Soil organic matter (SOM) is the largest reservoir of terrestrial carbon and plays an important role in the global carbon cycle. Carbon isotope systematics of SOM have been widely used to constrain the dynamics of this important carbon reservoir though interpretation of carbon isotope data remains controversial. It has been widely observed that the 13 C/ 12 C ratio of SOM increases systematically with soil depth though there is little consensus as to which of several mechanisms may be responsible for this pattern. Here we present a process‐based theoretical model of steady state δ 13 C SOM versus depth profiles, which is coupled with a carbon concentration model. We show that widely observed δ 13 C SOM versus depth profiles can be modeled using a reasonable range of model parameters. More importantly, we show that coupling carbon isotope data with carbon concentrations in soils allows for tighter constraints on model parameters that have biological and environmental significance.
A compilation of 68 studies from throughout many of the world9s mountain belts reveals an empirically consistent and linear relationship between change in elevation and change in the isotopic composition of precipitation along altitudinal transects. The isotopic composition of precipitation decreases linearly with increasing elevation in most regions of the world except in the Himalayas and at elevations >5000 m. There are no significant differences in isotopic lapse rates from most regions of the world (∼0.28 permil/100 m) except at the extreme latitudes where isotopic lapse rates are higher. Given information on past changes in the isotopic composition of precipitation preserved in pedogenic or authigenic minerals, this global isotopic lapse rate can be used to place numerical constraints on the topographic development of some ancient mountain belts or plateaus. There are many complicating factors that can confound interpretation of paleoelevation change based on stable isotopes, and many of these are unique to specific mountain belts or time periods. Relevant to all stable isotope based paleoelevation change studies is the temperature dependent isotope fractionation between a pedogenic or authigenic mineral and the water from which it forms. In cases where isotopic proxy minerals are sampled from localities where temperature will change simultaneously with elevation change, the apparent change in the isotopic composition of precipitation may be dampened by several permil. This suggests that samples taken from the rainshadow side of an emerging orographic barrier may be more likely to preserve isotopic changes resulting from mountain uplift than samples taken from atop a rising mountain range or plateau.
The Southern Alps are developing as a consequence of oblique collision between the Pacific and Australian plates. The Southern Alps lie on the west side of the South Island of New Zealand and create a massive rain shadow where greater than 12 m/year of rain falls on the west coast and semiarid conditions exist to the east. The rain-out effect across the mountains causes precipitation west of the Southern Alps to have δD and δ18O values averaging −30‰ and −5.5‰, whereas precipitation in the rain shadow to the east is isotopically lighter (δD=−72‰ and δ18O=−9.8‰). Such large differences in the isotopic composition of precipitation would not have existed prior to the development of significant topography. We have examined the topographic evolution of the Southern Alps using oxygen isotope analyses of authigenic kaolinites formed in the rain shadow to the east of the mountains between the Cretaceous (low topography) and the Pleistocene. Changes in the isotopic composition of authigenic clay minerals forming in equilibrium with meteoric water in the stratigraphic sequence record the development of Southern Alps topography and the resultant rain shadow. Our oxygen isotope analyses of authigenic kaolinites show a 5–6‰ decrease in the early Pliocene, from ∼18.2‰ in older rocks, to ∼12.3‰ in younger rocks. In addition, smectite is abundant in all samples from the Late Miocene to Recent, but is conspicuously absent in most older rocks, suggesting a change to a generally drier climate roughly coincident with the isotopic shift in kaolinites. This method may be useful in unraveling timing of development of mountain belts elsewhere in the world.
The Plains Sill is a thick diabase-granophyre body that intruded the wet sediments of the Middle Proterozoic Prichard Formation of the Belt-Purcell Supergroup. The diabase is a high-iron tholeiite geochemically compatible with large-volume mantle melting in an intracratonic rift environment. Evidence of emplacement into wet sediments includes thick zones of homogenized granosediments adjacent to the sill, soft-sediment deformation at sill contacts, and sedimentary ovoid structures possibly formed by local fluidization of sediments. Utilizing sediment pore water and driven by heat from the sill, the diabase was metamorphosed during crystallization and cooling, leaving hornblende as the dominant mafic phase. Continued retrograde alteration resulted in overgrowths of secondary hornblende and variable alteration of plagioclase to epidote. A miarolitic granophyre layer, up to 150 m thick, caps the diabase and appears igneous in origin. Locally the granophyre is anomalously thick, perhaps reflecting updip migration of granophyric fluid where the Plains Sill cuts upsection through the Prichard Formation stratigraphy.
Illite/smectite (I/S) and clinoptilolite mineral separates from drill holes in 11 Ma old altered volcanic tuffs of Yucca Mountain, Nevada were analyzed for δ18O values. We reconstruct the diagenetic and paleohydrologic conditions using the isotopic composition of clay minerals combined with other independent geological observations. Our isotopic data show that the δ18O values of clay minerals preserve paleohydrologic information from a Middle to Late Miocene episode of hydrothermal alteration at Yucca Mountain. We provide additional evidence for a dual decoupled groundwater circulation system at Yucca Mountain ca. 11 Ma ago. A near-surface system dominated by downward percolation of groundwater is separated from a deeper hydrothermal system by profound thermal and isotopic discontinuities near the R0–R1 I/S transition. Given the set of inferred formation temperatures and the isotopic composition of formation water, we show that the current set of isotopic compositions of clinoptilolite is significantly lower than those at the time of formation, and are not significantly different from the equilibrium values defined by the current groundwater δ18O and geothermal gradient. We conclude that, unlike I/S, clinoptilolite has not preserved its original isotopic composition over the past 11 million years, but we do not know how long is necessary to reset the isotopic signature of clinoptilolite under near-surface conditions. The current groundwater at Yucca Mountain is 4%. more enriched in 18O than the paleogroundwater 10–11 Ma ago as inferred from our reconstruction. This shift may be attributed to a change in the pattern of atmospheric circulation, such as by surficial uplift of the Sierra Nevada, which caused it to become an orographic barrier to moisture penetrating inland from the west.
Research Article| February 01, 2000 Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals C. Page Chamberlain; C. Page Chamberlain 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA Search for other works by this author on: GSW Google Scholar M. A. Poage M. A. Poage 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (2): 115–118. https://doi.org/10.1130/0091-7613(2000)28<115:RTPOMB>2.0.CO;2 Article history received: 09 Jun 1999 rev-recd: 27 Sep 1999 accepted: 05 Oct 1999 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation C. Page Chamberlain, M. A. Poage; Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals. Geology 2000;; 28 (2): 115–118. doi: https://doi.org/10.1130/0091-7613(2000)28<115:RTPOMB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The paleorelief of mountain belts can be estimated from the δ18O value of authigenic minerals. Development of relief during mountain building often creates lee-side rain shadows in which precipitation is depleted in 18O and D. The magnitude of this rain-shadow effect is strongly correlated to relief. A compilation of δ18O data from surface waters throughout the globe shows a linear relationship between net elevation change and Δδ18O (R2 = 0.79). Through the use of this relationship, we investigated the timing and magnitude of elevation change in the Southern Alps of New Zealand and the Sierra Nevada of California. The δ18O values of kaolinites from New Zealand show an ∼6‰ decrease in the early Pliocene that corresponds to an ∼2 km elevation change in the Southern Alps. The δ18O of smectites from the Sierra Nevada show little change since 16 Ma, suggesting that these mountains have been a long-standing topographic feature. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
'/%3e%3cuse xlink:href='%23a' transform='rotate(72 0 0)'/%3e%3cuse xlink:href='%23a' transform='rotate(-72 0 0)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1) rotate(72)'/%3e%3c/g%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h1200v600H0z'/%3e%3cpath stroke='%23FFF' stroke-width='60' d='m0 0 600 300M0 300 600 0' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='40' d='m0 0 600 300M0 300 600 0' clip-path='url(%23c)'/%3e%3cpath stroke='%23FFF' stroke-width='100' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='60' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(45.4 0 0 45.4 900 120)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(30 0 0 30 900 120)'/%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 519.022 -457.666) scale(40.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 519.022 -457.666) scale(25)'/%3e%3c/g%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 668.57 -327.666) scale(45.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 668.57 -327.666) scale(30)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(50.4 0 0 50.4 900 480)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(35 0 0 35 900 480)'/%3e%3c/svg%3e)
'/%3e%3cpath fill='%23fff' d='m8.751-17.959 10.11 11.373L3.997-9.844l13.94-6.1-7.692 13.129z'/%3e%3c/svg%3e)