출처:
Fig. 1. The East Sea is surrounded by the northeastern Asia and Japanese islands with four shallow straits of less than 140 m sills (the Korea, the Tsugaru, the Soya and the Tartar). The Tsushima warm Current enters the sea through the Korea Strait and exits mostly via the Tsugaru and Soya straits. This sea was nearly closed due to a glacio-eustatic drop in sea level and maintained a partial connection to the northern East China Sea via the Korea Strait during the LGM. The light gray area was exposed at that time. Solid circles indicate the core locations (C-3 and J-11) used in previous studies (Oba et al., 1991, 1995; Gorbarenko and Southon, 2000). Bathymetry is in meters.
그림 1. 동해는 문턱 수심 140 m 이하의 네 해협(대한, 쓰가루, 소야, 타타르)으로 둘러싸여 있다. 쓰시마난류는 대한해협을 통해 유입되고 주로 쓰가루·소야 해협으로 유출된다. 이 바다는 빙하성 해수면 하강으로 거의 닫혔으나 LGM 동안 대한해협을 통해 북부 동중국해와 부분 연결을 유지했다. 연한 회색 영역은 당시 노출지를 나타낸다. 검은 원은 선행 연구에서 사용된 코어 위치(C-3, J-11)이다(Oba et al., 1991, 1995; Gorbarenko and Southon, 2000). 수심 단위는 미터.
ISSUE
문턱 수심이 얕은 해협은 그를 통과하는 유입·유출이 해수면 변동에 의해 좌우되므로 민감한 수로다. LGM(약 23,000–19,000 cal. BP)의 전지구적 해수면 하강은 대한해협 일대의 지형·해양 조건을 크게 변화시켜 해협 단면 축소와 해협문턱 유동 감소를 초래했을 것이다. 대한해협은 (1) 내륙붕(≤80 m)·중륙붕(80–120 m)과 (2) 외륙붕(>120 m, 최대 약 230 m의 수로 포함)으로 구분된다(Park and Yoo, 1988; Yoo and Park, 2000; Yoo et al., 2003). 내륙붕은 두꺼운(약 30 m) 세립질 진흙(만수위 계열)으로, 중·외륙붕(수로 제외)은 얇은(약 5 m) 조립질 퇴적(해진 계열)으로 피복된다. 내·중륙붕은 육상 노출되었지만 외륙붕은 LGM 동안 대체로 수중에 남아 있었다.
LGM의 낮은 해수면에도 동해의 표면적은 상당히 유지되었고, 동해가 매우 가파르고 깊기 때문에 현재 표면적의 약 15%만 노출되었을 것으로 추정된다. 후기 제4기에 동해 주변 네 해협 모두가 이러한 해수면 변동을 겪었으며, 대한해협을 제외한 문턱 수심이 얕은 세 해협은 LGM 동안 닫혔다는 견해가 일반적이다(Yasuda, 1984; Oba et al., 1991; Ono and Naruse, 1997; Kim et al., 2000; Ono et al., 2004). 고고학적으로, Kuzmin et al.(2002)은 최소 23,000 cal. BP 이래 동북아에서 사할린을 거쳐 홋카이도로의 인적·물적 교류가 있었음을 보였는데, 이는 타타르·소야 해협의 육교를 시사한다. 30,000 BP 이후 혼슈로의 육로 형성도 제시되었고(Oda, 1990; Motohashi, 1996), 이는 쓰가루 해협의 노출을 뜻하며, 수중 암석의 ^10Be 노출연대로도 조사되었다(Kim and Imamura, 2004). 반면 폭 약 200 km·문턱 약 140 m의 대한해협은 당시 서측 일부가 잠긴 부분 개방 상태였을 가능성이 제기된다(Matsui et al., 1998; Gorbarenko and Southon, 2000; Park et al., 2000; Lee and Nam, 2003; Yoo et al., 2003). 지역 해수면 곡선은 약 25–15 ka BP 동안 해수면이 현재보다 약 130 m 낮았음을 보이며, 북부 동중국해와의 연결을 시사한다(Park and Yoo, 1988; Park et al., 2000).
얕은 황해가 광범위하게 노출되면서 중국 동부 해안선은 제주도 일대로 후퇴했을 것이다. 담수는 LGM 동안 북부 동중국해로 흘러들었고(Oba et al., 1991; Tada, 1999; Tada et al., 1999), LGM 당시 대만·류큐 일대의 육교 형성으로 쿠로시오의 동중국해 유입은 크게 제한되었다(Ujiie et al., 1991; Ahagon et al., 1993; Ujiie and Ujiie, 1999). 이로써 쓰시마난류 유입도 제한되어 당시 동해로의 염수 유입이 상대적으로 감소했다. LGM 동안 대한해협을 통과한 해수는 순수 염수가 아니라 쓰시마난류와 담수가 섞인 ‘고(古) 쓰시마난류’였고, 표층 희석을 초래하여 표층–심층 간의 밀도 성층을 형성하고 연직 혼합을 크게 제한했을 것이다. 그 결과 LGM 동해의 심층에는 무산소 환경이 발달했다(Oba et al., 1991; Keigwin and Gorbarenko, 1992; Gorbarenko and Southon, 2000; Ishiwatari et al., 2001).
Paleo-Tsushima Water and its effect on surface water properties in the East Sea during the last glacial maximum: Revisited
최종빙기 절정기 동안 동해 표층 수물성에 미친 고(古) 쓰시마난류의 영향: 재검토
Eunil Lee[a, *], Seongjoong Kim[b], Seungil Nam[c]
이은일[a, *], 김성중[b], 남승일[c]
- a. National Oceanographic Research Institute, 1-17, 7Ga Hang-dong, Jung-gu, Incheon 400-800, Republic of Korea;
- b. Korea Polar Research Institute, Ansan, P. O. Box 29, Seoul 425-600, Republic of Korea;
- c. Petroleum and Marine Resources Division, Korea Institute of Geoscience & Mineral Resources, Daejon 305-350, Republic of Korea
- a. 국립해양조사원, 대한민국 인천광역시 중구 항동 7가 1-17, 400-800;
- b. 한국해양과학기술원 극지연구소, 안산, P.O. Box 29, 서울 425-600;
- c. 한국지질자원연구원 석유해양자원본부, 대전 305-350
Abstract
The semi-enclosed deep marginal East Sea is known by limited sill flow and low sea-surface salinity during the last glacial maximum (LGM) when sea level was about 130 m lower than the present level. Although three straits (the Tsugaru, the Soya and the Tartar) with shallower than 130 m sills were completely closed, the Korea Strait with a maximum sill depth of 140 m seems to have persisted as a partial connection to the East Sea, allowing a sill flow. The volume transport at the Korea Strait during the LGM is estimated at approximately 0.3–1.1 × 10^12 m^3/yr, by using bathymetry, seismic reflection profiles and current data. The low sea-surface salinity has been explained by the East China Sea Coast Water (ECSCW) and high precipitation. However, the existing geological observations indicate that precipitation was reduced in the glacial East Sea. The high-resolution numerical simulation results predict that evaporation (2.16 mm/day) exceeded precipitation (1.43 mm/day), further suggesting net evaporation (evaporation minus precipitation) rates (0.2 × 10^12 m^3/yr) over the LGM East Sea. This signifies that the precipitation was not the factor lowering surface paleosalinity and that the paleo-Tsushima Water carried a huge amount of freshwater from the ECSCW than previously expected. The calculated surplus evaporation (0.2 × 10^12 m^3/yr) and sill flow (0.3–1.1 × 10^12 m^3/yr) are not identical, but they could be oceanographically considered as similar. The comparison between both values implies that most of the throughflow ultimately escaped the East Sea through the evaporation process during the LGM. The regional sea level in the almost isolated East Sea might be largely maintained by a rough balance between incoming throughflow and outgoing evaporation during the LGM. The geographic restriction due to lowered sea level and lower surface salinity by limited vertical mixing in the glacial East Sea are analogous to modern oceanographic features in the Black Sea.
동해는 반폐쇄성의 심해성 변연해로, 해수면이 현재보다 약 130 m 낮았던 최종빙기 절정기(LGM)에 해협문턱 유동이 제한되고 표층 염도가 낮았던 것으로 알려져 있다. 문턱 수심이 130 m보다 얕은 쓰가루·소야·타타르 해협은 완전히 닫혔지만, 최대 문턱 수심이 약 140 m인 대한해협은 동해와의 부분 연결이 유지되어 해협문턱 유동이 허용되었을 가능성이 크다. 수심도·탄성파 반사단면·유속 자료를 바탕으로 추정한 LGM 당시 대한해협의 체적 수송은 연간 약 0.3–1.1 × 10^12 m^3이다. 낮은 표층 염도는 동중국해 연안수(ECSCW) 유입과 많은 강수로 설명되어 왔지만, 기존 지질 관측은 빙기 동해에서 강수가 오히려 감소했음을 시사한다. 고해상도 수치모의 결과에 따르면 증발(2.16 mm/일)이 강수(1.43 mm/일)를 초과하며, 이에 따라 LGM 동해의 순증발(증발–강수)은 연간 0.2 × 10^12 m^3로 제시된다. 이는 표층 저염의 주 요인이 강수가 아니며, 고(古) 쓰시마난류가 기존 예상보다 많은 동중국해 연안수 기원의 담수를 실어 왔음을 뜻한다. 계산된 초과 증발(0.2 × 10^12 m^3/년)과 해협문턱 통과 유량(0.3–1.1 × 10^12 m^3/년)은 일치하지 않지만 해양학적으로 유사한 규모로 간주할 수 있다. 두 값의 비교는 LGM 동안 통과류의 대부분이 최종적으로 증발을 통해 동해에서 빠져나갔음을 시사한다. 거의 고립된 동해의 지역 해수면은 LGM에 유입 통과류와 증발 간의 대략적 균형으로 유지되었을 가능성이 크며, 낮아진 해수면과 제한된 연직 혼합으로 인한 저염 표층수라는 점에서 빙기 동해는 현대 흑해의 해양학적 특성과 유사했다.
1. Introduction
Glacial-caused global falling of sea level in the late Quaternary seems to be a generally synchronous event, and might have induced geographic and oceanographic changes especially in the small, semi-enclosed seas such as the East Sea, connected with the open ocean through shallow sill systems. The paleoenvironmental changes due to the late Quaternary sea-level drop have been presented in various areas of the world oceans (Wang and Wang, 1980; Ryan et al., 1997; Zhuo et al., 1998; Karaca et al., 1999; Petit-Maire et al., 2000; Aksu et al., 2002b; Siddall et al., 2004). Here we attempt to describe the implication of the sea-level drop to the water properties of the East Sea during the last glacial maximum (LGM).
제4기 후반의 빙하성 전지구 해수면 하강은 대체로 동시적 사건이었고, 얕은 문턱을 통해 외해와 연결되는 동해 같은 소규모 반폐쇄성 바다에서 지리·해양학적 변화를 유발했을 가능성이 크다. 이러한 해수면 하강에 따른 고환경 변화는 세계 여러 해역에서 보고되었다 (Wang and Wang, 1980; Ryan et al., 1997; Zhuo et al., 1998; Karaca et al., 1999; Petit-Maire et al., 2000; Aksu et al., 2002b; Siddall et al., 2004). 여기서는 LGM 동안 해수면 하강이 동해의 수물성에 미친 함의를 서술한다.
The East Sea is a typical semi-enclosed marginal sea and is linked to the South Sea of Korea by the Korea/Tsushima Strait (about 140 m in sill depth), to the northwest Pacific Ocean by the Tsugaru Strait (130 m), and to the Sea of Okhotsk by the Soya (55 m) and Tartar (15 m) straits (Fig. 1). This sea has an area of approximately 1,000,000 km^2 and an average depth of 1680 m with a maximum depth of 3700 m. During the glacial periods of the late Quaternary, the East Sea must have experienced oceanographic alteration because the straits are shallow. Particularly during the LGM, the global drop of about 130 m in sea level might have led to a reduction of the surface area and closure of three shallow straits (the Tsugaru, the Soya and the Tartar) in the East Sea (Yasuda, 1984; Oba et al., 1991; Ono and Naruse, 1997; Kim et al., 2000; Ono et al., 2004).
동해는 전형적인 반폐쇄성 변연해로, 대한/쓰시마 해협(문턱 수심 약 140 m)으로 남해와, 쓰가루 해협(130 m)으로 북서태평양과, 소야(55 m)·타타르(15 m) 해협으로 오호츠크해와 연결된다(그림 1). 면적은 약 100만 km^2, 평균 수심은 1680 m, 최대 수심은 3700 m이다. 제4기 후반 빙기 동안 해협이 얕았기 때문에 동해의 해양환경은 변화를 겪었을 것이다. 특히 LGM에는 전지구 약 130 m의 해수면 하강으로 표면적 감소와 세 얕은 해협(쓰가루·소야·타타르)의 폐쇄가 일어났을 가능성이 크다 (Yasuda, 1984; Oba et al., 1991; Ono and Naruse, 1997; Kim et al., 2000; Ono et al., 2004).
Complete closure of the Korea Strait during the LGM is still debated, but the general opinion is that the deepest sill of the Korea Strait might have been in part inundated, allowing a continuous, but highly limited sill flow (Morley et al., 1986; Keigwin and Gorbarenko, 1992; Matsui et al., 1998; Tada, 1999; Park et al., 2000; Lee and Nam, 2003). Volume transport at the Korea Strait was reduced during the LGM. Despite the marked decrease in the influx, the East Sea is characterized by low sea-surface salinity (20–29‰; Oba et al., 1995; Matsui et al., 1998; Tada, 1999; Gorbarenko and Southon, 2000; Itaki et al., 2004) associated with abnormally light δ18O values of planktonic foraminifera during the LGM as shown in Fig. 2 (Oba et al., 1991; Tada, 1999; Gorbarenko and Southon, 2000).
LGM 동안 대한해협의 완전 폐쇄 여부는 아직 논쟁적이지만, 대한해협의 가장 깊은 문턱 일부가 잠겨 연속적이되 매우 제한적인 해협문턱 유동이 허용되었을 가능성이 크다는 견해가 일반적이다 (Morley et al., 1986; Keigwin and Gorbarenko, 1992; Matsui et al., 1998; Tada, 1999; Park et al., 2000; Lee and Nam, 2003). LGM 시기의 대한해협 체적 수송은 감소했다. 유입이 크게 감소했음에도 동해는 표층 염도 20–29‰의 저염으로 특징지어지며 (20–29‰; Oba et al., 1995; Matsui et al., 1998; Tada, 1999; Gorbarenko and Southon, 2000; Itaki et al., 2004), 그림 2에서 보이듯 LGM 동안 부유성 유공충의 δ^18O가 비정상적으로 낮았다 (Oba et al., 1991; Tada, 1999; Gorbarenko and Southon, 2000).
Fig. 2. Downcore variations in δ18O of planktonic foraminifera and salinity. (a) Salinity (Tada, 1999) and δ18O (Oba et al., 1995) are extracted from Core C-3 with calendar ages. Open circles are sea-surface salinity, displaying low values (down to 20‰) between 15 and 20 cal. ka BP. (b) δ18O trends are derived from Core J-11 with 14C age dates (Gorbarenko and Southon, 2000). The shaded areas show the time intervals of low sea-surface paleosalinity associated with light δ18O with different chronological determinations.
그림 2. 부유성 유공충의 δ^18O와 염도의 코어 하향 변화. (a) 염도(Tada, 1999)와 δ^18O(Oba et al., 1995)는 달력연대로 환산한 C-3 코어에서 취했다. 빈 원은 표층 염도로, 15–20 cal. ka BP 사이에 20‰까지 낮은 값을 보인다. (b) δ^18O 경향은 ^14C 연대를 사용한 J-11 코어에서 산출했다(Gorbarenko and Southon, 2000). 음영 구간은 서로 다른 연대 결정에 따른 낮은 표층 고염도(=저염) 시기를 나타낸다.
There are three possible freshwater sources in the LGM East Sea: (1) freshwater input through the Korea Strait, (2) enhanced local precipitation and (3) river runoff from the surrounding land areas. The low surface paleosalinity event has been explained primarily by two factors: an increased freshwater supply from the East China Sea Coastal Water (ECSCW) (Oba et al., 1991; Tada, 1999; Tada et al., 1999; Gorbarenko and Southon, 2000); and from excess precipitation (Keigwin and Gorbarenko, 1992; Tada, 1995; Gorbarenko and Southon, 2000). It has been argued that precipitation may not be the factor decreasing surface salinity (Lee and Nam, 2003) because decreased temperatures during the glacial periods usually lead to reduction of global precipitation, compared with interglacial time (Kim et al., 2003, 2006). CLIMAP (1981) reconstructions show that the sea-surface temperature (SST) around the East Sea decreased by about 4–8 °C during the LGM. In addition, the lower SSTs have been inferred mainly based on δ18O values of planktonic foraminifera (Oba et al., 1991; Tada, 1999; Gorbarenko and Southon, 2000). The cooled SSTs (less than 8 °C) in the East Sea were also suggested by the dominance of N. pachyderma during the LGM (Kim et al., 2000). Using the aeolian dust accumulation in sediment cores, Irino and Tada (2002) reconstructed stronger wind and reduced precipitation in the East Sea. Saito (1998) suggested that sediment supply by the Yellow River largely decreased due to low precipitation during the LGM. Comparatively, modern meteorological data (Yanagi, 2002) show that evaporation (3.27 mm/day) over the East Sea is higher than precipitation (2.96 mm/day). The colder, windier and drier conditions seem to have been maintained even in the glacial East Sea. Subsequently, the discussion concerning regional precipitation proposes that the effect of sill flow on the low paleosalinity in the LGM East Sea should be reevaluated. The effect of river runoff from the northeastern Asia and the Japanese islands has not been considered.
LGM 동해의 잠재적 담수원은 (1) 대한해협을 통한 담수 유입, (2) 지역 강수 증가, (3) 주변 육지의 하천 유출 세 가지다. 표층 저염은 주로 동중국해 연안수(ECSCW) 기원의 담수 공급 증가(Oba et al., 1991; Tada, 1999; Tada et al., 1999; Gorbarenko and Southon, 2000)와 과도한 강수(Keigwin and Gorbarenko, 1992; Tada, 1995; Gorbarenko and Southon, 2000)로 설명되어 왔다. 그러나 빙기에는 기온 하강으로 간빙기에 비해 전지구 강수가 일반적으로 감소하므로 강수가 표층 염도 저하의 주 요인이 아닐 수 있다는 주장이 제기되었다(Lee and Nam, 2003; Kim et al., 2003, 2006). CLIMAP(1981) 복원은 LGM 동안 동해 주변의 SST가 약 4–8 °C 낮아졌음을 보이며, 낮은 SST는 주로 부유성 유공충의 δ^18O 값에 근거해 추정되었다(Oba et al., 1991; Tada, 1999; Gorbarenko and Southon, 2000). LGM 동안 N. pachyderma의 우세는 SST < 8 °C의 냉각을 시사한다(Kim et al., 2000). 코어의 풍성분 축적을 이용해 Irino와 Tada(2002)는 더 강한 바람과 감소한 강수를 복원했고, Saito(1998)는 LGM 동안 강수 감소로 황하 기원의 퇴적물 공급이 크게 줄었음을 제시했다. 현대 기상자료(Yanagi, 2002)는 동해에서 증발(3.27 mm/일)이 강수(2.96 mm/일)보다 큼을 보이며, 냉량하고 바람이 강하며 건조한 조건이 빙기 동해에도 유지되었음을 시사한다. 따라서 지역 강수 논의에 비추어 LGM 동해의 저염에서 해협문턱 유동(통과유량)의 효과를 재평가해야 하며, 동북아 및 일본 열도의 하천 유출 효과는 아직 충분히 고려되지 않았다.
This study is primarily concerned about the major cause of the low sea-surface salinity in the East Sea during the LGM. The role of hydrological parameters (e.g. evaporation and precipitation) and throughflow at the Korea Strait that might play a role in reducing sea-surface salinity and in regulating regional sea-level conditions in the nearly isolated East Sea during the time of the lowest sea level in the late Quaternary will be addressed. To solve this problem, the relative importance of evaporation and precipitation is investigated with geologic records and climatic simulation. Volume transport at the Korea Strait is also estimated based on seismic reflection data, bathymetric mapping and the present current data. This study includes high-resolution numerical simulation results for the LGM condition of the East Sea. The simulation results are compared with the existing geological records to better understand the late Quaternary paleoceanography of the East Sea.
본 연구는 LGM 동해의 낮은 표층 염도의 주된 원인을 규명한다. 후기 제4기 최저 해수면기의 거의 고립된 동해에서 표층 염도를 낮추고 지역 해수면 상태를 조절했을 수 있는 수문 인자(증발·강수)와 대한해협 통과류의 역할을 다룬다. 이를 위해 지질 기록과 기후 모의를 통해 증발과 강수의 상대적 중요성을 평가하고, 탄성파 반사자료·수심도·현대 유속 자료를 바탕으로 대한해협의 체적 수송을 산정한다. 또한 LGM 조건의 동해에 대한 고해상도 수치모의 결과를 제시하고, 기존 지질 기록과의 비교를 통해 후기 제4기 동해의 고해양학을 더 잘 이해하고자 한다.
2. Background
The deep marginal East Sea is located between the Asian mainland and the Japanese islands and is connected with the Okhotsk Sea, the northwestern Pacific Ocean and the East China Sea (ECS) through the South Sea by four shallow straits (Fig. 1). This sea consists of a narrow shelf, steep slope and deep basin. The modern East Sea is vertically stratified in two water masses: surface layer and deep water. The SSTs range from 0 to 25 °C (Moriyasu, 1972). Temperatures (16–24 °C) and salinities (31.3–34.5‰) have been measured in the surface waters of the southern East Sea during the summer seasons (Kang et al., 1997). Low salinity values (down to 31.3‰) may be caused by freshwater influxes originating from the southeastern coast of Korea (e.g. the Nakdong River) and the Yangtze River (Delcroix and Murtugudde, 2002; Isobe et al., 2002). The surface water circulation is dominated by the warm Tsushima Current in the south and the cold Liman Current in the north. The warm Tsushima Current, a branch of the western boundary Kuroshio Current, enters the East Sea through the Korea Strait and then exits the sea mostly through the Tsugaru and Soya straits (Toba et al., 1982; Lim and An, 1985; Isobe, 1999). This current generally carries warm (26 °C in summer and 14 °C in winter) and saline water to the sea, with a mean velocity of about 10–90 cm/s (Lee and Jung, 1977; Korea Hydrographic Office, 1982; Isobe et al., 1994; Teague et al., 2002). The most acceptable volume transport of the Tsushima Current is about 2 Sv (1 Sv = 10^6 m^3/s) (Toba et al., 1982; Isobe, 1994, 1999) although its measured values range from 1.3 to 2.7 Sv (Yi, 1966; Toba et al., 1982; Lim and An, 1985; Isobe, 1994, 1997; Isobe et al., 2002; Teague et al., 2002; Chang et al., 2004). The Tsushima Current normally is faster in summer than in winter (Lee and Jung, 1977; Korea Hydrographic Office, 1982; Ichiye, 1984; Isobe et al., 1994). The nearly homogeneous deep water, known as the East Sea Proper Water (ESPW), is cold (0–1 °C) and well-oxygenated (5–6 ml/l) (Kim et al., 1996; Itaki et al., 2004; Kang et al., 2004). The deep water is subdivided into three water masses: the East Sea Central Water (ESCW), the East Sea Deep Water (ESDW) and the East Sea Bottom Water (ESBW) (Kang et al., 2004). The cold deep water seems to be formed in the northwestern East Sea because of strong cooling of surface water and sea-ice formation during winter (Nitani, 1972; Gamo et al., 1986; Martine et al., 1992; Kawamura and Wu, 1998).
동해는 아시아 대륙과 일본 열도 사이의 심해성 변연해로, 남해를 통하여 네 개의 얕은 해협(그림 1)으로 오호츠크해·북서태평양·동중국해(ECS)와 연결된다. 이 바다는 좁은 대륙붕·급경사 사면·심해 분지로 이루어지며, 현대 동해는 표층수와 심층수의 두 수괴로 뚜렷이 성층한다. 해수면 온도(SST)는 0–25 °C(Moriyasu, 1972)이고, 여름철 남부 표층수의 온도는 16–24 °C, 염도는 31.3–34.5‰로 관측되었다(Kang et al., 1997). 최저 31.3‰까지 낮은 염도는 한반도 남동 해안(낙동강 등)과 양자강에서 유입된 담수로 설명될 수 있다(Delcroix and Murtugudde, 2002; Isobe et al., 2002). 표층 순환은 남쪽의 따뜻한 쓰시마난류와 북쪽의 찬 리만 해류가 지배한다. 쓰시마난류는 서경계 쿠로시오의 분지로 대한해협을 통해 동해로 유입되어 주로 쓰가루·소야 해협을 통해 빠져나가며(Toba et al., 1982; Lim and An, 1985; Isobe, 1999), 여름 26 °C·겨울 14 °C의 따뜻하고 염분이 높은 물을 평균 10–90 cm/s의 유속으로 운반한다(Lee and Jung, 1977; Korea Hydrographic Office, 1982; Isobe et al., 1994; Teague et al., 2002). 쓰시마난류의 체적 수송은 보통 2 Sv(1 Sv = 10^6 m^3/s)로 알려졌으나(Toba et al., 1982; Isobe, 1994, 1999), 관측치는 1.3–2.7 Sv 범위다(Yi, 1966; Toba et al., 1982; Lim and An, 1985; Isobe, 1994, 1997; Isobe et al., 2002; Teague et al., 2002; Chang et al., 2004). 쓰시마난류는 일반적으로 여름에 겨울보다 빠르다(Lee and Jung, 1977; Korea Hydrographic Office, 1982; Ichiye, 1984; Isobe et al., 1994). 거의 균질한 심층수인 동해 고유수(ESPW)는 0–1 °C의 저온과 5–6 ml/l의 높은 용존산소를 보이며(Kim et al., 1996; Itaki et al., 2004; Kang et al., 2004), 심층수는 중앙수(ESCW)·심층수(ESDW)·저층수(ESBW)로 세분된다(Kang et al., 2004). 겨울철 강한 표층 냉각과 해빙 형성 때문에 북서부 동해에서 심층수가 형성되는 것으로 보인다(Nitani, 1972; Gamo et al., 1986; Martine et al., 1992; Kawamura and Wu, 1998).
A strait with a shallow sill depth is a sensitive seaway because inflow and outflow passing through it are basically controlled by fluctuating sea levels. Global sea-level drop during the LGM (ranging from 23,000 to 19,000 cal. BP; see Mix et al., 2001) might considerably alter geographic and oceanographic conditions around the Korea Strait region, resulting in a reduction of cross-sectional area and sill flow. The Korea Strait is divided into two regions: (1) inner (down to 80 m) and mid-shelf (80–120 m) and (2) outer shelf (deeper than 120 m) including a trough as deep as about 230 m (Park and Yoo, 1988; Yoo and Park, 2000; Yoo et al., 2003). The inner shelf is characterized by thick (~30 m) fine-grained muds (highstand systems tract); in contrast, the mid- and outer shelf (except the trough) are largely blanketed by thin (~5 m) coarse sediments (transgressive systems tract). Although the inner and mid-shelf were subaerially exposed, the outer shelf remained largely below sea level during the LGM.
문턱 수심이 얕은 해협은 그를 통과하는 유입·유출이 해수면 변동에 의해 좌우되므로 민감한 수로다. LGM(약 23,000–19,000 cal. BP)의 전지구적 해수면 하강은 대한해협 일대의 지형·해양 조건을 크게 변화시켜 해협 단면 축소와 해협문턱 유동 감소를 초래했을 것이다. 대한해협은 (1) 내륙붕(≤80 m)·중륙붕(80–120 m)과 (2) 외륙붕(>120 m, 최대 약 230 m의 수로 포함)으로 구분된다(Park and Yoo, 1988; Yoo and Park, 2000; Yoo et al., 2003). 내륙붕은 두꺼운(약 30 m) 세립질 진흙(만수위 계열)으로, 중·외륙붕(수로 제외)은 얇은(약 5 m) 조립질 퇴적(해진 계열)으로 피복된다. 내·중륙붕은 육상 노출되었지만 외륙붕은 LGM 동안 대체로 수중에 남아 있었다.
Despite the lower sea level during the LGM, a large surface area of the East Sea would be maintained. Only about 15% of the present surface area was exposed to air because the East Sea is quite steep and deep. It is generally agreed that all four straits around the East Sea have experienced such sea-level variations in the late Quaternary. The three straits with shallower sill depths, except the Korea Strait, are postulated to have been closed during the LGM (Yasuda, 1984; Oba et al., 1991; Ono and Naruse, 1997; Kim et al., 2000; Ono et al., 2004). Archaeologically, Kuzmin et al. (2002) revealed human exchange and migration from northeastern Asia through Sakhalin to Hokkaido at least since 23,000 cal. BP, indicating a land bridge across the Tartar and Soya straits. A land route to Honshu since 30,000 BP has also been suggested (Oda, 1990; Motohashi, 1996), indicating emergence of the Tsugaru Strait; exposure has been further investigated by ^10Be dating of underwater rocks (Kim and Imamura, 2004). By contrast, the Korea Strait (~200 km wide; sill ~140 m) might not be completely shut down but remain partially submerged in the west at that time (Matsui et al., 1998; Gorbarenko and Southon, 2000; Park et al., 2000; Lee and Nam, 2003; Yoo et al., 2003). The regional sea-level curve shows ~130 m lower sea level during ~25–15 ka BP, suggesting a connection to the northern ECS (Park and Yoo, 1988; Park et al., 2000).
LGM의 낮은 해수면에도 동해의 표면적은 상당히 유지되었고, 동해가 매우 가파르고 깊기 때문에 현재 표면적의 약 15%만 노출되었을 것으로 추정된다. 후기 제4기에 동해 주변 네 해협 모두가 이러한 해수면 변동을 겪었으며, 대한해협을 제외한 문턱 수심이 얕은 세 해협은 LGM 동안 닫혔다는 견해가 일반적이다(Yasuda, 1984; Oba et al., 1991; Ono and Naruse, 1997; Kim et al., 2000; Ono et al., 2004). 고고학적으로, Kuzmin et al.(2002)은 최소 23,000 cal. BP 이래 동북아에서 사할린을 거쳐 홋카이도로의 인적·물적 교류가 있었음을 보였는데, 이는 타타르·소야 해협의 육교를 시사한다. 30,000 BP 이후 혼슈로의 육로 형성도 제시되었고(Oda, 1990; Motohashi, 1996), 이는 쓰가루 해협의 노출을 뜻하며, 수중 암석의 ^10Be 노출연대로도 조사되었다(Kim and Imamura, 2004). 반면 폭 약 200 km·문턱 약 140 m의 대한해협은 당시 서측 일부가 잠긴 부분 개방 상태였을 가능성이 제기된다(Matsui et al., 1998; Gorbarenko and Southon, 2000; Park et al., 2000; Lee and Nam, 2003; Yoo et al., 2003). 지역 해수면 곡선은 약 25–15 ka BP 동안 해수면이 현재보다 약 130 m 낮았음을 보이며, 북부 동중국해와의 연결을 시사한다(Park and Yoo, 1988; Park et al., 2000).
The coastline of East China might have moved to around Cheju Island due to a subaerial emergence of the shallow Yellow Sea. Freshwaters drained into the northern ECS during the LGM (Oba et al., 1991; Tada, 1999; Tada et al., 1999). Inflow of the Kuroshio Current into the ECS was largely restricted due to land-bridge formation around Taiwan and the Ryukyu islands during the LGM (Ujiie et al., 1991; Ahagon et al., 1993; Ujiie and Ujiie, 1999). This further limited the Tsushima Current influx, resulting in a relatively decreased influx of saline water into the East Sea at that time. The water passing through the Korea Strait during the LGM might not be entirely saline, but rather mixed with the Tsushima Current and freshwater (hereafter “the paleo-Tsushima Water”), diluting surface water. Subsequently, the lowered sea-surface salinity might create the density stratification between surface layer and deep water, severely limiting vertical mixing. As a result, anoxic conditions have been developed in the deep water of the East Sea during the LGM (Oba et al., 1991; Keigwin and Gorbarenko, 1992; Gorbarenko and Southon, 2000; Ishiwatari et al., 2001).
얕은 황해가 광범위하게 노출되면서 중국 동부 해안선은 제주도 일대로 후퇴했을 것이다. 담수는 LGM 동안 북부 동중국해로 흘러들었고(Oba et al., 1991; Tada, 1999; Tada et al., 1999), LGM 당시 대만·류큐 일대의 육교 형성으로 쿠로시오의 동중국해 유입은 크게 제한되었다(Ujiie et al., 1991; Ahagon et al., 1993; Ujiie and Ujiie, 1999). 이로써 쓰시마난류 유입도 제한되어 당시 동해로의 염수 유입이 상대적으로 감소했다. LGM 동안 대한해협을 통과한 해수는 순수 염수가 아니라 쓰시마난류와 담수가 섞인 ‘고(古) 쓰시마난류’였고, 표층 희석을 초래하여 표층–심층 간의 밀도 성층을 형성하고 연직 혼합을 크게 제한했을 것이다. 그 결과 LGM 동해의 심층에는 무산소 환경이 발달했다(Oba et al., 1991; Keigwin and Gorbarenko, 1992; Gorbarenko and Southon, 2000; Ishiwatari et al., 2001).
3. Volume transport via the Korea Strait during the LGM. LGM 동안 대한해협의 체적 수송
A lowered sea level might considerably diminish the cross-sectional area of the Korea Strait, limiting the Tsushima Current influx into the East Sea during the LGM. The significantly reduced volume transport at the Korea Strait is estimated based on the cross-sectional passage obtained by bathymetry, seismic reflection profiles and the present-day current meter data. Sediment thickness was extracted from the seismic profiles, assuming that the seismic velocity for the latest Pleistocene–Holocene sediments is 1500 m/s (Cho, 1985; Kim and Suk, 1985).
해수면 저하는 대한해협의 단면적을 크게 감소시켜 LGM 동안 쓰시마난류의 동해 유입을 제한했을 것이다. 대한해협에서 현저히 줄어든 체적 수송은 수심도, 탄성파 반사단면, 현대 유속계 자료로 얻은 통로 단면에 기초해 추정했다. 후기 플라이스토세–홀로세 퇴적물의 탄성파 속도를 1500 m/s로 가정하고(Cho, 1985; Kim and Suk, 1985), 반사단면에서 퇴적 두께를 산정했다.
The cross-sectional area at the Korea Strait is assessed at about 10 km wide and 10 m deep when sea level was the lowest during the LGM (see Fig. 3). To determine the accurate sill depth during the LGM, sediment thickness (less than 7 m) deposited since the LGM was estimated by analyzing seismic reflection profiles (Fig. 3b, c). Similarly, previous studies (Park and Yoo, 1992; Yoo and Park, 2000) found that sediment sequence in the shelf margin (120–130 m deep) is less than 5 m in thickness. Molluscan shells selected at less than 1 m depth in sediment cores were dated by 15,000–21,000 ^14C ages (Yoo and Park, 2000; Yoo et al., 2003). This indicates that sediment accumulation around the shelf margin is presumably much less than 5 m. Thus, about 2 m of the sediment thickness was additionally subtracted, and then the corrected cross-section area (120,000 m^2) during the LGM is obtained by multiplying about 10 km wide and 12 m deep.
LGM 최저 해수면 시기의 대한해협 단면은 폭 약 10 km, 수심 약 10 m로 평가된다(그림 3 참조). LGM 당시의 정확한 문턱 수심을 구하기 위해 탄성파 반사단면(그림 3b, c)을 분석하여 LGM 이후 퇴적 두께(<7 m)를 추정했다. 선행 연구(Park and Yoo, 1992; Yoo and Park, 2000) 역시 대륙붕 경계(수심 120–130 m)의 퇴적층 두께가 5 m 미만임을 보고했다. 퇴적 코어에서 심도 1 m 이내의 연체동물 패각은 15,000–21,000 ^14C 연대를 보였는데(Yoo and Park, 2000; Yoo et al., 2003), 이는 대륙붕 경계 부근의 누적 퇴적이 대체로 5 m보다 훨씬 적음을 시사한다. 따라서 퇴적 두께 약 2 m를 추가 공제했고, 폭 10 km와 수심 12 m를 곱해 LGM 당시 보정 단면적 120,000 m^2을 얻었다.
Fig. 3. The Korea Strait region with a general bathymetry. The paleo-strait is indicated by the area between two thick dotted lines (a). Sparker seismic reflection profiles (b and c) collected within the narrow channel in the Korea Strait shelf region. About 2 m of sediment layer is assumed to have accumulated since the LGM. The sediment thickness is estimated on the basis of the seismic velocity of about 1500 m/s measured in Pleistocene sediments.
그림 3. 대한해협 해역의 일반 수심도. 고(古) 해협은 두꺼운 점선 두 줄 사이의 영역으로 표시했다(a). 스파커 탄성파 반사단면(b, c)은 대한해협 대륙붕의 협소한 수로에서 취득했다. LGM 이후 약 2 m의 퇴적이 축적된 것으로 가정했다. 퇴적 두께는 플라이스토세 퇴적물에서 측정된 약 1500 m/s의 탄성파 속도를 기준으로 산정했다.
Ujiie and Ujiie (1999) suggested that the Kuroshio Current system had moved further southward during the LGM. This might weaken the intrusion of the Kuroshio Current into the north ECS and hence affect the Tsushima Current. For this reason, the current velocity at the LGM Korea Strait appears to have decreased although surface currents in open oceans are usually faster in the glacial than in the interglacial due mainly to the stronger winds (Crowley and North, 1991). Although sands (up to 90%) predominate, gravelly sediments (grain sizes of 2–16 mm) commonly are present on the mid and outer shelf in the Korea Strait region (ranging from 80 to 170 m in water depths), a route for the paleo-Tsushima Water (Park and Yoo, 1992; Yoo and Park, 2000; Yoo et al., 2003). The coarse-grained deposits predominantly are oriented northeast, indicating flow direction and intensity of the paleo-Tsushima Water toward the East Sea. These sediments are interpreted to have deposited between 21 and 15 ^14C ka BP which corresponds to a period of lowest sea level (Yoo and Park, 1997, 2000; Park et al., 2000). According to the diagram relating current velocity to grain size (Sundborg, 1956), most grains of 10 mm do not move until water velocity reaches ~100 cm/s. Moreover, sediment particles (about 2–5 mm in diameter) are usually not in motion at a flow velocity of about 50 cm/s. In consideration of the abundant sands with some gravelly particles, the lower values (10–30 cm/s) of the modern current speeds for this study are adopted to assume the paleo-current velocities for the LGM because it is not possible to directly measure the glacial current velocities. The volume transport at the Korea Strait during the LGM is evaluated as approximately (0.3–1.1) × 10^12 m^3/a, by multiplying 120,000 m^2 by (10–30) cm/s.
Ujiie와 Ujiie(1999)는 LGM 동안 쿠로시오 계통이 더 남하했다고 제안했다. 이는 쿠로시오의 동중국해 북부 침투를 약화시켜 쓰시마난류에도 영향을 미쳤을 수 있다. 이러한 이유로 개방해 표층류가 바람 강화로 빙기에 간빙기보다 보통 더 빠르다 하더라도(Crowley and North, 1991), LGM 당시 대한해협의 유속은 감소했을 가능성이 크다. 대한해협 중·외륙붕(수심 80–170 m)은 모래(최대 90%)가 우세하지만 자갈질 퇴적(입경 2–16 mm)도 흔하며, 이 구간은 고(古) 쓰시마난류의 통로이다(Park and Yoo, 1992; Yoo and Park, 2000; Yoo et al., 2003). 거립 퇴적은 북동향 배열을 보이며 동해를 향하는 고 쓰시마난류의 흐름 방향과 세기를 지시한다. 이 퇴적은 최저 해수면 시기에 해당하는 21–15 ^14C ka BP 동안 퇴적된 것으로 해석된다(Yoo and Park, 1997, 2000; Park et al., 2000). 유속–입경 관계도(Sundborg, 1956)에 따르면 10 mm 입자는 유속이 약 100 cm/s에 도달하기 전에는 대체로 이동하지 않으며, 지름 약 2–5 mm의 입자도 유속 약 50 cm/s에서는 보통 이동하지 않는다. 모래가 우세하고 자갈 입자가 일부 섞인 점을 고려해, 빙기 유속을 직접 측정할 수 없으므로 본 연구에서는 현대 유속 하한(10–30 cm/s)을 LGM 고유속의 가정값으로 채택했다. 이에 따라 대한해협의 LGM 체적 수송은 보정 단면적 120,000 m^2에 10–30 cm/s를 곱해 대략 (0.3–1.1) × 10^12 m^3/년으로 산정된다.
Previous studies also suggested that the water mass flowing through the Korea Strait diminished by more than 95% during the LGM (Tada, 1995; Matsui et al., 1998). This means that only about 5% (3.0 × 10^12 m^3/a) of the present transport volume entered the East Sea at that time period. This sill flow during the LGM is assessed, based on the suggested volume transport (2 Sv) passing through the Korea Strait. The comparison between two calculated values of (0.3–1.1) × 10^12 m^3/a and 3.0 × 10^12 m^3/a shows that these estimates for the volume transport at the strait during the LGM seem to be reasonable.
선행 연구는 LGM 동안 대한해협을 통과하는 수괴가 현재보다 95% 이상 감소했다고도 제시한다(Tada, 1995; Matsui et al., 1998). 이는 당시 현재 수송의 약 5%(3.0 × 10^12 m^3/년)만 동해에 유입되었음을 뜻한다. 이러한 LGM기의 해협문턱 통과 유량 평가는 대한해협을 지나는 체적 수송 2 Sv 가정에 기초한다. (0.3–1.1) × 10^12 m^3/년과 3.0 × 10^12 m^3/년의 두 계산값을 비교하면 LGM 동안 해협 체적 수송에 대한 이 추정들이 타당함을 보인다.
4. Effect of surface evaporation and throughflow on low paleosalinity. 낮은 고(古) 표층 염도에 대한 표면 증발과 통과류의 효과
The paleo-hydrological values (evaporation and precipitation) for the LGM are taken from a high-resolution numerical simulation by Kim et al. (2006) who used NCAR CCM3 at spectral truncation of T170, corresponding to a grid cell size of roughly 75 km. The simulation results reveal that evaporation (3.43 mm/day) exceeds precipitation (2.85 mm/day) in the present East Sea. Additionally, excessive evaporation (2.16 mm/day) over precipitation (1.43 mm/day) is calculated for the LGM East Sea. The net evaporation rates in the LGM East Sea are evaluated at 0.2×10^12 m^3/a, using evaporation minus precipitation (0.73 mm/day) and the surface area (0.85×10^6 km^2).
LGM의 고(古) 수문값(증발·강수)은 Kim et al.(2006)의 NCAR CCM3 T170(격자 약 75 km) 고해상도 수치모의에서 취했다. 모의 결과, 현대 동해에서는 증발(3.43 mm/일)이 강수(2.85 mm/일)보다 크며, LGM 동해에서도 증발(2.16 mm/일)이 강수(1.43 mm/일)보다 크다. 증발–강수(0.73 mm/일)와 표면적(0.85×10^6 km^2)을 이용해 계산한 LGM 동해의 순증발은 0.2×10^12 m^3/년이다.
Both numerical model results and geological records show that evaporation was higher than precipitation in the East Sea during the LGM. This attests that precipitation may not be the factor lowering the sea-surface salinity in the LGM East Sea. The excessive surface evaporation during the LGM indicates that more throughflow via the Korea Strait is required to sufficiently offset the evaporation effect during the LGM than previously suggested. This further supports that the freshwater entered the East Sea was sustained by the ECSCW derived from the Yellow and Yangtze Rivers.
수치모형과 지질기록 모두 LGM 동해에서 증발이 강수보다 컸음을 보여 준다. 이는 LGM 동해의 표층 염도를 낮춘 주 요인이 강수가 아님을 의미한다. LGM의 과잉 표면 증발은 이전 제안보다 더 큰 대한해협 통과류가 증발 효과를 충분히 상쇄했어야 함을 시사한다. 이는 또한 황하와 양자강에서 기원한 동중국해 연안수(ECSCW)가 동해로 유입되는 담수를 지속적으로 공급했음을 지지한다.
According to the studies of the planktonic foraminiferal assemblage, the Kuroshio Current might not have flowed into the ECS due to the land bridge during the LGM (Ujiie et al.,1991; Ujiie and Ujiie, 1999; Li et al., 2001; Ujiie et al., 2003). The isolation of the ECS from the Pacific Ocean is supported by a significantly reduced abundance of Pulleniatina obliquiloculata and an increase in the frequency of the coastal water species (e.g. Globigerina bulloides). On the other hand, the limited propagation of the Kuroshio Current over the Okinawa Trough region has been proposed based on the existence of N. dutertrei during the LGM (Xu and Oda, 1999; Ijiri et al., 2005). Recent palynological studies reported the occurrence of Phyllocladus in the northern ECS sediment deposited between 25 and 8 ka BP (Kawahata and Ohshima, 2004). This pollen genus originating from the tropical areas seems to have been transported to the ECS by the Kuroshio Current during the last Glacial period. Whether the ECS was completely isolated or not, the ECSCW prevailed in the ECS region due to the land bridge formation and southward shift of the Kuroshio Current pathway, especially over the Okinawa Trough during the LGM. The dominance of the ECSCW over the Okinawa Trough region is also supported by the positive correlation between d18O and d13C signals of planktonic foraminifera (Globigerinoides ruber) during the LGM, implying freshwater supply from the Yellow and the Yangtze Rivers (Ijiri et al., 2005). This work thus indicates that the ECSCW extended to the East Sea through the Korea Strait, and then decreased surface salinity during the LGM. The ECSCW expansion to the East Sea is further evidenced by the occurrence of Paralia sulcata in the East Sea because P. sulcata is a useful diatom species for less saline river water (Koizumi, 1989; Tada et al., 1999).
부유성 유공충 군집 연구에 따르면 LGM 동안 육교 형성으로 쿠로시오 해류가 동중국해(ECS)로 유입하지 못했을 가능성이 크다(Ujiie et al., 1991; Ujiie and Ujiie, 1999; Li et al., 2001; Ujiie et al., 2003). 태평양과 ECS의 고립은 Pulleniatina obliquiloculata의 현저한 감소와 연안수 지시종(예: Globigerina bulloides)의 증가로도 지지된다. 한편 오키나와 해구 일대에서의 쿠로시오 제한 전파는 LGM 동안 N. dutertrei의 존재에 근거하여 제안되었다(Xu and Oda, 1999; Ijiri et al., 2005). 화분학 연구는 북부 ECS의 25–8 ka BP 퇴적에서 열대 기원의 Phyllocladus 산출을 보고했는데(Kawahata and Ohshima, 2004), 이는 마지막 빙기 동안 쿠로시오를 따라 운반되었음을 시사한다. ECS가 완전히 고립되었는지 여부와 무관하게, 특히 LGM 기간 오키나와 해구에서는 육교 형성과 쿠로시오 경로의 남하로 ECS 연안수가 우세했다. LGM 동안 부유성 유공충(Globigerinoides ruber)의 δ^18O와 δ^13C 신호가 양의 상관을 보인다는 사실 역시 황하·양자강의 담수 공급을 시사한다(Ijiri et al., 2005). 따라서 ECS 연안수(ECSCW)가 대한해협을 통해 동해로 확장되어 LGM 동안 표층 염도를 낮추었음이 지시된다. 동해에서 관찰되는 Paralia sulcata 산출은 이 종이 저염 하천수의 유용한 지시 규조류라는 점에서 이러한 확장의 추가 증거다(Koizumi, 1989; Tada et al., 1999).
River runoff from the surrounding land areas has been little studied because the ECSCW has been regarded as an important freshwater source for the LGM East Sea. Presently, the Amur River flows to the northern part of the East Sea along the Russian coast near the Tartar Strait (Nijssen et al., 2001). Southward transport of freshwater from the Amur River has been measured about 2600 m^3/s at a channel around the Tartar Strait (Yakunin, 1975). Simulation results reported that freshwater discharges by the Amur River were increased by about 80% during the LGM (Kim et al., 2003). This increased river runoff suggests that the Amur River could, to some extent, contribute to the increase in freshwater in the East Sea. It is not possible to quantitatively estimate how much the paleo-Tsushima Water carried freshwater and how much the river runoff discharged in the East Sea during the LGM.
ECSCW가 LGM 동해의 중요한 담수원으로 간주되어 온 탓에 주변 육지의 하천 유출에 대한 연구는 상대적으로 적다. 현재 아무르강은 타타르 해협 인근 러시아 연안을 따라 동해 북부로 흘러들며(Nijssen et al., 2001), 타타르 해협 주변 수로에서 남향 담수 수송이 약 2600 m^3/s로 관측되었다(Yakunin, 1975). 수치모의는 LGM 동안 아무르강 담수 유출이 약 80% 증가했음을 보고한다(Kim et al., 2003). 이러한 유량 증가는 아무르강이 동해의 담수 증가에 일정 부분 기여했을 가능성을 시사하지만, LGM 당시 고(古) 쓰시마난류가 운반한 담수와 하천 유출의 상대적 기여를 정량적으로 분리해 추정하는 것은 불가능하다.
Accordingly, the excessive surface evaporation indicates that the paleo-Tsushima Water carried huge amounts of freshwater originated from the ECSCW than previously expected. The extensive invasion of the ECSCW to the East Sea reflects at least the partial opening of the Korea Strait during the LGM (Morley et al., 1986; Koizumi, 1989; Oba et al., 1995; Tada et al., 1999; Park et al., 2000; Lee and Nam, 2003).
따라서 과잉 표면 증발은 고(古) 쓰시마난류가 기존 예상보다 훨씬 많은 ECS 연안수 기원의 담수를 운반했음을 뜻한다. 동해로의 광범위한 ECS 연안수 확장은 LGM 동안 대한해협이 적어도 부분적으로 개방되어 있었음을 반영한다(Morley et al., 1986; Koizumi, 1989; Oba et al., 1995; Tada et al., 1999; Park et al., 2000; Lee and Nam, 2003).
5. Regional control on sea-level condition. 해수면 상태에 대한 지역적 제어
The comparison between the surplus evaporation (0.2 × 10^12 m^3/a) and volume transport (0.3–1.1 × 10^12 m^3/a) shows that two values are not quantitatively identical (Fig. 4). However, the two values could be considered as similar from an oceanographic point of view. This comparison implies that most of the throughflow eventually escaped the East Sea via evaporation during the LGM. It, furthermore, may be inferred that the sea level in the LGM East Sea might be primarily maintained by the incoming paleo-Tsushima Water and the outgoing evaporation. It is hard to tell whether there was an outflow from the East Sea to the northern ECS through the paleo-Korea Strait. Because the sill depth was only about 12 m, it may have been difficult for outflow to exist. There is no geological evidence on the countercurrent in the strait region at that time period even though the inflow of the paleo-Tsushima Water into the East Sea has been reported (Park and Yoo, 1992; Yoo et al., 1996; Yoo and Park, 1997). Tide influence on the East Sea as well as the northern ECS seems to have been negligible because of the land bridge formation between the Taiwan and the Ryukyu Islands. As a result, regionally excessive evaporation may serve as the most important factor regulating sea-level conditions in the nearly isolated East Sea during the LGM.
초과 증발(0.2 × 10^12 m^3/년)과 체적 수송(0.3–1.1 × 10^12 m^3/년)의 비교는 두 값이 정량적으로 동일하지 않음을 보여 주지만(그림 4), 해양학적으로는 유사한 규모로 간주될 수 있다. 이 비교는 LGM 동안 통과류의 대부분이 증발을 통해 동해에서 이탈했음을 시사한다. 더 나아가 LGM 동해의 해수면은 유입되는 고(古) 쓰시마난류와 유출되는 증발의 균형으로 주로 유지되었을 가능성이 크다. 고(古) 대한해협을 통해 동해에서 북부 동중국해로 유출이 있었는지는 판단하기 어렵다. 문턱 수심이 약 12 m에 불과해 유출이 성립하기 어려웠을 수 있으며, 당시 해협역 역류에 대한 지질학적 증거도 없다(고 쓰시마난류의 동해 유입은 보고됨: Park and Yoo, 1992; Yoo et al., 1996; Yoo and Park, 1997). 대만–류큐 사이의 육교 형성으로 동해와 북부 동중국해에서의 조석 영향은 미미했을 것으로 보인다. 결과적으로 지역적 과잉 증발이 LGM의 거의 고립된 동해에서 해수면 상태를 규정한 가장 중요한 요인이었을 수 있다.
Fig. 4. Simplified diagrams illustrate that the Tsushima Current flows into the East Sea through the Korea Strait and flows out through the Tsugaru and Soya Straits (a). The sill flow significantly decreased due to the reduced cross-sectional passage when sea level was the lowest during the LGM (b). The throughflow ultimately escaped the sea mostly through excessive surface evaporation, maintaining a rough balance between them. This further indicates that regional factors might play an important role in sea level conditions in nearly isolated East Sea during the LGM.
그림 4. 도식도는 쓰시마난류가 대한해협을 통해 동해로 유입되어 쓰가루·소야 해협으로 유출됨(a), LGM 최저 해수면기에 단면 축소로 해협문턱 유동이 크게 감소함(b), 그리고 통과류가 과잉 표면 증발로 대부분 이탈하면서 양자 간 거친 균형이 유지되었음을 보여 준다. 이는 LGM의 거의 고립된 동해에서 해수면 조건을 지역 요인이 좌우했을 가능성을 시사한다.
6. Comparison between the LGM East Sea and the present Black Sea. LGM 동해와 현대 흑해의 비교
The Black Sea is a small semi-closed sea which links to the Sea of Marmara through the Bosporus Strait (about 3 km wide and 50 m sill depth) and further to the Mediterranean Sea via the Dardanelles Strait. This sea occupies an area of 422,000 km^2 with a maximum depth of about 2200 m. The Black Sea is a large anoxic marine basin due to significantly limited mixing between much less saline (down to 20‰) surface layer water (down to 100 to 150 m deep) and highly saline (38–39‰) deeper water (Latif et al., 1992; Ozsoy and Unluata, 1997; Aksu et al., 2002a). Oceanographic conditions in the Black Sea are primarily regulated by changes in the inflowing Mediterranean water and outflowing Black Sea water through the narrow and shallow Bosporus Strait. The cold and less salty surface (20–30 m thick) water flows out of the Black Sea via the Bosporus Strait toward the Mediterranean Sea, whereas warm and saline Mediterranean water flows into the Black Sea below the surface outflow (Ozsoy et al., 1995; Polat and Tugrul, 1996). Presently, about 300 km^3/a of water flows out of the Black Sea to the Mediterranean Sea through the Bosporus Strait. This outflow is a result of surplus precipitation and high river discharge, which exceed evaporation in the Black Sea area (Ozsoy et al., 1995; Aksu et al., 2002b). During the last glacial maximum, globally lowered sea level led to closure of the Bosporus Strait, ceasing the Mediterranean inflow. The Black Sea then became a freshwater lake at least in the upper layer (Degens and Ross, 1974; Ryan et al., 1997; Karaca et al., 1999; Aksu, et al., 2002b).
흑해는 보스포루스 해협(폭 약 3 km, 문턱 수심 약 50 m)을 통해 마르마라해와 연결되고, 다시 다르다넬스 해협을 거쳐 지중해와 이어지는 소규모 반폐쇄성 바다이다. 면적은 422,000 km^2, 최대 수심은 약 2200 m이다. 흑해는 표층(염도 최저 ~20‰, 깊이 100–150 m)과 심층(염도 38–39‰) 간의 혼합이 크게 제한되어 대규모 무산소 분지가 형성되어 있다(Latif et al., 1992; Ozsoy and Unluata, 1997; Aksu et al., 2002a). 흑해의 해양학적 상태는 좁고 얕은 보스포루스를 통한 지중해수 유입과 흑해수 유출의 변동에 의해 주로 조절된다. 찬 저염의 표층수(두께 20–30 m)는 보스포루스를 통해 지중해로 유출되고, 따뜻하고 염분이 높은 지중해수는 표층 유출 아래에서 흑해로 유입된다(Ozsoy et al., 1995; Polat and Tugrul, 1996). 현재 연간 약 300 km^3의 물이 보스포루스를 거쳐 지중해로 유출되는데, 이는 흑해 지역에서 과잉 강수와 큰 하천 유입이 증발을 초과하기 때문이다(Ozsoy et al., 1995; Aksu et al., 2002b). 최종빙기 절정기에는 전지구 해수면 하강으로 보스포루스가 닫혀 지중해 유입이 중단되었고, 흑해는 적어도 상층에서 담수호가 되었다(Degens and Ross, 1974; Ryan et al., 1997; Karaca et al., 1999; Aksu et al., 2002b).
Both the present Black Sea and the glacial East Sea are characterized by similar oceanographic features because they are connected with the neighboring marine environments by narrow, channel-like straits, the Bosporus Strait and the Korea Strait, respectively. One of the striking characteristics in both seas is a low sea-surface salinity of down to about 20%, approximately half as saline as the normal ocean (34–35%). The low surface salinity in the Black Sea is due to the river runoff and excessive precipitation (Ozsoy et al., 1995; Aksu et al., 2002b). On the other hand, precipitation was not a factor in the paleo-East Sea. The paleo-Tsushima Water influx together with the Amur River discharge might have contributed to the reduction in the surface salinity during the LGM. The low surface salinity results in the strong stratification and inhibits vertical mixing between surface layer and deep water. Thus, anoxic conditions could have been established in deep sediments in the LGM East Sea as in the modern Black Sea (Degens and Ross, 1974; Oba et al., 1991; Jones and Gagnon, 1994; Gorbarenko and Southon, 2000; Ishiwatari et al., 2001; Aksu et al., 2002b).
현대 흑해와 빙기 동해는 각각 보스포루스 해협과 대한해협이라는 좁은 수로형 해협을 통해 주변 해역과 연결되어 있어 유사한 해양학적 특성을 보인다. 두 해역의 두드러진 공통점 하나는 표층 염도가 약 20%까지 낮다는 점으로, 이는 일반 해양(34–35%)의 약 절반 수준이다. 흑해의 낮은 표층 염분은 하천 유출과 과도한 강수에 기인한다(Ozsoy et al., 1995; Aksu et al., 2002b). 반면 고(古) 동해에서는 강수가 요인이 아니었다. LGM 동안 표층 염분 저하에는 고(古) 쓰시마난류 유입과 아무르강 유출이 함께 기여했을 수 있다. 낮은 표층 염도는 강한 성층을 초래해 표층과 심층 사이의 연직 혼합을 억제한다. 그 결과 LGM 동해의 심부 퇴적에서는 현대 흑해와 마찬가지로 무산소 상태가 형성되었을 수 있다(Degens and Ross, 1974; Oba et al., 1991; Jones and Gagnon, 1994; Gorbarenko and Southon, 2000; Ishiwatari et al., 2001; Aksu et al., 2002b).
The local effects (e.g. river runoff, precipitation, evaporation) may have a critical factor on sea level for each sea. In the Black Sea, unusually high river discharges along with excessive precipitation not only dilute the surface layer, but increase the water level, about 30 cm higher than that of the adjacent Marmara Sea (Besiktepe et al., 1994). In contrast, excessive evaporation prevailed over the East Sea, and seems to have been roughly balanced with the throughflow, regulating the sea level during the LGM. Consequently, the comparison between both seas strongly indicates that regional conditions could serve as an important parameter regulating surface water properties and sea level in an almost isolated sea.
각 해역의 지역적 요인(예: 하천 유출, 강수, 증발)은 해수면에 결정적일 수 있다. 흑해에서는 비정상적으로 큰 하천 유량과 과도한 강수가 표층을 희석시킬 뿐 아니라 수위를 인접한 마르마라해보다 약 30 cm 높게 만든다(Besiktepe et al., 1994). 반대로 동해에서는 과잉 증발이 우세했으며 통과류와 대략 균형을 이루면서 LGM 동안 해수면을 조절했을 것으로 보인다. 따라서 두 해역의 비교는 거의 고립된 바다에서 표층 수물성과 해수면을 규제하는 중요한 매개로 지역적 조건이 작용할 수 있음을 강하게 시사한다.
7. Conclusions
Sea-level fall during the LGM altered geographic and oceanographic conditions, resulting in exposure of shallow-water areas, reduction of sill flow with a relatively increased freshwater, freshening of surface water, and restriction of vertical mixing in the East Sea. About 15% of surface area might have been subaerially exposed and three straits shallower than 130 m in sill depths were closed, blocking water exchanges through them in the LGM East Sea. By contrast, the Korea Strait was partially open and allowed the reduced sill flow (0.3–1.1 × 10^12 m^3/a) which might be mixed with the ECSCW and seawater (“the paleo-Tsushima Water”) during the time of the lowest sea level. The model results show that the evaporation exceeded the precipitation by 0.2 × 10^12 m^3/a in the East Sea during the LGM, and are well-correlated with existing geological records. This means that the precipitation was not the factor decreasing sea-surface salinity. Moreover, the excessive evaporation indicates that the paleo-Tsushima Water transported substantial amounts of freshwater to lower the surface salinity. The comparison between surface evaporation and volume transport suggests that most of the throughflow ultimately escaped the East Sea through evaporation during the LGM. This may imply that regional sea level might be largely maintained by a rough balance between incoming paleo-Tsushima Water and outgoing evaporation in the latest Quaternary. The modern Black Sea can be considered as an analog of the glacial East Sea. The two seas are characterized by limited connection through a narrow and shallow strait to an adjoining marine environment, and by low sea-surface salinity. Sea-level variations are primarily influenced by local effects in the modern Black Sea and in the glacial East Sea. The former is controlled by high river runoff and surplus precipitation, leading to outflow from the Black Sea to the Mediterranean Sea. The later glacial sea is regulated by excessive evaporation, roughly balanced with the amount of throughflow.
LGM의 해수면 하강은 동해의 지리·해양 조건을 변화시켜 천해 노출, 담수 상대 증가를 동반한 해협문턱 유동 감소, 표층 저염화, 연직 혼합 제한을 초래했다. 표면적의 약 15%가 육상 노출되고 문턱 수심 ≤130 m의 세 해협이 폐쇄되어 LGM 동해에서 해수 교환이 차단되었다. 반대로 대한해협은 부분 개방되어 최저 해수면기에도 해협문턱 통과 유량(0.3–1.1 × 10^12 m^3/년)이 유지되었고, 이 유량은 동중국해 연안수와 해수가 섞인 ‘고(古) 쓰시마난류’를 형성했을 수 있다. 모형 결과는 LGM 동해에서 증발이 강수보다 연간 0.2 × 10^12 m^3만큼 컸으며, 기존 지질 기록과도 잘 부합한다. 이는 표층 저염의 주된 요인이 강수가 아님을 의미한다. 또한 과잉 증발은 고 쓰시마난류가 상당한 담수를 운반했음을 시사한다. 표면 증발과 체적 수송의 비교는 통과류의 대부분이 증발로 동해를 이탈했음을 보이며, 후기 제4기 동해의 해수면이 유입(고 쓰시마난류)–유출(증발)의 거친 균형으로 유지되었을 가능성을 시사한다. 현대 흑해는 빙기 동해의 유용한 상사체로, 두 해역 모두 좁고 얕은 해협을 통한 제한적 연결과 낮은 표층 염도를 특징으로 한다. 다만 현대 흑해는 높은 하천 유출과 과잉 강수가 지배하여 지중해로의 유출을 유도하는 반면, 빙기 동해는 과잉 증발이 통과류와 대략 균형을 이루며 지배했다.
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