chemical reaction in earth surface
TRANSCRIPT
-
7/29/2019 Chemical Reaction in Earth Surface
1/23
Reactions at the earthssurface
-
7/29/2019 Chemical Reaction in Earth Surface
2/23
Geochemistry of the earths surface is
dominated by aqueous solution and their
interactions with rock.
In this chapter, we focus on water and its
interaction with solids at the Earths surface
-
7/29/2019 Chemical Reaction in Earth Surface
3/23
We can broadly distinguish two kinds of
aqueous solutions:
1) Continental water: ground water, fresh
surface waters (river, stream and lake waters),
and saline lake waters.
2) Seawater
Hydrothermal fluids are third class of water produced when water
is heated and undergoes accelerated interactions with rock and
often carry a much higher concentration of dissolved
constituents than fresh water.
-
7/29/2019 Chemical Reaction in Earth Surface
4/23
Redox in Natural Waters
The surface of the Earth represents a boundary between
regions of very different redox state.
The atmosphere contains free oxygen and therefore is highly
oxidizing
In the Earths interior, however, there is no free oxygen
Natural waters exist in this boundary region and their redox
state, perhaps not surprisingly, is highly variable.
Biological activity is the principal cause of the different redox
state.
The redox state of solutions and solids at the Earths surface is
largely governed by the balance between photosynthesis and
respiration.
-
7/29/2019 Chemical Reaction in Earth Surface
5/23
Water in equilibrium with atmospheric
oxygen has a pe of +13.6 (at pH = 7).
At this pe, thermodynamics tells us that all
carbon should be present as CO2 (or
related carbonate species), all nitrogen as
NO3-, all S as SO4
2-, all Fe as Fe3+, and
all Mn as Mn4+.
-
7/29/2019 Chemical Reaction in Earth Surface
6/23
-
7/29/2019 Chemical Reaction in Earth Surface
7/23
-
7/29/2019 Chemical Reaction in Earth Surface
8/23
Biogeochemical Redox Reactions
photosynthesis and atmospheric exchangemaintains a high p in surface waters.
Once oxygen is consumed, a variety of
specialized bacteria continue to consumeorganic matter and respire utilizing oxidants
other than oxygen. Thus p will continue to
decrease.
-
7/29/2019 Chemical Reaction in Earth Surface
9/23
Figure 13.1. Important biogeochemical redox
couples in natural waters.
-
7/29/2019 Chemical Reaction in Earth Surface
10/23
Eutrophication
The extent to which the redox sequence
depends on a several factor:
1. Temperature structure2. Nutrient levels
-
7/29/2019 Chemical Reaction in Earth Surface
11/23
In lakes with high nutrient levels:
Regions where dissolved oxygen is present aretermed oxic, those where sulfide or methaneare present are called anoxic.
Regions of intermediate p are called suboxic. Lakes are where suboxic or anoxic conditions
exist as a result of high biological producitivy
are said to beeutrophic
. The process is called eutrophication
-
7/29/2019 Chemical Reaction in Earth Surface
12/23
Redox Buffers and Transition Metal
Chemistry
The behavior of transition metals in aqueous
solutions and solids in equilibrium with them
is particularly dependent on redox state. Many transition metals have more than one
valence state within the range of p of water.
Redox conditions influence a strong control onthe concentrations of these elements in natural
waters.
-
7/29/2019 Chemical Reaction in Earth Surface
13/23
For the example: Fe and Mn
There two common circumstances wherehigher Fe and Mn concentrations in water
occur:
1) The first is when sulfide ores are exposedby mining and oxidized to sulfate
2) The second circumstance where higher Fe
and Mn concentrations occur is under suboxicor anoxic conditions that may occur in deep
waters of lakes.
-
7/29/2019 Chemical Reaction in Earth Surface
14/23
-
7/29/2019 Chemical Reaction in Earth Surface
15/23
Weathering, Soils, and
Biogeochemical Cycling
-
7/29/2019 Chemical Reaction in Earth Surface
16/23
Weathering is the process by which rock is
physically and chemically broken down into
relatively fine solids (soil or sediment
particles) and dissolved components.
Weathering plays a key role in the exogenic
geochemical cycle (i.e., the cycle operating at
the surface of the Earth).
Weathering related to atmospheric CO2
-
7/29/2019 Chemical Reaction in Earth Surface
17/23
What are the factors that control on weathering
rates?
1) Temperature (Berner et al. (1983) and
Berner (1991))
2) Tectonic uplift and exposure of fresh rock
(Edmond et al. (1995))
-
7/29/2019 Chemical Reaction in Earth Surface
18/23
Soil Profiles
-
7/29/2019 Chemical Reaction in Earth Surface
19/23
Chemical Cycling in Soils
-
7/29/2019 Chemical Reaction in Earth Surface
20/23
-
7/29/2019 Chemical Reaction in Earth Surface
21/23
Biogeochemical Cycling
The biota plays a substantial role in the
weathering process and in controlling the
composition of streams.
The biota will influence both soil and water
chemistry within a water-shed and can be asignificant reservoir for some elements.
-
7/29/2019 Chemical Reaction in Earth Surface
22/23
There are four sources of nutrients in an
ecosystem:
1) The atmosphere
2) Dead organic matter
3) Water
4) Rock
-
7/29/2019 Chemical Reaction in Earth Surface
23/23
WEATHERING RATES AND REACTIONS
The Watershed Approach