Epeiric Seas and Global Biogeochemical Cycles

School of Earth & Environment
Epeiric Seas and Global
Biogeochemical Cycles
(or, Palaeogeography Matters!)
Rob Newton
Collaborators:
Nefeli Kafousia, Eoghan Reeves, Paul Wignall, & Simon
Bottrell.
Structure
1. C isotopes in;
A modern analogue – Florida Bay & Bahama Banks
An ancient example – Late Ordovician, Laurentia
2. Mo/TOC relationships
modern anoxic environments
3. Early Jurassic European Epicontinental Sea.
A sulfate isotope proxy
Mo/TOC relationships
4. How might epeiric seas modify GBC’s?
What do we mean by Epeiric Seas?
The flooded margins and interiors of the continents.
Modern examples are the Baltic, the North Sea,
Hudson Bay
Their extent is dependent on sea level
The areal extent of Epeiric Seas
through time
The proportion of the continents whose surface is
flooded is at an all time Phanerozoic low
→ So we don’t
know if
modern
environments
are good
analogues.
Hay et al, 2006
Modern analogues
Little Bahama
Bank
Atlantic Reef Tract
• Deeper water very close to
open ocean.
Bahama Banks
• Not laterally disconnected
Florida
Florida
Bay
Atlantic
Reef
Tract
Great
Bahama
Bank
from open ocean.
Andros
Florida Bay
• Partially isolated from
open ocean by barrier
islands.
δ13C & δ18O Florida bay molluscs - Lloyd, 1964
δ13C DIC Florida bay, Bahama Banks - Patterson & Walter, 1994
Modern analogues - δ13C DIC
Atlantic Reef tract
Great & Little
Bahama Banks
Florida Bay
Florida
-1‰ difference
-4‰ difference
-6‰ difference
Differences read at average ocean salinity (blue vertical line = 35‰)
Red shading = range of open ocean DIC δ13C values
(Broecker & Maier-Raimer, 1992; Kroopnick, 1985)
Data from Patterson & Walter, 1994
La
ur
en
tia
Late Ordovician Palaeogeography
Ia
tu
pe
s
Image from University of Northern Arizona website.
Author - Ron Blakey
(http://jan.ucc.nau.edu/~rcb7/global_history.html)
Late Ordovician – study area
• Samples
constrained in time
by two laterally
extensive Kbentonite layers
(altered ash layers)
• Bentonites have
identical Ar-Ar ages
of 454 within error
• Thought to represent no more than 0.5 Ma
although sampling designed to reduce this
substantially
Holmden et al, 1998,
Panchuk et al, 2005
Late Ordovician δ13C timeslice
Iapetus = +1.3 to +1.4‰
Range = -2.0 to +2.5 ‰
• Analysed bulk carbonate
samples
• Both +ve and –ve
gradients in δ13C away
from the Iapetus ocean.
• Isotopic relationships are
also preserved in organic
matter suggesting that
they are not the result of
diagenesis
Iapetus
Panchuk et al, 2005
Late Ordovician δ13C timeslice
-ve relative to Iapetus
Lower productivity, shallow
water, affected by org-C
remineralization
+ve relative to Iapetus
Higher productivity, deeper
water, affected by burial
export of org-C.
Fanton & Holmden, 2007
Late Ordovician sea level rise
Low stand
As sea level rises the balance
of processes affecting the δ13C
of DIC changes
From Fanton & Holmden, 2007
Late Ordovician sea level rise
High stand
The change of C cycling
regime around circled site
promotes a positive shift of up
to 4‰
From Fanton & Holmden, 2007
Comparisons with global δ13C curve
Indiscriminate
measurements
without
reference to
palaeogeography
lead to large
deviations in
perceived global
signal
Panchuk et al, 2005
Mo in the modern ocean
Residence time ~800 Kyr
Concentration ~105 ±5 nmol/kg
OXIC environments – behaves conservatively. Present as the stable
unreactive molybdate ion (MoO42-)
ANOXIC environments - transformed to particle reactive thiomolybdate
(MoOxS2-(4-x), x=0-3). Removed by adsorption to humics/oxyhydroxides
and precipitated into Fe-sulfides.
Fastest removal under sulfidic conditions.
Oxic-suboxic facies most important sink for Mo in modern ocean, BUT
the burial flux much greater under anoxic conditions.
Mo/TOC in modern anoxic basins
Algeo & Lyons, 2006
Clear relationship
between Mo and TOC
but varies depending
upon location
Saanich inlet > Cariaco Basin > Framvaren > Black Sea
Mo/TOC in modern anoxic basins
• Good correlation between
deepwater dissolved Mo
concentration and Mo/TOC
in sediments
Algeo & Lyons, 2006
Mo/TOC in modern anoxic basins
• Independent estimates of
deepwater renewal co-vary
with Mo/TOC
• Long renewal = low Mo/TOC
• Short renewal = high Mo/TOC
This relationship between
deepwater renewal time and
Mo/TOC promises to provide
important new information for
ancient systems and is of
special importance to the study
of Eperic Sea dynamics.
Algeo & Lyons, 2006
Application of Mo/TOC to ancient
environments.
• Open shelf location
• Approximate slope of
Mo/TOC would imply
deepwater restriction
similar to the Black Sea (!).
• Surface samples, not core
from single location →
incorporates lateral
variations in conditions.
Algeo & Lyons, 2006
• Anoxic setting – but water
column only rarely euxinic.
• Moderate levels of H2S are → Need to demonstrate euxinicity
important for Mo removal
before interpreting deepwater
renewal times in ancient systems.
Early Toarcian palaeogeography
YORKSHIRE
Image from University of Northern Arizona website.
Author Ron Blakey
(http://jan.ucc.nau.edu/~rcb7/global_history.html)
TIBET
Early Toarcian anoxic event
(Data from Kemp et al, 2005; Newton & Bottrell, 2007; McArthur 2000; Cohen et al, 2004)
Sections: Yorkshire coast and Tibet
CAS analysed as a
marine-SO4 isotope
proxy
• Yorkshire:
belemnites
• Tibet: whole rock
carbonates
Yorkshire SO4 isotope data
δ34S
• Stable background
at ~16-17‰
• ~+6‰ excursion
starts in the middle
of the OAE interval
Excursion occurs over a
maximum time interval of
1.5 Ma.
VERY RAPID
Low early Jurassic seawater sulfate
concentration?
Modern 29
Early Jurassic
12-16
Yes, but is it low enough?
Horita et al, 2002
Low early Jurassic seawater sulfate
concentration?
Modern ocean residence time = ~20 Ma
Early Jurassic residence time = ~8-11 Ma
Still too slow to reconcile with a 6‰ shift in
1.5 Ma
→ Yorkshire data on their own argue for
an isolated European Sea
Tibet SO4 isotope data
δ34S
No systematic change in
δ34S, but same average
as Yorkshire (+19.1 vs
+19.3‰)
Grey shading denotes tentative
estimate of the timing of the
anoxic event based on C isotopes
& biostrat.
Data comparison
• Comparison of δ34S
•
consistent with common S
source but different
isotopic evolution
δ18O differences
consistent with increased
internal S cycling within
the European Sea
compared to open ocean
Vertical dashed line & shading
denotes Tibetan average ±1sd
Early Toarcian Mo/TOC
• Data from same
•
•
•
Yorkshire section as
CAS isotope data
Interval 2 = OAE
(shaded)
Interval 3 = post
OAE sediments
Evidence for euxinia
at this level.
Pearce et al argue that this is
a signal of global euxinia, but
it is much more likely that
this represents evidence of
regional deepwater isolation.
Pearce et al, 2008
Early Toarcian summary
1. Both a seawater sulfate isotope proxy and Mo/TOC
relationships are consistent with extremely slow deep
water renewal during the OAE interval in the
European eperic sea.
2. Watermass isolation can occur even in apparently
well connected shallow seaways and even affect the
second most abundant anion in seawater.
How might epeiric seas modify
global biogeochemical cycles?
Eperic Seas:
• Maintain a crucial position between weathering fluxes &
the open ocean.
• Have shorter timescale of burial than continental margin
sediments – A shorter return to the Earths surface
environment controlled by sea level.
• May well respond differently to the effects of climatic
events, so the response of earth system will be different
depending on the areal extent of epeiric seas. E.g. A
warming climate might induce anoxia & carbon burial in
Eperic seas but not necessarily in other marine
environments.
Conclusions
1. Epeiric Seas were not just extensions of the open
ocean: A large difference in chemistries should be
considered the norm rather than the exception.
2. Their geochemical response to environmental change
is extremely variable and related to palaeogeography
and sea level and can amplify or diminish the
magnitude of changes seen in proxies for GBC’s.
3. Because of their role as conduit for weathering fluxes
between the land and the oceans Epeiric sea
environments may act as modifiers of GBC’c but we
have yet to fully understand their effect.