The ratio of total organic carbon to total nitrogen (TOC:TN) and the stable carbon isotope ratio of organic matter (δ13Corg) are widely applied for inferring the origin of organic matter (OM) in Quaternary marine sediments. A plot of TOC:TN vs. δ13Corg is useful for such studies but is strongly based on qualitative constraints. This study is based on the qualitative characterization of the source of Quaternary OM via analysis of TOC:TN and δ13Corg signatures, but also proposes a probability parameter, which combines both signatures, to infer the amount of Terrestrial OM Input (TOMI). This index provides a method for quantifying the proportion of terrestrial OM vs. marine OMin a more comprehensive manner. The TOMI index concept was applied to a study area in theJoetsuBasin, eastern margin of theJapanSea, where previous studies have characterized theOMfrom the Last Glacial Maximum (LGM) to the present. The upwards increase in TOC indicates thatOMproduction during the Holocene was higher than during the LGM. The enriched δ13Corg signature upwards and decrease in TOC:TN suggest predominantly marine phytoplankton OM during the Holocene. Throughout the LGM, low OM production with depleted δ13Corg values and high TOC:TN values in the sediments suggest a predominantly C3 terrestrial plant source for the OM. Using these data, it was possible to calculate a proxy for a sea level variation curve during that period and to investigate the influence of the proximity of the coastal line to the continental slope on the input of terrestrial material to the basin. The proposal provides information for the application of sequence stratigraphic concepts. The TOMI index could confirm that the proximity to the shoreline and shelf break has a strong influence on the input of terrestrial material during lowstand periods.
The identification of the sources of organic matter (OM) in marine sediments is important for inferring the contribution of terrestrial material since both terrestrial vegetation and soils can be delivered to the deeper parts of a basin along the geological time. During glacial stages, for example, the sea level was lowered by tens of meters and the input of terrestrial material became high at distal sites. During lowstands the mouths of the rivers were much closer to the continental shelf break, increasing the input of terrestrial material. Therefore, sea level curves can be generated using the terrestrial OM input as a proxy.
During such a sea level decline the occurrence of turbidite flows is common. The resulting sandy deposits associated with turbidities, if present, can be used to infer lowstands. A common interpretation is that the coarser the grain size, the closer the slope or coastal zone. On the other hand, in regions where there are no sandy sediments they represent distal parts of turbidite flows, so clayey sediments are dominant. In these cases, it is difficult to infer the provenance or location of the sediment source.
Therefore, the identification of terrestrial OM can be helpful for inferring both the input of continental clayey sediments and paleoenvironmental settings. Traditionally, the use of microfossil assemblages is an important tool, sometimes the only one, for reconstructing past sea level changes and paleoenvironmental conditions in marine regions. However the absence of microfossils from sediments seriously hampers interpretation. This is critical for marine regions located below the carbonate compensation depth (CCD), where the carbonate body of marine organisms is totally or partially dissolved in the water column. As an alternative, the stable carbon isotope values (δ13Corg) combined with the ratio of total organic carbon (TOC) to total nitrogen (TN) has been used to infer the nature of the OM in Quaternary marine sediments.
A criterion for differentiating the origin of OM on the basis of δ13Corg values was previously proposed [1-4]. In general, the OM present in marine sediments is classified into three groups. The first is dominated by marine organisms with δ13Corg values between −20‰ and −22‰; the second has δ13Corg values from −22‰ to −25‰, and may contain a mixture of terrestrial and marine OM; the third shows δ13Corg values lower than −25‰, implying a predominant supply of terrestrial OM.
This criterion is based on the fact that both photosynthetic processes and source of carbon are different between marine organisms and terrestrial plants. The primary carbon source for marine phytoplankton is seawater bicarbonate with a δ13C of ca. 0‰. In contrast, land plants use atmospheric CO2 as carbon source, with δ13C of ca. −7‰ [4-6]. The difference in δ13Corg of ca. 7‰ between marine primary producers and land plants has been successfully used to elucidate the origin of recent OM in sediments [5,7].
Burdige [
According to Prahl et al. [
The relationship between TOC content (wt%) and the TN content (wt%) gives an idea about both marine OM productivity as well about terrestrial OM input, whereas both δ13Corg (‰) and TOC:TN provide inferences about the origin of OM [
The main objective of this work was to calculate a Terrestrial OM Input (TOMI) probability index by combining the parameters TOC:TN and δ13Corg, plotting them vs. depth in one single graph, providing a way of constructing a proxy for sea level variation on the basis of semi-quantitative data. Based on the data of Freire et al. [
The Japan Sea is a typical back-arc basin formed behind the island-arc system of the Japanese islands and initiated by the rifting of the eastern margin of the Eurasian continent at around 25 Ma [
According to Oba et al. [
oxic to euxinic conditions [
Several piston cores, 6 to 9 m long, were collected for gas hydrate research from the Joetsu Knoll, Umitaka Spur and surrounding areas since 2005 by the R/V Umitaka Maru of the Tokyo University of Marine Science and Technology and by the R/V Kaiyo of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). These studies have been conducted by the University of Tokyo and other institutions, providing improvement in the geological knowledge of the eastern margin of the Japan Sea, particularly the Joetsu Basin [16, 17,21,25-27]. This study used three representative piston cores: PC701 (Oki Trough), PC702 (Joetsu Knoll) and PC505 (Umitaka Spur) to compare the OM at the three different locations, as related to the distance to the present shelf brake (
From the three cores 223 samples were collected, each of ca. 5 ml. The sampling interval was ca. 10 - 15 cm in the upper part of the cores, until the presence of the first thinly laminated dark gray layer, and 5 - 10 cm between and within two thin laminated layers (TLs), to characterize the geochemical signatures for oxic and anoxic environments [16,17]. Lithologic units are described in Section 3.1.
For TOC, TN and δ13Corg analysis, sediment samples were powdered and treated using 10% HCl solution to remove carbonate. An aliquot of each sample was preserved for analysis of TC, with no acid treatment, to calculate TIC and to control the quality of the acid treatment by the comparison of both TOC and TIC values. The results are not discussed here, but are discussed in detail by Freire [
In order to calculate the Terrestrial Organic Matter Input (TOMI) probability index the following procedures were adopted:
1) Values suggesting a probability field for OM of terrestrial origin were empirically inferred from Lamb et al. [
2) Twelve grid points (
3) An interpolated surface grid of regularly spaced probability values was then generated by way of a kriging method, using software Surfer version 8.02, from the Golden Software, Inc. For the TOC:TN axis (x-axis), the grid line geometry was defined as a minimum of 0 and a maximum of 100, with spacing of 0.1 and a total of 1001 cells. For the δ13Corg axis (y-axis) the grid line geometry adopted a minimum of −34‰ and a maximum of −10‰, with a spacing of 0.1‰ and a total of 241 cells.
4) Each pair of TOC:TN and δ13Corg values from analysis of a core sample was plotted onto the interpolated grid and the corresponding value of TOMI index was obtained and listed as an output file (Supplementary data 1).
Five lithologic units were identified during core descriptions from the bottom to the top (
Two tephra were identified in PC701 (Oki Trough) and their glass shards and composition were correlated [17,25] with the Atlas of Tephras in and around Japan [
Two unknown tephra were recognized in the upper and middle part of unit 4 (TL-2) in the two cores from the Joetsu Basin (
A total of four foraminifera samples were collected from PC701 for 14C dating [
A depth-age conversion was made on the basis of tephrochronology, 14C of foraminifera and lithologic unit correlation [
High sedimentation rate values were observed for PC701 (
Graphs comparing age vs. TOC (wt%), age vs. 13Corg (‰ VPDB), age vs. TN (wt%) and age vs. TOC:TN were constructed to promote an age-based correlation between Joetsu Basin and Oki Trough (
TOC content increases from 0.5 to 1.5 wt% and 13Corg varies from −23.8‰ at the base to −21.7‰ at the top of unit 3, representing the LGM/Holocene geochemical transition. TN content rapidly increases from 0.06 wt% at the base to 0.19 wt% at the top, resulting in a decrease in TOC:TN from 46 at the base of unit 3 to around 10 at the top. Unit 2 (TL-1) is characterized by TOC content from 1.0 to 2.5 wt%, and δ13Corg varying from −21.0‰ to −24.0‰. TN content increases up to 0.3 wt%, resulting in TOC:TN < 20. The shallower unit 1 is characterized by TOC content with a maximum value of 2.5 wt% at the base to a minimum of around 1.0 wt% at around 6.5 ka
cal BP. The δ13Corg varies from −22.4‰ at the base to −20.1‰ in the upper part, representing the present Holocene sedimentation in the area. TN content decreeses from 0.25 wt% at the base to 0.11 wt% in the top, resulting in TOC:TN of nearly 10 on average.
The Holocene sediments of the Japan Sea are characterized by high TOC and TN contents, low TOC:TN values and enriched δ13Corg signatures (
Such paleoenvironmental interpretation is only possible if the analytical results represent a proxy for the chemical setting at the time of deposition. The use of δ13Corg and TOC:TN is a well establish stratigraphic tool and is widely applied to infer the origin of OM in Quaternary marine sediments. There is no doubt about the relationship between those parameters and paleoenvironmental interpretation [1-4,6,7,10-13,15]. However, recent studies indicate that bias resulting from sample acidification during pre-treatment with HCl [
1) All the samples were analyzed for both TC (total carbon—with no HCl treatment) and TOC (with HCl treatment) for inferring total inorganic carbon (TIC), giving good quality control;
2) There are no macroscopic or microscopic indicators of early diagenesis, like carbonate concretions, cementation levels or framboidal pyrite;
3) Benthic and planktonic foraminipheral tests are well preserved along the section, with no dissolution and border erosion;
4) The presence of well preserved phytoclasts, cuticles and light orange amorphous OM, as well as the presence of non-ecloded copepods eggs, indicate rapid burial and preservation;
5) The high sedimentation rate values induced rapid burial and protection of OM from aerobic oxidation at the sea floor;
6) The absence of an increased and linear trend of results with depth/age, despite the lithology being almost the same (clay minerals) along the whole section;
7) Good correlation between the cores over long distances (>100 km), preserving the same geochemical pattern for each lithologic unit. In a general, diagenesis is a local phenomenon and not correlated over long distances.
These observations suggest that the analytical data represent the depositional conditions and that diagenesis was not significant for disturbing the original record.
Spatially, analysis of the graphs (
The increased pattern in the TOC curve suggests that productivity rapidly increased from around 18 ka cal BP to 12.5 ka cal BP (
In an attempt to see the ranges of marine OM at higher resolution, a crossplot TN/TOC vs. 13Corg cross plot is shown in
4(b)). Using this approach, it was easy to separate samples plotting in the marine OM range. However, both graphs show that is difficult to separate marine DOC from C3 terrestrial plants, because they overlap (
As discussed in Section 4.1, TOC:TN and δ13Corg are frequently used to provide a qualitative distinction between terrestrial and marine OM. Lamb et al. [
In an attempt to combine both TOC:TN and δ13Corg data in a single curve, we propose the TOMI index. This is an approach to quantifying the amount of terrestrial OM in marine sediments through a probability calculation based on the ranges compiled by Lamb et al. [
Relative changes in the sea level and rate of sediment supply are the main factors controlling transgression and regression events, which can be predicted within the
context of sequence stratigraphy. Regression is produced by relative sea level fall (forced regression) and/or excessive sediment supply (normal regression) [
Two graphs were constructed here and can be used to infer sea level variation in the eastern margin of the Japan Sea during the last 30 ka, in particular at the Joetsu Basin and surrounding areas. The first plots the TOMI index vs. depth (
To create an accurate depth-age conversion the most effective way is to have an age control for all the cores. Unfortunately, there is no age control for PC702 and PC505, so the age control is based only on the correlation with PC701 (
and 70% - 80% for PC505, reflecting the proximity to the shelf break and the shoreline during the LGM lowstand (LST). The farther the distance from the terrestrial source the lower the TOMI index because it represents the amount of terrestrial OM. According to the index, the maximum lowstand occurred ca. 23 to 20 ka cal BP, when the terrestrial input was higher (
From 17 to 16 ka cal BP a strong and rapid decrease occurs in the values of the index from 65% to 35% for PC701 indicating a shift to open sea conditions, reflecting a large input of plankton species from the Pacific Ocean [
The same pattern was observed for both PC702 (70% to 40%) and PC505 (85% to 50%), suggesting that the inflow of Pacific Ocean water enriched in marine organisms was greater to Joetsu Basin. Using the TOMI index as indicative of sea level changes (
Between 16 and 11 ka cal BP, the TOMI index decreases from 40% to 25% for PC701, 50% to 35% for PC702 and 55% to 35% for PC505. As a proxy for the sea level variation, these trends suggest that sea level rose continuously during this period (
This is important to note the average difference in the TOMI index between PC701, PC702 and PC505 during the period, reflecting the strong influence of distance from the shoreline/shelf break on the input of terrestrial organic material.
At around 11 to 10.5 ka cal BP a minimum in the index of ca. 20% is observed and, from then on until the present, it oscillates around 25% - 35% on average at all sites (
Two minimum peaks occurs around 9 ka cal BP and 5 ka cal BP, when the TOMI index is lower than the average at all sites. This may represent maximum flooding surfaces (MFSs) during the Holocene highstand. One coincides with the observation by Nakada et al. [
The index reflects the terrestrial OM content present in marine sediments based on TOC, TN and δ13Corg. Thus, refining these parameters, with respect to the characteristics of both marine and terrestrial organisms, is critical for improvement in the use of the index curve as a proxy for sea level variation. Special attention should be given to a better characterization of marine DOC with respect to TOC:TN and δ13Corg. The use of TOMI index is recommended at non-disturbed sediments. In the case of mounds and pockmarks, where the crystallization and dissociation of gas hydrates can alter the volume of sediments, causing an uplift of older sediments [
From this work, we conclude that:
1) The upwards increase in TOC indicates that OM production during the Holocene was higher than during the LGM in the study area. The enriched signature of δ13Corg upwards and the decrease in TOC:TN suggest predominantly phytoplankton-derived marine OM during the Holocene. Throughout the LGM, low OM production with depleted δ13Corg values and high TOC:TN suggests a predominantly C3 terrestrial plant source for the OM in the eastern margin of the Japan Sea.
2) The Terrestrial OM Input (TOMI) index is a promising tool for semi-quantifying the proportion of terrestrial OM in marine sediments. It can be used for validating the sea level curve based on other proxy measurements. Because the bulk parameters involved in this proxy are easy to determine, it can be applied for understanding paleoenvironmental conditions.
3) TOMI index could confirm that proximity to the shoreline and shelf break has a strong influence on the input of terrestrial material during lowstand periods, although it is not as important during highstand periods.
The authors are thankful to R. O. Kowsmann, D. J. Miller, J. V. P. Guzzo and M. Arai for comments. Thanks go to the anonymous reviewers for their comments and suggestions.
TOMI index for samples collected in the Japan Sea.
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