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12.1 Introduction
Lac Pavin is a crater lake in the Massif Central (France,
located at 45°29.740 N, 2°53.280 E), with a maximum water
depth of 92 m. It is meromictic, i.e. permanently stratified
with two layers. The upper layer, called mixolimnion,
extends from 0 to ~50–60 m depth and contains abundant
dissolved oxygen (O 2 ). According to the season, and in par-
ticular to the luminosity, there is a strong peak in oxygen
concentrations in the zone of maximum photosynthetic pro-
duction, between surface water and ~20 m depth. There is
diffusional O 2 exchange and O 2 transfer deeper into the lake
by mixing during seasonal overturns. The oxygen chemo-
cline ranges from ~50 to ~65 m depth, depending on the effi-
ciency of water mixing. Some year the mixolimnion may be
partially mixed, only down to ~30 m depth (Aeschbach-
Hertig et al. 2002 ). Others authors noted that the spring over-
turn can affect the water column down to 60–70 m depth,
depending on the year (Restituito 1987 ). The autumn over-
turn is much less intense (Restituito 1987 ). From ~70 to
92 m depth, a stable layer, called the monimolimnion, does
not mix with oxygenated upper water and exchanges solutes
by diffusion only. The high organic matter (particulate
organic carbon, POC) flux from the surface waters in Lac
Pavin results in a complete consumption of oxygen in the
monimolimnion. A purported deep ferruginous spring prob-
ably feeds this anoxic zone with dissolved ferrous iron
(Meybeck et al. 1975 ; Assayag et al. 2008 ; Jézéquel et al.
2011 ). Between the mixolimnion and the monimolimnion,
an intermediate layer, called the mesolimnion, is character-
ized by a sharp gradient of salinity and dissolved chemical
species. There are varved, Fe-rich sediments in the bottom of
the lake, making Lac Pavin a potential modern analogue to
the Earth’s oceans and an archive of recent environmental
change in the Massif Central (Stebich et al. 2005 ; Schettler
et al. 2007 ; Busigny et al. 2014 ). Numerous studies have
been published on biogeochemical cycling at Lac Pavin and
a wealth of data cover a wide variety of topics such as geo-
chemistry (Michard et al. 1994 ; 2003 ; Viollier et al. 1995 ,
1997 ; Albéric et al. 2000 , 2013 ), biology and microbiology
(Amblard and Bourdier 1990 ; Biderre-Petit et al. 2011 ;
Lehours et al. 2007 , 2009 ), hydrology and dynamic of the
water column (Meybeck et al. 1975 ; Aeschbach-Hertig et al.
2002 ; Assayag et al. 2008 ; Jézéquel et al. 2011 ; Bonhomme
et al. 2011 ), mineralogy and sedimentology (Schettler et al.
2007 ; Chapron et al. 2010 ; Cosmidis et al. 2014 ).
The Fe cycle at Lac Pavin is driven by redox cycling:
there is extensive ferric iron reductive dissolution in the
anoxic layer and at the bottom of the lake, upward diffusion
of Fe2+, oxidation and precipitation as Fe(III) at the redox
interface, and finally Fe particles settling (Michard et al.
2003 ). This cycling, termed “iron wheel” or “ferrous wheel”,
is known for other anoxic water systems, where sulfate and
sulfide concentrations are low enough so that dissolved Fe
can accumulate (Campbell and Torgersen 1980 ; Davison
1993 ). The ferrous wheel in anoxic basins has a strong
impact on biogeochemical cycles (e.g. Hongve 1997 ) and
plays a key role for bacteria and archaea that derive their
chemical energy from redox reactions.
Lac Pavin is a relatively unique analogue to Earth’s early
oceans—in particular Archean marine systems (Martin
1985 ; Busigny et al. 2014 ). At that time, oceans were anoxic,
sulfate-poor and ferruginous, and Fe represented a central
element driving other biogeochemical cycles such as carbon,
sulfur, nitrogen or phosphorus (Konhauser et al. 2011 ). There
is evidence for Archean ferruginous conditions in a wide
range of Fe-rich sedimentary deposits, including some shales
and iron formations (Poulton and Canfield 2011 ).
Determining the significance of the geochemical signatures
recorded in these sedimentary rocks, and extrapolating
paleo-environment and -ecosystem conditions, is compli-
cated by the lack of studies on modern analogues (e.g.,
Crowe et al. 2008 ; Staubwasser et al. 2013 ) and laboratory
experimental calibration (e.g., Konhauser et al. 2002 ; 2007 ).
Lac Pavin represents a natural laboratory in which redox sen-
sitive element cycling under ferruginous conditions can be
studied.
In the present paper, we present chemical and isotopic
data available for dissolved and particulate matter, and recent
mineralogical advances on the water column and bottom
sediments of Lac Pavin. In particular, we describe the Fe bio-
geochemical cycle and its interactions with phosphorus (P),
manganese (Mn), sulfur (S) and organic matter. A geochemi-
cal model consistent with chemical, isotopic and mineralogi-
cal data supports our current view of the Fe cycle in Lac
Pavin and allows us to quantify some chemical and physical
parameters such as chemical reaction rates, the velocity of
falling particles, and Fe isotope fractionations associated
with various processes in the water column. We also high-
light evidence for microbial mediation and outline the links
between the water column Fe cycle and sediment Fe-bearing
phases.12.2 Methods
12.2.1 SamplingWater samples were collected either from a sampling plat-
form anchored near the center of the lake or from a boat,
using various types of sampler (syringes, Niskin bottles),
equipped with an electronic depth gauge providing a preci-
sion of ±0.2 m on the depth position in the water column
profile. Waters were filtered to 0.2 μm and acidified with aV. Busigny et al.