Relationships Between Composition of Organic Matter, Depositional Environments, and Sea-Level Changes in Backarc Basins, Central Japan

Abstract

Sedimentary organic matter is a potential indicator of paleoenvironments. In this study we examine the relationship among composition of kerogen (insoluble organic matter), sedimentary environments, and sea-level changes in Miocene to Pleistocene sediments of the Niigata and Akita backarc basins, central Japan. Our primary analytical tool is a ternary diagram with apexes consisting of woodycoaly organic matter, herbaceous with pollen and spores, and amorphous organic matter (AOM) with alginite (organic-walled marine microfossils). The composition of kerogen plots into distinct regions on the ternary diagram: fluvial to estuarine, prodeltaic, shelf, slope and basin-floor submarine fans, and distal basin-floor sediments. The fluvial and estuarine sediments have high proportions of woody-coaly and herbaceous organic matter with pollen and spores, and a lesser proportion of AOM ؉ alginite, because pollen and spores were mainly deposited in estuaries. Because abundant coarse-grained, terrigenous organic matter was supplied by delta distributaries, the prodeltaic sediments have high proportions of woody-coaly organic matter. The composition of kerogen in the shelf sediments is similar to the kerogen in slope and basin-floor submarine-fan sediments, as plotted on the ternary diagram. Both kinds of kerogen accumulations have high proportions of woody-coaly organic matter and AOM with alginite and lesser proportions of herbaceous organic matter with pollen and spores. This relationship suggests that turbidity currents supplied the terrigenous sediments. The sediments on the distal basin-floor contain high proportions of AOM.Each pattern in the ternary diagram reflects a difference in hydrodynamic behavior, distance from land, and the supply of terrigenous organic matter. The sub-ternary diagram, which has apexes of WFA (weakly fluorescent amorphous organic matter), NFA (nonfluorescent amorphous organic matter) ؉ FA (fluorescent amorphous organic matter), and alginite, further suggests the origin of AOM. The NFA in shelf sediments and WFA in distal basin-floor sediments are inferred to consist of terrestrial higher plant and marine organic matter, respectively. A ␦ 13 C value of kerogen rich in NFA (Ϫ24.6 and Ϫ27.3‰) suggests land plants, whereas kerogen rich in WFA (Ϫ20.0 to Ϫ23.6‰) suggests marine plankton. These inferences agree with those derived from the sub-ternary diagram. Furthermore, compositional changes of the kerogen in turbidites reflect relative sea-level changes, as seen by shifts in compositions on the ternary diagram. The use of ternary diagrams like those used here is recommended for future studies of kerogen, depositional environment, and sea-level relationship.

High-resolution sequence stratigraphy in an incised-valley system on the basis of sedimentary organic matter, sulfur content and fossil diatom: An example from Miocene to Pliocene Tatsunokuchi Formation, Iwate Prefecture, Northeast Japan

“…The present study follows the classification of kerogen and the procedures of Omura and Hoyanagi (2004) for the preparation and observation of organic matter (Table 1). Kerogen is classified into vitrinite (derived from woody phytoclasts), cutinite (cuticle phytoclasts), sporinite (pollen or spores), and alginite (algae or marine plankton).…”

“…Kerogen is classified into vitrinite (derived from woody phytoclasts), cutinite (cuticle phytoclasts), sporinite (pollen or spores), and alginite (algae or marine plankton). AOM (amorphous organic matter) is subdivided into NFA (non-fluorescent amorphous kerogen), FA (fluorescent amorphous kerogen), and WFA (weakly fluorescent amorphous kerogen) on the basis of their fluorescence (Sawada and Akiyama, 1994;Omura and Hoyanagi 2004). The origins of NFA and WFA in Pleistocene to Miocene shelf and deep-sea deposits of Niigata and Akita prefectures, which are on the back-arc side of the Japanese archipelago are estimated to be terrestrial higher plants and marine plankton, respectively (Sawada and Akiyama, 1994;Omura and Hoyanagi, 2004).…”

“…AOM (amorphous organic matter) is subdivided into NFA (non-fluorescent amorphous kerogen), FA (fluorescent amorphous kerogen), and WFA (weakly fluorescent amorphous kerogen) on the basis of their fluorescence (Sawada and Akiyama, 1994;Omura and Hoyanagi 2004). The origins of NFA and WFA in Pleistocene to Miocene shelf and deep-sea deposits of Niigata and Akita prefectures, which are on the back-arc side of the Japanese archipelago are estimated to be terrestrial higher plants and marine plankton, respectively (Sawada and Akiyama, 1994;Omura and Hoyanagi, 2004). While, Omura et al (2006) analyzed stable carbon isotope of Pleistocene shelf sediments in a fore-arc setting and pointed out that NFA in this area is derived from both terrigenous and marine plankton.…”

“…Our stable carbon isotope analytical procedures are as follows: (1) After the procedures mentioned above, a portion of the separated kerogen was treated with a saturated ammonium carbonate solution in order to remove any fluoride, (2) NaBH 4 was used to eliminate sulfide, and (3) a few milligrams of the dried sample were heated to 950℃ in an elemental analyzer (FlashEA 1122, ThermoQuest, Ltd.), Table 1 Classification of sedimentary organic matter using fluorescent visual kerogen analysis (modified from Omura and Hoyanagi, 2004) and the resulting purified CO 2 gas was fed directly into a mass spectrometer (Delta Plus, ThermoQuest, Ltd.) using pure helium carrier gas. Our results are expressed as per mil (‰) relative to the V-PDB standard.…”

“…These studies were mainly conducted on siliciclastics (e.g., Hart et al, 1994;Tyson, 1996;Wood and Gorin, 1998;Omura and Hoyanagi, 2004;Pellaton and Gorin, 2005) and carbonates (e. g., Gorin and Steffen, 1991;Steffen and Gorin, 1993;Bombardiere and Gorin, 1998) deposited in shelf to deep-sea environments. On the other hand, several studies about organic-matter type and preservation have been conducted on estuary to delta deposits (e.g., Hart 1994;Pittet and Gorin, 1997;Carvalho et al, 2006;Omura et al, 2000Omura et al, , 2006Yoshida et al, 2006Yoshida et al, , 2007Yoshida et al, , 2009).…”