Effects of hydrothermal biogeochemistry on the oceans over ...

<strong>Effects</strong> <strong>of</strong> <strong>hydrothermal</strong> <strong>biogeochemistry</strong>
on the oceans over geologic time
Lee Kump
Penn State
www.lpi.usra.edu

• Mantle-dominated Archean Ocean
• Hydrothermal fluids in the absence <strong>of</strong> sulfate (redox, no
anhydrite)
• Submarine volcanism and the rise <strong>of</strong> atmospheric
oxygen
• Phanerozoic major-element chemistry and vent fluids
• Wilson cycles <strong>of</strong> seafloor production, Mg/Ca and CO 2
• Fluid inclusions and modeling
• Problems
• Conclusions concerning role <strong>of</strong> <strong>hydrothermal</strong> geochemistry
in evolution <strong>of</strong> ocean chemistry
www.lpi.usra.edu

HIGH Fe, H 2 , H 2 S
LOW 87 Sr/ 86 Sr
Stanley and Hardie, 1999

Shields & Veizer (2002)

Shields & Veizer (2002)

Knoll and Holland (1995)

Huston & Logan (2004)

Kump and Seyfried (2005)

Cooling path for vent fluid derived from
sulfate-bearing seawater
Kump and Seyfried (2005)

Cooling path for vent fluid derived from
sulfate-free seawater
Kump and Seyfried (2005)

Why are ridges today near the
critical curve for seawater?
de Wit, 1998

Effect <strong>of</strong> pressure and temperature on dissolved Fe in 3.2% NaCl
(seawater) coexisting with basalt and its alteration products
14
12
Modern Seafloor
410°C
Modern Subseaflor Reaction
Zones
778
667
Fe, mmolal
10
8
6
4
380
400
390
556
444
333
222
Fe, ppm
2
0
350 360 370
111
0
150 200 250 300 350 400 450 500 550
Termination <strong>of</strong> isotherms at low pressure
defines the two-phase boundary <strong>of</strong> seawater
Pressure, bars
Data from: Seyfried (1987), Seyfried and
Janecky (1985); Rosenbauer and Bisch<strong>of</strong>f
(1983)
Kump and Seyfried (2005)

Kump and Seyfried (2005)

Dales Gorge, Paleoproterozoic

Why did atmospheric O 2 rise
at ~2.4 Ga?
• This is when cyanobacteria
invented oxygenic
photosynthesis; or
• Cyanobacteria evolved
oxygenic photosynthesis ≥
2.7 Ga, but O 2 sinks
exceeded sources until 2.4
Ga

Rise <strong>of</strong> oxygen occurs when sources exceed sinks
org C, pyr S
burial
CO 2 + H 2 O
CH 2 O + O 2
Atmospheric O 2
org C, pyr S
weathering
oxidation <strong>of</strong>
volcanic fluids
Holland’s (2002) “f” value: reducing power <strong>of</strong> volcanic gas or
<strong>hydrothermal</strong> fluid relative to biospheric demand: f > 1 allows
H 2 to accumulate in atmosphere (i.e., no O 2 )

Phanerozoic Trends

Lowenstein et al. 2003 (Geology)

Published by AAAS
T. K. Lowenstein et al., Science 294, 1086 -1088 (2001)

Fantle (unpub)

Hydrothermal uptake <strong>of</strong> Mg, release <strong>of</strong> Ca
∝ (spreading rate, [Mg 2+ ] )

Gaffin sea-level inversion, no feedback
T. K. Lowenstein et al., Science 294, 1086 -1088 (2001)
Published by AAAS

Growing consensus:
variations in seafloor spreading
rate control:
CO 2 fluctuations
climate variations
(hothouse/greenhouse)
seawater chemistry
carbonate mineralogy

But problems!
alkalinity balance
seafloor production rates haven’t varied
significantly through time! (?)
or if they have, they’ve INCREASED over
last 30-40 million years

Gaffin sea-level inversion, no feedback
Gaffin sea-level inversion, feedback
T. K. Lowenstein et al., Science 294, 1086 -1088 (2001)
Published by AAAS

Rowley (2002)

Implications
No CO 2 driver
No seafloor chemistry driver
No sea-level driver
Expected result: global rate <strong>of</strong> seafloor
production ultimately tied to mean rate <strong>of</strong>
heat production in mantle (varies slowly in
time)

But wait!
Demicco (2004)
spreading rates today vary from 7.3 to 180
mm/yr
Kominz (1984) showed that spreading rates for
preserved seafloor clearly varied through time
sublithospheric flows non-steady
ridge jumps, abrupt changes in plate motions,
subduction <strong>of</strong> ridges => steady state model
unlikely (?)

Demicco (2004)

Cogne and Humler, 2006

new
old
Miller et al. (2006)

Anoxia, pyrite, dolomite
Oxic
Anoxia, pyrite, dolomite
Miller et al. (2006)

Conclusions
• Vent fluids today, and associated biological
communities, may not be good analogue for those
<strong>of</strong> the distant geologic past, especially if sulfate
content <strong>of</strong> seawater has varied.
• Archean <strong>hydrothermal</strong> systems were Fe- and H 2 -
rich and unclogged.
• Vent fluids played an important role in
controlling seawater chemistry early in Earth
history; role in the more recent geologic past
unclear.
www.dfo-mpo.gc.ca