Owens, N.J.P.
Plymouth Marine Laboratory, Plymouth,
Devon,
United Kingdom
The marine nitrogen cycle is one of the most complex elemental cycles on earth, in which nitrogen
‘cycles’ between 8 oxidations states (-3 to +5), mediated predominantly by micro-organisms. The use of the stable isotope of nitrogen, 15N, has been the most significant tool that has led to our current knowledge of this cycle.
From an historical context the introduction of 15N can be traced back to parallel developments in two fields: a biogeochemical context and developments in isotope chemistry. In biogeochemistry the proposals of Liebig - “Liebig’s Law of the Minimum” led to the recognition of the importance of nitrogen as a possible determinant of growth of living organisms. This was followed by early and important work of Brandt which initiated the study of nitrogen in the marine realm. The Founding Father’s of marine chemistry Brand, Cooper and Harvey firmly established the nitrogen cycling into
‘main-stream’ marine science. The parallel developments in isotope chemistry initiated by Wien in the 1890s with the discovery of positive ions, through to the development of mass spectrometers by Thomson and the pioneering work on stable isotopes by Aston, Nier, Urey, Rittenberg and others was to lead ultimately to Hoering’s seminal paper detailing the natural variations in 15N in “natural materials”. These dual developments led quickly to the use of 15N in deliberate marine studies. Thus, Benson and Parker in 1961 used natural variations in 15N to demonstrate unequivocally marine denitrification, and in 1967 Miyake and Wada published the first detailed study of variations in 15N in marine organisms and foodchains. Alongside these developments there was landmark work being done by the use of 15N as a deliberate tracer by Dugdale, Goering and others in the early 1960s which firmly established the use of 15N as an important part of the marine ‘armoury’.
As noted above 15N can be used in two ways to help resolve the nitrogen cycle: study of the natural variation in its abundance in natural materials; use as a deliberate tracer by the addition of measurable concentrations under experimental conditions.
In the latter case the resolution of the form of nitrogen used by phytoplankton has led to the fundamental paradigm in biological oceanography of ‘new’ production. This was introduced in the 1967 by Dugdale and Goering and is important because it describes in effect the carrying capacity of any part of the marine environment. It thus sets the upper limit of the amount of carbon that can be sequestered by marine phytoplankton (of major importance in determining the role of the oceans in the context of global climate change); it also sets the upper limit of fish production (of major importance in the context of sustainable marine bioresources). Because of its importance a considerable amount of work has been conducted, using 15N, over many years to establish the global level of ‘new production’. A reasonably comprehensive study of the literature revealed over 70 studies since 1967 that when combined results in a global estimate of new production of 20GtC year-1; this estimate is of considerable significance for models of the global C cycle.
Turning to the use of variations in natural abundance of 15N two examples serve to show its utility.
First: the unequivocal demonstration that nitrogen fixation is of major importance in the marine environment. Many studies across the world-ocean have shown that components of the marine food-
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chain are unusually ‘light’ in 15N content; this could only occur if the basis of the nitrogen was from the atmosphere, which has a relatively low 15N content. This finding is also of importance to global ecosystem models. Secondly, in resolving marine food-chains. The resolution of marine food-chains by 15N is based on the observation and known theory of isotope fractionation as nitrogen is transformed from one form to another. Thus, as nitrogen moves up through trophic levels it is expected that consumers would progressively become ‘heavier’ in 15N content. A large number of marine food-chains which otherwise would not be tractable to study have been resolved using this approach.
The above are limited examples of the use of 15N but it is obvious that 15N has been of enormous importance in resolving the marine nitrogen cycle. With the development of new techniques in molecular biology and the continuing development of more sensitive mass spectrometers we can expect a continuing development in its use and value to natural scientists.
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Using stable isotopes (C, N) to constrain organics in sub-basement fossil soils (Ocean Drilling Program-Leg 197; N. Pacific): a possible example of isolated atmosphere-land-ocean systems
Bonaccorsi, R.
University of Trieste,
Dept. of Geological, Environmental, and Marine Sciences (DiSGAM), Trieste,
Italy
Although the discovery of deep red-brown paleosols, or fossil soils, during Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legs dates back to the mid 1970s [1-3], the potential for preservation of organic matter in these igneous-derived silty-claystone units has been overlooked, and depositional settings have been inferred from only petrologic observations.
I present here naturally occurring stable isotopes of carbon and nitrogen (G13Corg,G15N) data of a well- preserved paleosol unit (Fig. 1) cored at Site 1206 (Koko Seamount) during the ODP Leg 197 (Emperor Seamounts, north Pacific Transect) [4-5]. The study of the sources and variation with depth of organic matter in sub-basement Fe-oxide-rich paleosol units from Leg 197 contributes to understanding the palaeoenvironmental history of the Emperor Seamounts prior to, during and after their burial and subsidence (ca. >48 to 55 Ma). Furthermore, preserved organic traces in such an isolated deep Earth system make them a useful test bed for future deep Earth's biosphere-relevant investigations [5-6].
FIG. 1. Core 197-1206A-40R soil unit. This sequence develops for a total length of 234 cm and consists of a silty-clay red-brown material that becomes progressively lighter in color upward i.e., dusky brown (5YR 2/2) to very dusky red (10R 2/2).
111 R. Bonaccorsi
This soil interval forms in the first part of Section 40R-3 (0-77 cm), it develops throughout the entire Section 40R-2, and ends as capped by a lava flow top at Section 40R-1, 104 cm. Throughout the fossil soil sequence two contacts are present: 1) the contact between the weathered lava flow top unit (white arrow), which represents the early/mid stage of soil formation; and 2) the contact between the top of the soil sequence (Unit 18A; [4]) and the overriding rock unit (black arrow) - i.e., the brecciated base of Unit 17 [4].
Measurements were made throughout Core 197-1206A-40R soil unit on organic-poor samples (Corg= 0.03-0.07%; 0.049 ± 0.011, n = 7) by using an Elemental Analyzer 1110 CHN coupled with a Finnegan Delta Plus MS-Conflo II.
This fossil soil system is characterized by a) the smallest difference for C (G13Corg=~1.5 delta ‰); b) the highest variability for N (G15Ntot = -9.5‰ to +2.5‰); c) even wider differences that occur between the fossil and the recent soils i.e., Hawaiian oxisols (13Corg = ~12 delta‰; 15N =~18 delta‰) as counterparts.
More specifically, the G13Corg (bulk) values for the paleosol beds are –25.3‰ (at 307.54 mbsf) to - 26.2‰ (at 308.92 mbsf) downcore. These values contrast to those obtained from an exposed Hawaiian oxisol sample (e.g., a Horizon-B sample at 100-105 cm-depth) with G13Corg = -23.0‰. Typical uncertainties for these measurements were ±0.1‰ to ±0.3‰.
First, G13Corg values of ca. -26‰ support a terrestrial, rather than marine source [7-8] of organics preserved in the paleosol interbeds from Core 197-1206A-40R. Thus, providing additional evidence for red claystone units as soil horizons subaerially formed on the top of Koko Seamount and buried by erupted lava flowing in a nearshore environment [1, 3-4]. The red soils probably underwent not only environmental changes (in paleolatitude and/or in paleoclimate) during formation, but also diagenetic changes after their burial.
However, G13C alone resulted inadequate at distinguishing among different carbon sources (i.e., microbials, algae and land plants, [e.g., 8]. Hence the need of adding 15N data to constrain the source of organics in these interesting fossil soils.
Second, the isotopic composition of carbon and nitrogen would represent a complex signal from such history since late Palaeocene to early Eocene time [5]. Specifically, the stable isotope signature of C and N (G13Corg =~ -26‰; G15Ntot= -9.5‰ to +2.5‰) could reflect mixed sources of organics (i.e., plant and primary/secondary bacterial) and microbial-induced processes within the Carbon cycle and the Nitrogen cycle (e.g., nitrogen fixation, nitrification, and denitrification).
Finally, the geochemical divergence of these deeply buried fossil soils from their still exposed counterparts (with G13Corg =~-17 ‰ to ~ -23 ‰; G15Ntot= up to +8.5‰) would indicate a possible example of isolated atmosphere-land-ocean systems and deep Earth systems where to test for potential deep subsurface biospheres.
REFERENCES
[1] KARPOFF, A.M., Init. Repts. DSDP 55 Washington (1980) 707-711.
[2] Shipboard Scientific Party, Site 871, Proc. ODP, Init. Repts. 144 (1993) 41-103.
[3] HOLMES, M.A., Proc. ODP, Scientific Results, 144 (1995) 381-398
[4] Shipboard Scientific Party, Proc. ODP, Init. Repts. 197 (2002) College Station, TX [5] BONACCORSI, R., et al., Abstract, IAU 2002 Bioastronomy Symposium (2002).
[6] FURNES, H., et al., Abstract, GSA 2001 Annual Meeting (2001).
[7] MEYERS, P.A., Chem. Geology, 144 (1994) 289-302.
[8] MEYERS, P.A., Org. Geochem. 27 5/6 (1997) 213-259.
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