3B2

Neassess solubiliy

too1 n . 8B3


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*hy do (e careK

In#luences o# dissolution rate la( include1"ertical pro#ile o# dissolution in sediments

impact o# CO2 release by benthic decomposition

total dissolution rate

shape o# lysocline


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Saturation state in the ocean

In#luence o# T, P, and CO2release

In situ [CO32 and the saturation hori!on


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Pressure,temperature, and

respiration e##ects

on [CO32

in situB

[CO32

lo(er indeeper, colder, and

>older? (aterB

:in: dominatesB

Temperature

0 5 10 15 20 25 30

CO3=umol/kg

200

250

Const Alk (2320) and DIC (19!)

Dept" (m)

0 1000 2000 3000 #000 5000

CO3=umol/kg

200

250T = 25 C

T = 3 C

$ = sur%a&e

'espraton DIC n&rease (umol/kg)

0 100 200

CO3=um

ol/kg

0

100

200

3 C 3*5 km no dssoluton


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&o( does [CO32 respond to chan:es in l= or DICK

CT . [&2CO3L 0 [ &CO3 0 [CO32

~ [ HCO3] + [CO

32] 4an approximation5

l= . [O& 0 [&CO3 0 2[CO

32 0 [@4O&5

+ [&0

~ [HCO3] + 2[CO

32] 4aB=BaB >carbonate al=alinity?5

So 4rou:hly51

[CO32] ~ Alk CT

CT , [CO

32] Alk , [CO

32]


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Photosynthesis

CO20 &

2O ./ >C&

2O? 0 O

2

CO2

. +8

l= . J

So [CO32 . l= + CO

2. J 4+85 . 08

[CO3

2] increases, increasesB

Nespiration

>C&2O? 0 O

2 ./ CO

20 &

2O

CO2 . 08l= . J

So [CO32 . l= + CO

2. J 8 . +8

[CO32] ecreases, ecreases!


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)eebe and *ol#+ladro(


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Calci#ication

Ca020 CO32 ./ CaCO

3

CO2

. +8

l= . +2

So [CO32 . +2 4+85 . +8

[CO3

2] ecreases, ecreasesB

Dissolution1

CaCO3 ./ Ca020 CO

32

CO2

. 08

l= . 02

So [CO32 . 2 8 . 08

[CO32] increases, increases!


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% CaCO3vs. water depth

lysocline onset ofdissolution

calcite compensation

depth


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Ta=ahashi and @roec=er 9GJE QOSQCS data


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Calcite

saturation

ra:onite

in situ


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@et!er et alB, 8FG

nother pu!!le1

RPac sediment traps loss o#

carbonate abo"e the

ara:onite saturation hori!onB

&i:h ara:onite%calcite ratio

in sin=in: #lux1

JB2 to 2J, most / 8


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Carbonate #lux in Rtl

sediment traps loss o#

carbonate abo"e the

ara:onite saturation hori!onB

;illiman et alB, 8FFF

ThermodynamicsK &i:h+

solubility phase4s5K

;a:nesian+calcite5

rti#actsK


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Sedimentary e"idence o# carbonate dissolution

$inetic and%or thermodynamic controls


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Sediment

e"idence #or

dissolutionK


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&ere, core+top ara:onite

distribution seems to

match the depth o# theara:onite saturation

hori!onB


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'arrell and Prell


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delsec= 8F

lab study o#

selecti"edissolution o#

plan=tonic

#oramini#era

ssembla:e chan:e

only a#ter substantial

carbonate dissolutionE

not a sensiti"eindicator


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@er:er et alB, 8FG2 Selecti"e dissolution in Q6Pac

Di##erential dissolution1 a ran:e o# susceptibility

ThermodynamicsK 4di##erent solubilitiesK5

$ineticsK 4di##erent area%:, crystallinityK5


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M radiolaria

M (hole menardii

M benthic

M (hole plan=tBM CoarseM CaCO3

Peterson and Prell a

>composite dissolution index?


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Peterson and Prell a>composite dissolution index? compared

(ith (ater column saturation stateB


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ohmann si!e+normali!ed

shell (ei:htB


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Si!e+normali!ed shell

(ei:ht su::estsdissolution startin: (ell

abo"e the calcite

saturation hori!onB


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050428 Carbonate Solubility Lecture 20 DM 2701

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