6 Comparison of alkenone-based core-top temperatures with observed temperatures at 0–30 m water depth (from the JODC database). 9 Comparison of temperatures based on Mg/Ca core to top ratio with observed temperatures at water depth 0 – 50 m (from JODC database). 13 Comparison of B-3GC temperatures based on core and top foraminiferal assemblages with observed SSTs (from the JODC database).
동중국해의 과거 지표수 온도를 재구성하는 데 사용된 알케논, Globigerinoides ruber, 플랑크톤 유공충 군집 및 글리세롤 디알킬글리세롤 테트라에테르(GDGT) 프록시의 Mg/Ca 비율을 비교했습니다. 먼저, 우리는 수층의 각 프록시와 관련된 유기체의 분포를 조사하여 이러한 프록시에 의해 복원된 온도가 나타내는 온도를 알아냈습니다. 플랑크톤 유공충 군집에서 회수된 여름 표면 수온은 가장 높았으며, 알케논으로 회수된 온도는 Mg/Ca 비율로 회수된 온도보다 낮았습니다.
지표수 온도는 복원 시 사용된 통계처리 방법이나 데이터베이스에 따라 달라지는 것으로 보인다. 따라서 알케논과 Mg/Ca 프록시는 동중국해의 표층수 온도를 복원하는 데 적합한 것으로 간주됩니다.
Introduction
However, other studies have suggested that the alkenone temperature represents spring-summer or spring-summer SSTs in the East China Sea (Ijiri et al., 2005; Zhou et al., 2007; Nakanishi et al., 2012a). The representative GDGT has not been extensively studied in the East China Sea, and the representative depth of the GDGT is unclear in this region (Nakanishi et al., 2012a). Thus, there is no consensus on what each proxy represents, and furthermore, no comparative assessment has previously been published regarding these proxies in the East China Sea.
Here, we compared these four temperature proxies for the reconstruction of SSTs in the East China Sea. To find out what the proxy temperatures represent, we examined the seasonal and vertical distribution pattern of each proxy-related producer in water column. Furthermore, the proxy temperature estimates from core top sediments were compared with observed SSTs to confirm that proxy temperatures reconstructed from marine sediments can represent current SSTs in the East China Sea.
Sediment cores
Chronology
The age of the cores was determined primarily from radiocarbon dating of planktonic foraminifera (Table 2). However, in core A7, a reservoir age of 700 years was used because the converted calendar ages agreed well with the K-Ah tephra age (Sun et al., 2005). The ages in core Z14-6 were determined using the correlation between the oxygen isotope curve of the cores and the δ18O stacking curve (Zhou et al., 2007).
Sedimentation rates
Temperatures from the SST proxies
- Habitat depth of alkenone producer
- Alkenone seasonality
- Comparison of core-top alkenone based temperatures with
- Foraminiferal Mg/Ca ratio based temperature
- Habitat depth of G. ruber
- Seasonality of G. ruber
- Comparison of core-top Mg/Ca ratio based temperatures with
- Planktonic foraminiferal assemblage based temperature
- Estimation techniques
- Comparison of core-top planktonic foraminiferal assemblage
- GDGTs based temperature
- Habitat depth of GDGTs producer
- Seasonality of GDGTs
- Comparison of core-top GDGTs based temperature with
In addition, Sawada et al. 1998) compared MB'37 of sediments in the upper part of the core with MB'37 of time-series sediment trap samples. The high baseline UK'37 is consistent with the flux-weighted annual mean UK'37 of trap samples. To examine whether temperatures based on alkenones reconstructed from marine sediments represent SST, alkenone temperatures estimated using core core samples were compared with annual mean temperatures in water depths of 0 - 30 m (Fig. 6).
This suggests that the core-top sediment of core MD982195 may not be well preserved. However, based on the similarity to SSTs seen in data from the other two cores, we suggest that alkenone-based temperatures reconstructed from core-top sediments may represent annual mean SST. Core-top Mg/Ca temperatures were reconstructed from cores MD012403, MD012404, and KY07-04 PC-01, from the southern, middle, and northern parts of the Okinawa Trough in the East China Sea, respectively.
The SIMMAX technique uses a similarity index to select core-top sediments that have a similar assemblage (Pflaumann et al., 1996). To investigate whether foraminiferal-based temperatures of marine sediments are consistent with current SSTs, core top summer and winter. In contrast, the winter temperature of the core top sediment was 25.0℃, which was 3℃ warmer than the observed February SST (22.1℃).
10 Comparison of DGKS9603 core-top foraminiferal assemblage based on summer and winter temperatures with observed SSTs (from JODC database). Thus, foraminiferal assemblage-based temperatures estimated using the RAM technique from MD012404 core-top sediments represent observed SSTs. 11 Comparison of MD012404 core-top foraminiferal assemblage based on summer and winter temperatures with observed SSTs (from JODC database).
In summary, the summer collection temperatures reconstructed from the upper core sediments can represent the observed August SST. To examine whether the GDGT-based reconstructed marine sediment temperatures are consistent with SSTs, the upper GDGT temperatures were compared with observed annual mean SSTs (JODC database) (Figure compared GDGT temperatures of top sediments from a global dataset with observed temperatures recorded at water depths of 0 – 4000 m. So we compared annual mean temperatures at 0 m depth (JODC data date) with GDGT temperature on top of core KY07-04 PL-01.
Paleo-temperature estimates
Last Glacial Maximum (18 - 21 kyr)
We compared 3-kyr mean temperatures reconstructed from alkenone, Mg/Ca, and foraminifera assemblage proxies during the late Holocene (0 – 3 kyr) with SSTs during the LGM (18 – 21 kyr), when the climate was dramatically different than the present (Fig. .16). Although only 1000 years of data exist on the Mg/Ca-based temperature for core KY07-04 PC-01 during the LGM, it appears that it was 4.1℃ cooler during the LGM than during the late Holocene. Assembly-based temperatures could not be compared because there is no data during the LGM.
In the middle Okinawa valley (Figs. 16b, 16e), alkenone-based temperatures of cores Z14-6 and DGKS9604 were 24.4℃ and 22.7℃ during the LGM, respectively, which are 1.9℃ and 3.4 ℃ cooler than their respective temperatures during alkene. late Holocene. There are no records of temperature indicators in the southern Okinawa valley during the LGM. Overall, temperature comparisons between the two periods suggest that SSTs during the LGM were lower than during the late Holocene.
Comparisons between SST proxies during the late Holocene and
It seems reasonable because the alkenone temperature represents the annual mean temperature, while the Mg/Ca temperature represents the summer-autumn temperature. The lower Mg/Ca temperatures compared to the summer temperatures seem reasonable based on the clustering because the Mg/Ca temperature represents the summer-autumn temperature. In summary, the summer composite SSTs were the warmest of the three proxy SSTs in both the late Holocene and the LGM.
Winter assemblage SSTs were lower than the alkenone and Mg/Ca temperatures during the late Holocene, but close to these two other proxy temperatures during the LGM. It is possible that the warm SSTs in the winter assemblage are caused by the uncertainty in estimating temperatures from foraminiferal assemblages.
Uncertainties of calibration equations
Foraminiferal Mg/Ca proxy
Foraminifera shells must be cleaned to remove clay minerals, organic matter, and so on, before samples can be geochemically analyzed to determine Mg/Ca ratios. Some authors believe that Mg/Ca ratios of foraminifera are very sensitive to the cleaning method used (Barker et al. 2003). In addition, all Mg/Ca temperatures for the East China Sea were calculated using the Hastings et al.
Thus, the issue of errors due to the use of a different equation does not arise.
Planktonic foraminiferal assemblage proxy
18 Comparison of temperatures based on foraminiferal assemblages calculated using two different statistical techniques: FP-12E (a), SIMMAX-28 (b) and the difference between them (c). Therefore, temperatures estimated using the foraminiferal assemblage proxy appear to be influenced by statistical techniques. 2010) used the MAT technique to reconstruct past East Sea SSTs for a global foraminifera dataset as well as a regional dataset. The MAT temperatures reconstructed using these two different foraminiferal assemblages (global data set vs. regional data set) were completely different.
We felt that in this case the temperature estimates obtained from the East Sea data set were more reliable because it is difficult to find core-top sediments that have foraminiferal assemblages similar to those from the global data set. The western Pacific and northwestern Pacific datasets were used to reconstruct past SSTs for the East China Sea (Table 4), except for the core DGKS9603 (global dataset). The winter temperatures of core DGKS9603 at the top of the core were 3℃ warmer than the observed February SST (Figure 10).
In addition, mean winter temperatures during the late Holocene (0–3 kyr) were also 3.4℃ warmer than observed February SST (Fig. 16b). Thus, winter temperatures based on foraminiferal assemblages appear to be largely influenced by the data set used.
Conclusions
Thus, alkenones and Mg/Ca ratios appear to be the most robust choices for paleothermometry in the East China Sea. Millennial-scale variability of the planktic foraminiferal fauna in the East China Sea over the past 40,000 years (IMAGE MD012404 from the Okinawa Trough). Monsoon hydrography and productivity changes in the East China Sea over the past 100,000 years: evidence from the Okinawa Trough (MD012404).
A comparison of three independent paleotemperature estimates from a high-resolution record of deglacial SST data in the tropical South China Sea. Paleoenvironmental changes in the northern East China Sea during the past 42,000 years. East Asian summer monsoon variations since the last deglaciation based on Mg/Ca and oxygen isotope of planktic foraminifera in the northern East China Sea.
Seasonal changes in the isotopic composition of planktonic foraminifera from sediment traps in the South China Sea. Distribution of glycerol dialkyl glycerol tetraethers, alkenones and polyunsaturated fatty acids in suspended organic matter in the East China Sea. Modern distribution patterns of planktonic foraminifera in the South China Sea and western Pacific: a new transmission technique for estimating regional sea surface temperatures.
Production and transport of long-chain alkenones and alkyl alkenoates in a seawater column in the northwest Pacific Ocean off central Japan. Last deglaciation in the Okinawa Trough: subtropical northwest Pacific Ocean, associated with the Northern Hemisphere and tropical climate. Coccolith flows and species assemblages at the shelf edge and in the Okinawa Trench of the East China Sea.
Planktonic foraminifera in the western North Pacific during the past 150 000 years: comparison of modern and fossil assemblages.
